Plant Biology - Advanced

Home , Antheridium, Bryopsida, Carolus (plant), Cooksonia, Cycad, Devonian explosion

Douglas Wilkin, Ph.D. Barbara Akre

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Printed: January 24, 2016 www.ck12.org Chapter 1. Plant Biology - Advanced

CHAPTER 1 Plant Biology - Advanced

CHAPTER OUTLINE 1.1 Why Study Plants? - Advanced 1.2 Evolution of Plants - Advanced 1.3 Classiﬁcation of Plants - Advanced 1.4 Plant Life Cycles - Advanced 1.5 Bryophytes - Advanced 1.6 Diversity of Nonvascular Plants - Advanced 1.7 Human Uses of Nonvascular Plants - Advanced 1.8 Vascular Plants - Advanced 1.9 Evolution of Vascular Plants - Advanced 1.10 Early Vascular Plants - Advanced 1.11 Ferns - Advanced 1.12 Seed Plants - Advanced 1.13 Seed Plants Evolution - Advanced 1.14 Diversity of Seed Plants - Advanced 1.15 Flowering Plants - Advanced 1.16 Characteristics of Angiosperms - Advanced 1.17 Evolution of Flowering Plants - Advanced 1.18 Diversity of Flowering Plants - Advanced 1.19 Eudicots - Advanced 1.20 Monocots - Advanced 1.21 Flowering Plant Tissues - Advanced 1.22 Plant Cells - Advanced 1.23 Dermal Tissue of Plants - Advanced 1.24 Ground Tissue of Plants - Advanced 1.25 Vascular Tissue of Plants - Advanced 1.26 Growth of Plants - Advanced 1.27 Roots - Advanced 1.28 Root Types - Advanced 1.29 Root Structure - Advanced 1.30 Root Function - Advanced 1.31 Root Growth - Advanced 1.32 Stems - Advanced 1.33 Stem Types - Advanced

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1.34 Stem Structure and Function - Advanced 1.35 Stem Growth - Advanced 1.36 Leaves - Advanced 1.37 Leaf Types - Advanced 1.38 Leaf Structure and Function - Advanced 1.39 Life Cycles of Non-ﬂowering Plants - Advanced 1.40 Life Cycles of Bryophytes - Advanced 1.41 Life Cycles of Seedless Vascular Plants - Advanced 1.42 Life Cycles of Gymnosperms - Advanced 1.43 Life Cycles of Angiosperms - Advanced 1.44 Structures of Flowering Plants - Advanced 1.45 Pollination of Flowering Plants - Advanced 1.46 Fertilization of Flowering Plants - Advanced 1.47 Fruits of Flowering Plants - Advanced 1.48 Seeds of Flowering Plants - Advanced 1.49 Fruit and Seed Dispersal - Advanced 1.50 Seed Dormancy and Germination - Advanced 1.51 Vegetative Reproduction in Plants - Advanced 1.52 Propagation in Plants - Advanced 1.53 Hydrophytes - Advanced 1.54 Xerophytes - Advanced 1.55 Epiphytes - Advanced 1.56 Carnivorous Plants - Advanced 1.57 Plant Hormones - Advanced 1.58 Abscisic Acid - Advanced 1.59 Auxins - Advanced 1.60 Cytokines and Gibberellins - Advanced 1.61 Ethylene and Brassonosteroids - Advanced 1.62 Plant Responses - Advanced 1.63 Hormones and Plant Growth - Advanced 1.64 Tropisms of Plants - Advanced 1.65 Nastic Movements of Plants - Advanced 1.66 Photoperiodism and Circadian Rhythms in Plants- Advanced 1.67 Protective Responses of Plants - Advanced 1.68 References

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Introduction

Can you see all the photosynthesis? This lush, green landscape is thickly carpeted with trees and a myriad of other plants. Much of Earth’s land is dominated by plants. Yet compared to our active existence as animals, plants are —literally —rooted to the ground. Their sedentary lives may seem less interesting than the active lives of animals, but plants are very busy doing extremely important work. All plants are chemical factories. Each year, they transform huge amounts of carbon (from carbon dioxide) into food for both themselves and virtually all other land organisms. Plants are complex organisms that carry out complex tasks. But unlike animals, they don’t have nerves, bones, or muscles to do their work. How do plants do it? These concepts will tell you.

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1.1 Why Study Plants? - Advanced

• Compare the human life cycle to the life cycles of the earliest plants. • Analyze the role of plants in supplying food and energy to their ecosystems. • Explore the importance of plants to atmospheric oxygen, ozone, and temperature. • Explain the signiﬁcance of plants for the water cycle and soil conservation. • Identify plants’ role in the nitrogen cycle and other biogeochemical cycles. • Describe the ways in which plants are interdependent with bacteria, fungi, and animals. • Relate the characteristics of plants to their needs.

Why study plants? There are numerous reasons. One is the importance of materials from plants. Food is obvious. Bamboo, shown above, is a renewable resource that can be used in many products.

Why Study Plants?

Imagine that human life cycles resemble those of the earliest plants. If you think about this analogy, you may begin to realize that many plants actually lead secret lives of surprising variety. In a human life cycle, you develop from an infant into a sexually mature adult. We all began as a single cell and gradually develop into trillions of cells organized into tissues, organs, and organ systems, which make us complex and individual beings. None of us would question that the beings we are today are the same beings that began as single cells; each of us has a unique identity that we keep throughout our entire lives, until death marks our end. Unlike the human life cycle, the plant cycle does not include the joining of genes from two parents. Instead, a mature plant releases thousands of haploid spore cells. Small spores become males, and large spores become females. At some time during their relatively long lives, the male and female beings produce sperm cells and egg cells. Depending on which kind of plant we chose as our model, sperm might swim on their own from the male to the

4 www.ck12.org Chapter 1. Plant Biology - Advanced female being, they might be blown by the wind, or they may be carried by an animal. After sperm and egg join, the zygote begins its life as a single cell and grows into an “adult,” eventually producing its own haploid spores. Why do plants lead such complex lives? Life cycles make up one of several topics we will explore in this lesson’s introduction to the plant kingdom. Members of the plant kingdom play many crucial and sometimes surprising roles in the drama of life on Earth. Why should you understand how plants live? Because their gifts to us include, but are certainly not limited to, the following:

1. Supplying food and energy. 2. Maintaining Earth’s atmosphere. 3. Cycling water and nurturing soils. 4. Contributing to the nitrogen cycle and other biogeochemical cycles. 5. Interdependence with animals. 6. Interdependence with fungi. 7. Interdependence among plants. 8. Resources for humans. 9. Aesthetics for humans. 10. Scientiﬁc use by humans. 11. Causing problems.

Supplying Food and Energy

The sources of most terrestrial food chains begin with food produced by plants during photosynthesis. Photosyn- thesis provides energy and carbon-containing organic molecules for nearly all land-based organisms. Humans are no exception - the vast majority of our nutrition is derived directly (e.g. cereals, grains, vegetables, fruits, nuts) or indirectly (e.g. chicken, turkey, lamb, beef, pork) from plants.

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Plants provide nearly all of human nutrition, either directly or indirectly through the food chain. Domesticated animal energy –for plowing or transport –also originally comes from plants. The above photo collage shows human foods and energy sources, but virtually all terrestrial life depends on plants for food and energy.

Maintaining Earth’s Atmosphere

Through photosynthesis, plants produce O2 and absorb CO2. Oxygen is essential for both animals and plants to perform vital process of aerobic respiration. Most biologists agree that plants, together with algae and bluegreen bacteria, are responsible for maintaining current levels of oxygen (21%) in the atmosphere. Moreover, these levels of oxygen maintain the layer of ozone in the stratosphere, which protects life from harmful ultraviolet radiation. Although CO2 represents a much smaller fraction of the atmosphere, its use and removal by plants is critical to regulating our planet’s temperature. As a greenhouse gas, CO2 traps heat within the atmosphere. As we burn more fossil fuels and increase deforestation, Earth’s CO2 levels rise past optimal levels, which have already begun to raise the average temperatures on Earth.

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FIGURE 1.1 This model depicts the ﬂow of the carbon cycle. Energy, oxygen, carbon dioxide, and water molecules move through plants, animals, and the environment via photosynthesis, respiration, and the burn- ing of fossil fuels. Trace the molecules (yellow = sunlight energy, blue = water, green = sugar, clear = CO2) through the plants and processes shown in the model. For more on the carbon cycle, see the Recycling Matter: The Carbon Cycle (Ad- vanced) concept.

Cycling Water and Nurturing Soils

Through transpiration, plants move enormous quantities of water from soil to air. Moreover, plants’ roots probe rocky substrates to help form soil, limit runoff of rainwater, and prevent soil erosion.

Contributing to Nitrogen and Other Biogeochemical Cycles

Many plants, such as those in the pea family, construct nodules in their roots to house speciﬁc types of bacteria which are critical to plant survival. They “employ” these bacteria to harvest nitrogen in its unusable form –atmospheric N2 gas - and convert it into usable ammonia. This makes nitrogen available not only to plants, but also to animals, who absorb the nitrogen through consumption. Plants play a similar role in providing phosphorus and other minerals to animals, although bacteria are less directly involved.

Interdependence with Animals

Plants are architects of the terrestrial world, second only to geologic processes. They provide both shelter and food and are the basis of many habitats for most land animals. Coevolution has led to a wide variety of symbiotic relationships between plants and animals –especially insects. Insects and bats help pollinate plants in return for food; birds, reptiles, and mammals disperse seeds in return for fruit, and ants provide defense and fertilizer in return for certain trees’ food, nectar, and shelter.

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Interdependence with Fungi

Some biologists estimate that as many as 95% of trees and many other plants depend on mycorrhizal fungi for absorption of water and minerals. In turn, these fungi depend on their “host” plants for food in the form of energy- rich carbohydrates. Some fungi also contribute to the defense against herbivores by producing toxins. However, since they occur underground and out of sight, the extensive relationships between plants and fungi are relatively unknown.

Interdependence Among Plants

Trees and other plants provide support and habitats for epiphytes (plants which live on other plants), such as certain orchids or bromeliads, vines such as ivy and strangler ﬁgs, and parasites such as mistletoe. As architects of the terrestrial world, they are essential to the preservation of biodiversity.

Plants structure terrestrial habitats for each other and all other terrestrial organisms. In tropical rainforests, trees form multiple layers of habitats for other plants and animals, as well as anchors for epiphytes and vines.

Resources for Humans

We use many products directly from plants: wood (ﬁrewood or timber), cloth ﬁbers (cotton, hemp), medicines (aspirin, morphine), pesticides (rotenone, pyrethrin), drugs (opium, cocaine), and poisons (hemlock, curare). We extract dyes and pigments, waxes and oils, tannins, latex and gums, resins (amber), and cork. We process plants to make many necessities, such as soap, paint, shampoo, cosmetics, rubber, linoleum, plastic, and basic chemicals for the organic chemical industry.

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More than 100,000 natural compounds come from plants, and most of these have yet to be explored. Some of the most powerful and useful compounds come from plants. Who knew they could help us unlock some of the biology’s mysteries - all using an approach of mapping biological pathways. For more information, see Solving Biology’s Mysteries with Plants at http://youtu.be/K9mhXBOhuHU?list=PLzMhsCgGKd1hooﬁKuifwy6qRXZs7NG6a .

MEDIA Click image to the left or use the URL below. URL: http://www.ck12.org/ﬂx/render/embeddedobject/144943

Scientiﬁc Use by Humans

Biologists use plants in basic research. For example, as you have learned, Gregor Mendel studied pea plants to pioneer the human understanding of genetics. Similarly, Barbara McClintock discovered “jumping genes” by studying corn. You may have learned about mitosis and energy for life by studying plants in your biology course. Plants are critical components of biosphere models (see the Figure 1.2 and http://www.biospherics.org/russia.html ), and they will undoubtedly play important roles in future space stations and colonies.

FIGURE 1.2 The Eden Project in Cornwall, England is one of the latest attempts to enclose and support entire biomes artiﬁcially. For additional information, see the Eden Project web site at http://www.edenproject.com/ . Experiences from such projects may contribute to the development of space stations and colonies, as well as more sustainable living on Earth. Plants have helped to meet many different scientiﬁc goals within these projects.

Causing Problems

Some plants are experts at growing where people do not want them to - we consider them "weeds." When we transport plants to new habitats, they often accidentally become weeds or invasive species. Plant pollens can cause allergies, and some plant chemicals are poisonous to humans and our livestock. Some drugs are also derived from plants, and drugs are disruptive to the economy, productivity, and health.

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Characteristics and Needs of Plants

What characteristics earn an organism membership in the plant kingdom?

1. Multicellularity, with eukaryotic cells. 2. Production of food by photosynthesis, using light energy and CO2 as a source of carbon. However, about 300 of the estimated 300,000 species of plants have become parasitic or saprophytic (absorbing food from dead or decaying organisms), losing their photosynthetic ability. 3. Storage of food as starch. 4. Cell walls made of cellulose. In contrast, most Fungi have cell walls made of chitin. 5. Presence of reproductive organs. Non-reproductive tissues surround plants’ reproductive cells. 6. Two membranes surrounding chloroplasts. The number of membranes suggests that plants acquired their chloroplasts directly through endosymbiosis of bluegreen bacteria. The membrane of the bluegreen bacteria became the inner membrane surrounding the chloroplast of the plant cell. Red and green algae are similar in this respect, and, for this reason, some botanists classify them together with plants. Other algae that have 3 or 4 membranes are believed to have acquired their chloroplasts secondarily, through endosymbiosis of red or green algae. 7. Chloroplasts that contain chlorophylls a and b. Green algae contain these pigments, but red algae contain only chlorophyll a and other, red pigments. Therefore, most botanists consider green algae to be more closely related to plants, and many classiﬁcation systems include green algae (but not red) in the plant kingdom. In this text, we include all algae in the Protists concepts and discuss only those green algae thought to be direct ancestors of plants in the Plants concepts.

Members of the plant kingdom are multicellular. Their eukaryotic cells (left) contain chloroplasts with two membranes and chlorophylls a and b. Cell walls made of cellulose surround their cells and, together with large central vacuoles, provide support. Nearly all plants make food by photosynthesis (right), using light energy and carbon dioxide (see the Photosynthesis chapter). These characteristics of plants deﬁne their needs. As you have learned, photosynthesis requires light, carbon dioxide, and water. Because most plants are terrestrial (although their evolutionary origin is aquatic, and some have returned to aquatic habitats), water is a critical need and often a limiting resource. In addition, plants require minerals, such as magnesium to build chlorophyll, nitrogen to build protein, and phosphorus to make DNA. Most plants must absorb these minerals and water from soil. Air provides light and carbon dioxide - but demands that plants provide support not needed by their aquatic ancestors. Many plants help support themselves by using the pressure of water in large, central vacuoles against the walls of their cells. In summary, to obtain their needs, most plants must adapt to two very different habitats at once, probing downward into the soil and reaching upward toward the light.

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Vocabulary

• biodiversity: The variety of life and its processes, including the variety of living organisms, the genetic differences among them, and the communities and ecosystems which they occupy.

• chloroplast: The organelle which carries out photosynthesis.

• coevolution: Evolution of interacting species in which each species is an important factor in the natural selection of the other species; a pattern in which species inﬂuence each other’s evolution and, therefore, evolve in tandem.

• epiphyte: A type of plant (or lichen) that grows on other plants for support.

• food chain: A pathway which traces energy ﬂow through an ecosystem.

• photosynthesis: The process by which carbon dioxide and water are converted into glucose and oxygen, using sunlight for energy.

• transpiration: A process by which plants lose water; this occurs when stomata in leaves open to take in carbon dioxide for photosynthesis and lose water to the atmosphere in the process.

Summary

• Food produced by photosynthesis is the basis for most terrestrial food chains. Most plants make food by photosynthesis and store food as starch. • Plant photosynthesis produces O2 and ozone and absorbs CO2, which helps regulate global temperatures. • Plant roots help form soil, limit runoff of rainwater, and prevent soil erosion. • Some plants help to “fix” nitrogen. They also begin many biogeochemical mineral cycles. • Many plants have mutualistic relationships with fungi, which assist with nutrient and water absorption. • Plants and animals have evolved mutualistic relationships for food, pollination, and dispersal. • Plants structure habitats for both animals and other plants. Thus, they are essential for biodiversity. • The development of spores promoted the dispersal of the first true plants to venture onto land. • Bryophytes were small and low to the ground, requiring moist habitats. • Vascular tissue to carry water up to the first leaves characterized early vascular plants similar to clubmosses. • Ferns used vascular tissue to develop true roots, stems, and leaves and the first trees. • In gymnosperms, pollen enclosed sperm cells in hard capsules and seeds protected embryos and stored food. • Woody trunks, needle-like leaves, and pollen adapted the conifers to large size and dry, cold climates. • Flowers and fruits enlisted animal mobility for pollination and dispersal in Angiosperm development.

Practice

Use this resource to answer the questions that follow.

• Introduction to the Plantae at http://www.ucmp.berkeley.edu/plants/plantae.html

1. How long ago did plants ﬁrst begin to resemble modern plants? 2. What is the most striking feature of plants, and what provides this feature? 3. What necessary foods do plants produce? 4. Give four examples of land plants.

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Practice Answers

1. 360 million years ago. 2. Their green color that results from the pigment chlorophyll. 3. Plants manufacture sugar, starch, and other carbohydrates. 4. Four examples of land plants include mosses, ferns, conifers, and ﬂowering plants.

Review

1. Summarize the differences between the human life cycle and the life cycles of early plants, such as liverworts. 2. Explain the importance of plants to the atmosphere, to soil and water, and to biogeochemical cycles. 3. Discuss the importance of plants to biodiversity, giving one example each of plant interdependence with fungi, animals, and other plants. 4. List the seven characteristics common to all plants. 5. What unique feature of most plants is determined by their terrestrial habitats?

Review Answers

1. Early plants have a part of their life cycle where the plant is haploid, compared to humans who are diploid for all of their lives. 2. Through photosynthesis, plants produce O2 and absorb CO2. Through this process, plants regulate oxygen levels in the atmosphere, which in turn prevents damage to the ozone layer. CO2 levels also must be regulated to maintain normal temperatures. Plants also house bacteria which harvest nitrogen into usable ammonia. 3. Plants are necessary in maintaining biodiversity because they structure many different terrestrial habitats. In addition, plants have developed interdependence with many organisms. An example of interdependence with animals is that insects and bats help pollinate plants in return for food. Mycorrhizal fungi assist many plants in absorbing water and minerals, and, in turn, plants provide food and nutrients. Plants also participate in relationships with each other - for example, vines wrap around tree trunks. 4. 7 Characteristics: Multicellularity and have eukaryotic cells, production of food by photosynthesis, use of light energy and CO2 as a source of carbon, storage of food as starch, cell walls made of cellulose, presence of reproductive organs, two membranes surround their chloroplasts, chloroplasts that contain both chlorophyll a and b. 5. A plant’s terrestrial habitat determines how much water a plant needs to survive.

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1.2 Evolution of Plants - Advanced

• Cover prerequisite events for the evolution of complex plants. • Describe the characteristics of stoneworts, one of the most primitive plants. • Trace the evolution of early vascular plants to modern, complex plants (ﬂowering plants, evergreens, grasses)

How did plants evolve? With small low-to-the-ground plants. Early plants could not transport water or minerals as effectively as later plants, nor did they have leaves, roots, or stems. The Lycopodium moss shown here is related to early land plants.

How Did Plants Evolve?

Given their importance to nearly all terrestrial life, you might think that plants evolved early in Earth’s history –or at least early in the history of life. However, as shown in the Figure 1.4, the fossil record shows that plants are relative

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FIGURE 1.3 From a simple, green alga ancestor that lived in the water, plants eventually evolved several major adaptations for life on land.

FIGURE 1.4 This diagram of geologic time projects the Earth’s 4.5 billion year history onto a 24- hour clock. Ga = gigaannum = 109, or one billion, years ago. Ma = megaannum = 106, or one million, years ago. The colored lines outside the clock indicate the appearance and persistence of major forms of life. Note especially the length of time between the appearance of bacterial life (purple), the evolution of photosynthesis (at 3.5 Ga), and the appearance of land plants (yellow-green). Note too that animals (bluegreen) appeared long before plants.

latecomers to life on Earth; they evolved on land millions of years after animals had long dominated the seas. Their complexity and the challenges of adapting to dry land help to explain their late arrival. Prerequisite for the evolution of complex plants included the following major evolutionary events:

1. Bluegreen bacteria developed photosynthesis roughly 3.5 billion years ago. The O2 produced ﬁrst oxidized minerals in the oceans, precipitating limestone and iron formations. Later (by 2 billion years ago), oxygen ﬁlled the atmosphere.

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2. Other bacteria evolved aerobic respiration for a more efﬁcient production of energy. 3. Endosymbiosis between bluegreen bacteria and other prokaryotes produced chloroplasts and eukaryotes. 4. Green algae developed multicellularity. 5. About 1 billion years ago, certain green algae developed differentiated tissues for specialized functions.

FIGURE 1.5 Stoneworts closely resemble the green algal ancestors of plants. Sterile protective tissues surround developing sex cells, forming specialized reproductive organs which are characteristic of the plant kingdom. On the right is a ﬁeld modern stoneworts. On the left is a division of green algae, a close relative to stoneworts and early plants.

Stoneworts

Many biologists believe that stoneworts ( Figure 1.5) most closely resemble these immediate ancestors of plants. Often classiﬁed as green algae but sometimes included in the plant kingdom, stoneworts are multicellular, with stalks of giant, multinucleated cells (instead of stiff stems like most modern plants). Whorled branches arise at intervals, and tiny hair-like rhizoids attach them to their freshwater homes (instead of true roots like most modern plants). Stoneworts’ most specialized structures are reproductive organs made of sterile cells, which surround and protect developing gametes (haploid sex cells). Make the effort to learn their names now, because your study of plant evolution and diversity will use them often. Archegonia (singular, archegonium) are the female sex organs which produce egg cells or ova (singular, ovum), and antheridia (singular, antheridium) are the male organs which produce sperm. Are you surprised that algae and some plants have sperm? Perhaps it will surprise you even more to learn that most plant sperm cells have not just one, but two or more ﬂagella!

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Algae and early plants produce motile sperm for sexual reproduction. The sperm cells of the earliest plants - mosses, liverworts, and clubmosses (Lycopodium) –are biflagellate, having two flagella. The sperm cells of later vascular plants, such as whisk ferns (Psilotum), ferns, and horsetails (Equisetum), have many flagella.

460 to 480 million years ago

The oldest fossils of land plants date back to the Ordovician period, 460 to 480 million years ago, although DNA comparisons among living plants (molecular clocks) suggest that plants may have colonized land as many as 700 million years ago. Bacteria and fungi had preceded them, but vast areas of barren land ﬂooded with sunshine awaited plants’ exploitation. Fungi may have helped plants invade the land, forming mycorrhizal symbioses as they do today. The development of spores –asexual reproductive cells with a protective coating that resisted drying and cold –promoted the dispersal of the ﬁrst true plants to venture onto land: the liverworts and other bryophytes. Like today’s liverworts ( Figure 1.6), they were small and low to the ground, requiring moist habitats with adequate water, in part, so their sperm could swim from antheridium to archegonium to join the egg.

Early Vascular Plants

Early vascular plants had characteristics that allowed them to flourish on dry land. A “plumbing system” of well- supported tubes ( vascular tissue) to carry water up to the first, elevated, tiny, flattened leaves gave early vascular plants an advantage over the bryophytes. The earliest vascular plants may have had just stems and rhizoids, like the whisk fern (Psilotum) today ( Figure 1.7), but the ability to carry water allowed them to branch into the air to

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FIGURE 1.6 The ﬁrst true land plants likely resembled the liverworts of today. Liverworts are restricted to moist habitats so that their tissues have sufﬁcient water and their sperm can swim to egg cells for successful sexual reproduction.

reach sunlight without drying out, and sporangia (singular, sporangium) allowed them to reproduce asexually and populate relatively dry land. During the Silurian, clubmosses ( Figure 1.7) added the ﬁrst leaves, called microphylls, which are tiny, almost needle-like solar collectors, each with a single vein to supply water.

FIGURE 1.7 The whisk fern Psilotum, a plant of the tropics and subtropics, may resemble the ﬁrst plants to “lift off” the ground in order to reach sunlight –the earliest vascular plants. Psilotum lacks roots and leaves, but has stems and rhizoids. DNA evidence suggests, however, that its “simple” features actually represent a loss of more complex features from fern ancestors.

416 to 359 million years ago

Late in the Devonian Period (416 to 359 million years ago), ferns’ ( Figure 1.8) vascular tissue allowed them to develop true roots, stems (underground, forming rhizomes), and broad leaves ( megaphylls) with lacy, branching veins. The ﬁrst modern tree, Archaeopteris, grew to about 10 meters tall, with a branching trunk up to 1.5 meters in diameter and wood with tracheids similar to those of later gymnosperms. Like the ferns, Archaeopteris photosynthesized with a large umbrella of fronds and reproduced by spores. However, as the ﬁrst plant to produce an extensive

17 1.2. Evolution of Plants - Advanced www.ck12.org system of true roots, it quickly became the dominant tree all over the Earth, signiﬁcantly changing soil chemistry as it spread. Many consider Archaeopteris to be a link between the ferns and the gymnosperms.

FIGURE 1.8 Ferns resemble early vascular plants, having some of the ﬁrst large leaves with branching veins (megaphylls). Their stems form horizontal, underground rhizomes, with rhizoids and true roots anchoring them into the earth. Brown sporangia on the undersides of the fronds produce spores for asexual reproduction, but motile sperm cells restrict sexual reproduction to wet habitats. During the Carboniferous period, tree ferns (right), clubmosses, and horsetail trees formed forests which later became the coal deposits we mine today.

Bamboo-like horsetails ( Figure 1.9) have jointed hollow stems which contain silica and sometimes wood for support. Throughout the Carboniferous period, these groups produced forests of tree-like species up to 30 meters (100 feet) tall, forming the coal swamps which gave the period its name. However, sexual reproduction restricted these early vascular plants to moist areas because their sperm cells still needed water to swim from antheridium to archegonium.

FIGURE 1.9 Carboniferous fossils (left) of a tree-like horsetail ancestor, Calamites, closely resemble the jointed, hollow stems of living horsetails (right). The fossil stems, however, measure 30 cm wide –far larger than those of today’s horsetails. Horsetails were among the earliest vascular plants.

Pollen and Seeds

Toward the end of the Paleozoic era, two major developments allowed plants to expand beyond moist habitats. Pollen enclosed sperm cells in hard capsules, which prevented them from drying out. Seeds protected embryonic plants and stored food within a tough seed coat. Together, these adaptations allowed plants to colonize much higher, drier land areas.

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Among the first successful seed plants were the cycads ( Figure 1.10) and ginkgos ( Figure 1.11). Cycads are evergreens with a large crown of pinnately compound leaves. Individual cycads are either male or female, and they were among the first plants to depend on insects, rather than wind, for pollination. Seeds in these early gymnosperms developed “naked” on modified leaves or scales arranged into cones.

FIGURE 1.10 Cycads (left) were among the ﬁrst plants to produce both pollen, which carried male sex cells that did not dry out, and seeds, which allowed embryonic plants to begin development inside a protective coat with a stored food supply. Note the bright red-orange seeds produced in the female cone (middle). Cycads are dioecious (plant in which the male and female reproductive structures are found in different individuals), so pollen is produced in cones on separate male plants (right). Speciﬁc insects carry the pollen from male to female, but motile sperm must still swim a short, last lap to reach the egg.

Long-lived ginkgos are wind-pollinated. Males produce cones and pollen, but females produce pairs of egg cells –and if fertilized, seeds - that form at the end of each stalk. After pollination, both cycad and ginkgo pollen release motile sperm, unlike other seed plants. Their amazing sperm cells, each encircled by a band of thousands of ﬂagella that behave more like cilia, must swim a short distance through a thin ﬁlm of moisture to reach and fertilize the egg cell.

251 to 65 million years ago

Together with the dinosaurs, gymnosperms (now familiar as conifers, as shown in the Figure 1.12) dominated the forests of the Mesozoic Era (roughly 251 to 65 million years ago). Many were giant, massive trees, and, even today, our largest, tallest, stoutest, and oldest plants are members of this group (these are the giant sequoia, coastal redwood, Montezuma cypress, and bristlecone pine, respectively). Woody trunks, needle-like leaves covered with a waxy cuticle, and pollen allowed the conifers to develop their to large size and adapt to dry, even cold climates. In fact, most were (and are) evergreen. Conifers and one other group of gymnosperms, the Gnetae ( Figure 1.12), transitioned into being land plants - within their pollen, sperm lost their ﬂagella, eliminating their dependence on moisture for swimming to the egg. Instead, like nearly all of today’s dominant gymnosperms and ﬂowering plants, the pollen grows a pollen tube to transfer sperm to the egg. For all conifers, wind carries pollen from male cones to female cones. Unlike earlier seed plants, some conifers bear both male and female cones on the same tree –they are monoecious.

Flowers and Fruits

The most recent, major adaptations in the plant kingdom are ﬂowers, designed to attract the freely mobile animals to carry their pollen and fruits. Their fruits initially enclose and protect seeds and later develop a wide variety

19 1.2. Evolution of Plants - Advanced www.ck12.org

FIGURE 1.11 Ginkgos ﬂourished together with cycads as the earliest seed plants toward the end of the Paleozoic era. Both had separate male and female plants, but, unlike cycads, ginkgos have seeds which develop without woody cones. The longitudinal section of a ginkgo seed shows the adaptive value of seeds: embryonic plants, provided with an initial food supply, begin their development inside a tough seed coat.

FIGURE 1.12 Two groups of Mesozoic gymnosperm seed plants, which used pollen tubes to carry sperm cells to eggs, were the Gnetae and the conifers; both were able to colonize drier regions throughout the world. Ephedra (left) is one of a very few genera of Gnetae still living; a developing seed is shown above and pollen cones below. Tamaracks (right) are monoecious conifers; the inset shows female cones (red) and male cones (pale) on the same plant, as well as the characteristic needle- like leaves. Unlike most conifers, which are evergreen, tamaracks are deciduous, losing their soft needles each fall.

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of dispersal mechanisms, also enlisting animal mobility. The angiosperms, or ﬂowering plants ( Figure 1.13), diversiﬁed during the Cretaceous period at the end of the Mesozoic era, coevolving with bees and other insects. By the end of the Mesozoic, angiosperms (e.g. magnolias, beeches, oaks, and maples) were the dominant plants.

FIGURE 1.13 Angiosperm’s enclosed reproductive organs in ﬂowers (top left) are designed to employ animals (often insects) as pollinators. They enclosed seeds in fruits (top right) to encourage other animals (often birds) to eat and disperse their seeds. Bees (the one in the lower photo covered with pollen) and ﬂowers are a classic example of coevolution.

Grasses

During the Cenozoic Era, grasses ( Figure 1.14) appeared among the angiosperms. With reduced ﬂowers, grasses relied on the wind to carry their pollen to other plants. Their coevolution involved large, grazing herbivorous mammals –and later, humans. Human domestication of plants within the past 10,000 years began with and still focuses on grasses with relatively large, edible seeds, also known as cereals. Three cereals –wheat, rice, and corn –provide more than half of the calories humans consume today, and over 70% of cultivated crops are grasses. Arguably, grasses remain the most economically important plants for humans.

Vocabulary

• angiosperm: Seed plant in which seeds develop within a vessel, which may later become the fruit.

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FIGURE 1.14 Grasses are angiosperms that coevolved with large mammalian herbivores and later with humans. Their ﬂowers are greatly reduced (inset above) and rely on wind for pollination. Over half of human nutritional calories are derived from the seeds of three grasses: wheat (above), rice, and maize (corn).

• antheridia (singular, antheridium): Haploid male reproductive organs which produce male gametes by mitosis.

• archegonia (singular, archegonium): Haploid female reproductive organs which produce female gametes by mitosis.

• ﬂower: A plant’s reproductive organ, often designed to attract pollinators.

• fruit: A plant’s ovary (female reproductive organ) which may later develop ("ripen") for dispersal.

• gamete: A sexually reproducing organism’s reproductive cells, such as sperm and egg cells.

• megaphylls: Larger, true leaves with branching veins, characteristic of ferns and seed plants.

• microphylls: Single-veined "tiny leaves" of club mosses, perhaps similar to the ﬁrst true leaves.

• ovum (plural, ova): A mature female gamete.

• rhizoids: Fine, hair-like outgrowths –either unicellular or multicellular - which anchor and support nonvascular plants.

• sporangia (singular, sporangium): Asexual reproductive organs which produces spores.

• spore: A haploid reproductive cell found in plants, algae, and some protists; they can fully develop without fusing with another cell.

• vascular plant: Plants with tissues for conducting water and minerals throughout the plant.

• vascular tissue: A type of tissue in plants that transports ﬂuids through the plant; the xylem and phloem are vascular tissues.

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Summary

• In order for plants to have evolved, conditions ﬁrst had to be ideal for them to perform photosynthesis and aerobic respiration and to develop chloroplasts, multicellularity, and specialized, differentiated tissues. These conditions were not met until relatively late. • Stoneworts are the earliest organisms to resemble plants; they developed specialized reproductive organs that allowed plants to perform sexual reproduction. • The oldest true land plants (liverworts and bryophytes) date back 460 to 480 million years ago. The development of spores allowed them to grow and reproduce successfully on dry land. • Late in the Devonian Period (416 to 359 million years ago), ferns developed vascular tissue which allowed them to grow true roots and stems, anchoring them into the land. • Towards the end of the Paleozoic era, plants were able to expand beyond moist habitats and into drier land areas. • 251 to 65 million years ago, massive conifers dominated forests. They were evergreens and extremely resistant to cold climates. • Flowers and fruits allowed plants to enlist aid from animals to spread their pollen. • Grasses appeared alongside angiosperms (ﬂowering plants) and coevolved with grazing herbivorous mammals. To this day, humans heavily consume grasses.

Practice

Use this resource to answer the questions that follow.

• Plant Evolution Timeline at http://www.plantsci.cam.ac.uk/timeline/

1. According to the timeline, what was the first plant to evolve? When did this plant evolve? 2. According to the timeline, when did angiosperms really flourish? 3. According to the timeline, what event occurred 65mya? 4. According to the timeline, when did cooksonia evolve? 5. According to the timeline, when did the first large trees begin to evolve?

Practice Answers

1. Green algae, 450mya. 2. Angiosperms were at their peak about 100mya. 3. 65 mya was the Cretaceous-Tertiary Extinction Event. 4. Cooksonia evolved about 428mya. 5. The ﬁrst large trees began to evolve about 270mya.

Review

1. What were the most immediate ancestors of plants? Describe their most specialized structures that allow them to reproduce. 2. What structures allowed early land plants to venture onto land? 3. Describe early vascular plants and list two examples. 4. What two major developments allowed plants to expand beyond moist habitats? 5. What important plant makes up a large part of human diet?

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Review Answers

1. Stoneworts. Reproductive organs which surround and protect developing gametes. 2. Spores. 3. Early vascular plants had tiny, ﬂattened leaves that allowed them to carry water up the plants. They had stems and rhizoids that allowed them to carry water into the air without drying out. Some examples include whisk ferns and clubmosses. 4. Pollen and seeds. 5. Over 70% of cultivated crops are grasses.

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1.3 Classiﬁcation of Plants - Advanced

• Outline the general methods of classifying plants. • Separate the kingdom Plantae into 10 separate divisions. • Brieﬂy explain how classiﬁcation is tied to our knowledge of evolution.

How are plants classiﬁed? Obviously, these redwood trees are quite different from a moss plant. Water from the roots has to make it to the top of these trees - sometimes over 350 feet away. Classiﬁcation is based on a number of characteristics, including how water and nutrients are transported.

How are Plants Classiﬁed?

Perhaps even more so than for other kingdoms, classification of plants is under constant revision, and botanists are currently considering a number of different proposals. As you have learned, the goal of classification is to reflect evolutionary relationships. For our purposes, just one general classification system is useful for review of the plant history presented above; we will explore some of the more familiar groups in more detail in the next few lessons. You should remember, however, that many of the groupings are controversial, and that they may change again in the future - based on new understanding about evolutionary relationships. Because their cells have well-developed nuclei and other organelles enclosed within membranes, plants are classified within the domain Eukaryota. Plants form the kingdom Plantae: they all have cell walls and chloroplasts and are multicellular with specialized tissues. Two major groups (sometimes given the ranks of subkingdoms) of plants are the green algae, which include their ancestors, and the land plants, or embryophytes.

10 Separate Divisions

Land plants, in turn, are informally grouped into nonvascular and vascular plants. In the past, these were the only two divisions (plant equivalents of animal phyla) in the plant kingdom: Bryophyta (“moss plants”) and Tracheophyta (“tube plants”). However, because their members represent several different major evolutionary branches, they are

25 1.3. Classification of Plants - Advanced www.ck12.org now grouped in more than 10 separate divisions –3 for the bryophytes, and a minimum of 7 for tracheophytes. After listing these divisions, we will defer discussion of the smaller levels of classification (class, order, family, genus, and species) to later lessons, which focus on specific groups. The three bryophyte (nonvascular) divisions are these:

• Marchantiophyta –the liverworts. • Anthocerotophyta –the hornworts. • Bryophyta –the mosses.

The seven major tracheophyte (vascular) divisions include these:

• Lycopodiophyta –clubmosses. • Pteridophyta - ferns and horsetails. • Pinophyta - conifers. • Cycadophyta - cycads. • Ginkgophyta - ginkgos. • Gnetophyta - gnetae. • Magnoliophyta - ﬂowering plants.

Within the tracheophyte divisions are two more informal groups –at least one of which some taxonomists recognize as a superdivision. The ﬁrst, less-closely related group reproduces with spores. The second group reproduces with seeds; these divisions are sometimes combined into the super division Spermatophyta, the spermatophytes (seed plants).

Classiﬁcation Reﬂects Our Understanding of Evolution

Of the three nonvascular plant divisions, we focused on liverworts (Marchantiophyta) as the first terrestrial plants. Hornworts and mosses are similar in their life cycles and habitats. They will be discussed in more detail in additional concepts. The earliest vascular plants still reproduced with spores, so they remained confined to somewhat moist habitats. Spore-producing vascular plants include the clubmosses (division Lycopodiophyta) and the ferns and horsetails (division Pteridiophyta), which are also discussed in additional concepts. The most recently evolved plants are the seed - producing vascular plants (superdivision Spermophyta). This includes the cycads, ginkgos, conifers, gnetae, and flowering plants. You should be able to recognize the general pattern of plant evolution in this classification of plants. If the major groups and their characteristics still seem overwhelming, additional Plant concepts will help clarify the classification.

Vocabulary

• bryophyte: A type of plant that lacks vascular tissues, such as a liverwort, hornwort, or moss.

• spermatophyte: A type of plant that reproduces by producing seeds.

• tracheophyte: A type of plant that has vascular tissues, such as a seed plant or ﬂowering plant.

Summary

• Plants are classiﬁed within the domain Eukaryota. Two major groups of plants are green algae and embryophytes (land plants).

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• Three bryophyte (nonvascular) divisions are liverworts, hornworts, and mosses. • Seven tracheophyte (vascular) divisions are clubmosses, ferns and horsetails, conifers, cycads, ginkgos, gnetae, and ﬂowering plants. • The earliest vascular plants reproduced with spores, and the most recently evolved plants are seed-producing vascular plants.

Practice

Use this resource to answer the questions that follow.

• Classiﬁcation of Plants at http://theseedsite.co.uk/class.html .

1. Gymnosperms would characterize what level of plant classification? 2. What are the monocotyledonae? 3. Pinaceae would distinguish what level of classification? 4. Rosoideae would distinguish what level of classification? 5. Some botanists claim there are 500 of this level of classification?

Use these resources to answer the questions that follow.

• http://www.hippocampus.org/Biology Non-Majors Biology Search: Nonvascular Plants ! !

1. What is a bryophyte? 2. Compare hornworts, liverworts, and mosses.

• http://www.hippocampus.org/Biology Non-Majors Biology Search: Vascular Plants ! !

1. What is a seedless vascular plant? Give an example. 2. What is a gymnosperm? What is an angiosperm? 3. Why are angiosperms vital to human life?

• http://www.hippocampus.org/Biology Non-Majors Biology Search: Monocots and Dicots ! !

1. What is the main difference between monocots and dicots? 2. What is the purpose of the vascular tissue in these plants? 3. How do the ﬂowers differ between monocots and dicots?

Practice Answers

1. Gymnosperms is a class in the classification of plants. 2. Monocots are plants with one seed leaf. 3. Pinaceae would belong to the family level of classification. 4. Rosoideae would belong to the subfamily level of classification. 5. Some botanists recognize nearly 500 plant families.

1. Bryophytes are plants without vascular tissue. 2. Hornworts have a spiky appearance, liverworts have a ﬂattened shape, and mosses grow in damp and dim places.

27 1.3. Classiﬁcation of Plants - Advanced www.ck12.org

1. A seedless vascular plant has vascular tissue, but does not produce seeds. An example of a seedless vascular plant is a fern. 2. A gymnosperm is a seeded vascular plant that doesn’t have ﬂowers, while an angiosperm is a seeded vascular plant that has ﬂowers. 3. Almost all the plants we consume and feed to animals are angiosperms.

1. Monocots have one cotyledon, while dicots have two. 2. The vascular tissue transport food and water in the plant. 3. Monocots have ﬂower parts in multiples of three, while dicots have them in multiples of four or ﬁve.

Review

1. What characteristics make plants form the kingdom Plantae? 2. What two separate divisions are plants separated into? 3. Spore-producing vascular plants include which speciﬁc plants? What kind of habitats did spores conﬁne plants to?

Review Answers

1. Plants are multicellular with specialized tissues and have cell walls and chloroplasts. 2. Nonvascular (bryophyte) and vascular (tracheophyte). 3. Spore-producing vascular plants include clubmosses, ferns, and horsetails. They conﬁned them to moist habitats.

28 www.ck12.org Chapter 1. Plant Biology - Advanced

1.4 Plant Life Cycles - Advanced

• Describe a signiﬁcant difference between plant and animal life cycles. • Outline the plant life cycle and describe the meaning of alternation of generations. • Discusse the differences, advantages, and disadvantages of sexual and asexual reproduction in plants.

Seeds turn into plants. How complicated can the life cycle be? A lot more complex than that. Shown above are sporophytes of the Juniper hair cap moss. Sporophytes are diploid parts of a plant that produce haploid non-gamete cells called spores. And that’s just the beginning.

29 1.4. Plant Life Cycles - Advanced www.ck12.org

An Overview of Plant Life Cycles

A signiﬁcant difference between the lives of most plants and those of many animals (especially humans) is the importance of major transitions in form from haploid to diploid stages in life –often including regular alternations between two completely different ways of reproducing (sexual and asexual). Plants inherited this tendency to switch from haploid to diploid –better known as alternation of generations –from their green algal ancestors. Tracing plants’ evolutionary changes in alternation of generations provides insight into plants’ complex life cycles.

FIGURE 1.15 Life Cycle of Plants. This diagram shows the general life cycle of a plant.

Alternation of Generations

Let’s begin by comparing a typical animal life cycle to the life cycle of green algae with alternation of generations. In animals, a single, diploid individual grows by mitosis from a zygote. Within that individual, meiosis produces haploid gametes (often sperm or egg cells). If the egg cell is fertilized by the sperm, a new zygote develops into a new individual, completing the life cycle. In plants ( Figure 1.16), part of the life cycle is quite similar to that of animals. As in animals, a zygote develops into a diploid individual by mitosis, and, within that individual, meiosis produces haploid cells. However, these haploid cells are not gametes (cells that fuse together to be fertilized) - they are called spores, and the diploid individual which produced them is called a sporophyte (“spore-producing plant”). Often, the spores will have two different sizes: small spores are male and larger spores are female. Each divides by mitosis to form new, separate, haploid individuals. Gametophytes, multicellular haploid organisms, are produced after the single-celled haploid spores divide by mitosis. The gametophytes then produce gametes at maturity. Recognize that meiosis is not necessary in alternation of generations –haploid gametophytes produce gametes by mitosis, rather than meiosis. Lastly, gametes join, and the diploid zygote is formed, completing the plant life cycle.

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FIGURE 1.16 In humans and most other animals, a single life cycle involves a dominant, diploid individual which arises from a single-celled zygote, produced by the joining of two haploid gametes (sperm and egg). Mitosis and meiosis occur only within that single, dominant individual. In plants (above), as in animals, meiosis occurs only once –as diploid individuals produce haploid cells (spores). However, mitosis occurs during two separate “lives”: ﬁrst, “interrupting” fertilization by growing haploid individuals (gametophytes) from spores and, second, growing diploid individuals (sporophytes) from zygotes –all within a single “life cycle.” This life cycle pattern is alternation of generations. Essentially, it is alternation of sexual reproduction (zygote/sporophytes have two parents) with asexual reproduction (spore/gametophytes have just one parent).

All plant life cycles include alternation of generations, but specifics of the haploid and diploid stages vary depending on the species. We will summarize these differences here, but you can find more detail (and diagrams) in later lessons, which focus on specific types of plants.

• In green algae, haploid and diploid forms are quite similar, as shown in the Figure 1.16. • In nonvascular plants (liverworts, hornworts, and mosses), the two stages (gametophyte and sporophyte) are usually quite different. Sporophytes and spores are adapted for drying and dispersal, while gametophytes are “vegetative” (green and photosynthesizing) and produce gametes only in moist habitats. The sporophyte often begins to develop on the female gametophyte, almost appearing parasitic. • In many early vascular plants (ferns, clubmosses, and horsetails), haploid gametophytes and gametes remain tied to moist habitats. The gametophytes, though independent, are extremely small and seldom-seen. Diploid sporophytes begin development on the female gametophyte after fertilization, but eventually they grow much larger, greener, and become photosynthetic. This is the dominant stage of early vascular plant life cycles. • In seed plants, diploid sporophytes are the only independent individuals, and they are often divided into males and females. The sporophytes produce haploid gametophytes by meiosis. The haploid gametophytes are multicellular, but are reduced to microscopic size and develop entirely within the sporophytes. Pollen (male gametophytes) is released from the male sporophyte and blown by the wind or carried to the female, where it produces male gametes (primarily nonmotile sperm nuclei, except in cycads and ginkgos which have motile sperm). Ovules (which contain female gametophytes) develop in female sporophytes, producing an egg cell which is fertilized by two sperm). After fertilization, the ovule develops into a seed, which leaves the parent sporophyte to begin a next generation of sporophytes.

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Sexual and Asexual Reproduction

Alternation of generations (as in green algae) allows both asexual and sexual reproduction –and depends on both haploid and diploid individuals. Think back to your study of genetics; what are the advantages and disadvantages of each? Asexual reproduction requires just one parent and produces offspring which are identical to that parent. It’s a quick and easy way to populate a large, uniform, unchanging environment. Sexual reproduction, however, requires two parents, meiosis, and fertilization; it’s much more complicated and risky, but the offspring show variation which promotes the survival of some in a changing environment. Would a lifestyle (life cycle) which involves both types of reproduction also guarantee both sets of advantages? If you had to choose, which type of reproduction do you think would be more valuable over time? As for diploid vs. haploid, do you remember which state is better protected from the effects of damaging but recessive mutations? As land plants evolved, they moved from dominance of haploid individuals (in nonvascular plants) to dominance of diploid individuals (in seed plants) and from dispersal of spores (products of asexual reproduction) to dispersal of seeds (products of sexual reproduction). Although they retain echoes of their “alternating” inheritance, the more recent, most successful plants today live almost completely diploid lives and, therefore, reproduce sexually. As you probably know, they have also evolved many ways to reproduce asexually while diploid as well. Using the ideas we’ve reviewed and discussed in this section, can you explain how the path that life cycle evolution has taken has helped plants survive?

Evolution and Alternation of Generations

As land plants evolved, they moved from dominance of haploid individuals to dominance of diploid individuals and from dispersal of spores to dispersal of seeds. What are the advantages of each? Because haploid individuals have just a single set of chromosomes, the effects of a harmful mutation are felt immediately. In diploid individuals, a second set of chromosomes may mask harmful effects of mutations in the ﬁrst set. Spores reproduce by means of asexual reproduction, so they usually produce clones. As long as the environment does not change, clones are a relatively quick and “inexpensive” (in terms of energy costs) way to reproduce. Because they resist drying and cold, spores promote the colonization of new land. Seeds also resist drying and cold, but they also contain food supplies to “jump start” the embryo in its new life. Although seeds are relatively more costly in terms of energy, they are products of sexual reproduction, which increases genetic variety and, therefore, the chance that at least some will survive if the environment changes.

Vocabulary

• alternation of generations: A life cycle that alternates between diploid and haploid phases.

• gametophyte: A haploid structure which produces gametes by mitosis. This is the gamete-producing phase in the alternation of generations life cycle.

• spore: A haploid reproductive cell found in plants, algae, and some protists; they can fully develop without fusing with another cell.

• sporophyte: A diploid structure which produces spores by meiosis. This is the spore-producing phase in the alternation of generations life cycle.

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Summary

• Plant cycles include alternation of generations, which is the tendency of plants to reproduce sexually and asexually and, thus, alternate between being haploid and diploid. • In alternation of generations, a diploid cell grows by mitosis from a zygote, then develops into a haploid cell, by meiosis, called a spore. • Asexual reproduction requires only one parent and produces offspring that are identical to that parent. Sexual reproduction requires two parents, and the offspring carries a combination of genes from both. • Sexual reproduction is much more complicated and risky, but it allows for more variation in the population. Seeds are products of sexual reproduction in plants, and they are also more resistant to harsh weather. • Spores produced by asexual reproduction are advantageous because they can reproduce in quickly and with little energy.

Practice

Use this resource to answer the questions that follow.

• Alternation of Generations at http://www.shmoop.com/plant-biology/alternation-generations.html .

1. What is a spore? 2. Distinguish between a sporophyte and a gametophyte. 3. Why is the plant life cycle an alternation of generations?

Practice Answers

1. A spore is a single, haploid cell that contains the genetic material and instructions necessary to make another plant. 2. A sporophyte is a plant that produces spores (asexual reproduction), and a gametophyte is a plant that produces gametes (sexual reproduction). 3. The plant life cycle alternates between generations because a sexually reproducing plant makes a plant that reproduces asexually, and then the asexually reproducing plant makes a sexually reproducing plant, repeating the cycle.

Review

1. What is the main difference between plant and animal reproductive cycles? 2. What are spores and what individuals are they produced by? 3. Describe the differences between the two stages of reproduction in nonvascular plants. 4. Why is sexual reproduction more advantageous than asexual reproduction? 5. Why is asexual reproduction sometimes more beneﬁcial than sexual reproduction?

Review Answers

1. Plant cycles include alternation of generations, which is the tendency to perform both asexual and sexual reproduction. A single life cycle involves alternating between a haploid and a diploid phase. 2. Spores are haploid cells that are produced by diploid individuals called sporophytes. Spores are not gametes; they are produced by meiosis within the zygote. 3. Sporophytes and spores are adapted for drying and dispersal, while gametophytes are adapted to produce gametes in moist habitats.

33 1.4. Plant Life Cycles - Advanced www.ck12.org

4. Sexual reproduction requires two parents and meiosis, which results in more variation in the population. 5. Asexual reproduction only requires one parent. It is a quick, energy-efﬁcient way to reproduce in mass quantities.

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1.5 Bryophytes - Advanced

• Describe some of the characteristics of the division bryophytes. • Describe the differences between Bryophyta and bryophytes. • Examine the reproductive cycles of bryophytes.

What’s a bryophyte? Bryophytes were the ﬁrst plants to evolve. So, they are not very big. Shown here is a mossy undergrowth in a mountain forest. Notice how the moss appears as a shallow layer just covering the ground, rocks, and trees.

Bryophytes

In the introductory concepts on plants, you learned that bryophytes (bryo = moss; phyte = plant) were once combined to form one of two major divisions (the plant kingdom equivalent of animal phyla). This division recognized the following similarities:

• Development of tissues. • Enclosed reproductive organs. • Reproduction by spores. • Absence of vascular tissue. • Absence of ﬂowers. • Absence of seeds.

Many of these similarities acknowledge the absence rather than the presence of speciﬁc characteristics. Even the common name, “nonvascular” describes a negative feature. Perhaps, then, it was not too surprising when recent studies of the differences among bryophytes suggested that they form three distinct groups: liverworts, hornworts, and mosses.

35 1.5. Bryophytes - Advanced www.ck12.org

As you know, biological classification works to reflect evolutionary relationships, forming taxonomic groups which reflect our understanding of the history of life. We form these groups based on our interpretations of similarities and differences in characteristics. Ideally, a division (or any taxonomic group) would include all descendants of a single common ancestor –and only descendants of that common ancestor.

Bryophyta vs. bryophytes

Studies of similarities and differences using modern biochemistry and molecular clocks suggest to botanists that mosses are more closely related to vascular plants than are liverworts and hornworts. Liverworts and hornworts form distinct groups with separate evolutionary histories. As such, each is now considered a separate division within the plant kingdom. In this lesson, Bryophyta (note the capitalization) refers to the moss division, and bryophytes (lower case b) is a less formal group which includes three divisions: mosses (Bryophyta), liverworts, and hornworts. In other words, we will use the term “bryophyte” as the equivalent to “nonvascular plant,” recognizing that this grouping does not precisely reﬂect their evolutionary history. The Figure below diagrams the differences in evolutionary history implied by the old system and the one we will use here.

FIGURE 1.17 Our understanding of the evolutionary history of early plants has changed, and these changes prompt us to change our classiﬁcation system. In the past, botanists believed that all bryophytes shared a more recent common ancestor with each other than they did with vascular plants (right). Modern data suggests that liverworts diverged ﬁrst and have evolved independently since that time. Mosses and vascular plants share a more recent common ancestor –they are more closely related –than mosses and other “bryophytes.” Botanists now assign each independently evolving group to its own separate division.

Characteristics of Bryophytes

Nonvascular plants share several positive characteristics, many of which are related to the absence of vascular tissue. Despite the dominance of vascular plants, as many as 20,000 species of bryophytes continue to ﬂourish and reproduce today, as they are older than vascular plants. Most bryophytes are small, with heights measured in centimeters or even millimeters ( Figure 1.18). They occupy niches in moist, shady, terrestrial habitats. Like all plants, their life cycles involve alternation of generations. However, in bryophytes, the haploid gametophyte stage - the larger, greener photosynthetic “plant” you would be mostly likely to notice - is dominant, and the diploid sporophyte is not much more than a sporangium, which produces spores by meiosis ( Figure 1.19). Fine, hair-like outgrowths called rhizoids anchor and support the gametophytes. Gametophytes produce gametes by mitosis within reproductive organs, which surround them with sterile protective tissue. In males, these organs are

36 www.ck12.org Chapter 1. Plant Biology - Advanced

FIGURE 1.18 Nonvascular plants live in moist, shady, terrestrial habitats and are generally quite small; some would say they are Lil- liputian. Sphagnum mosses (right) are among the “tallest,” measuring up to 30 cm (12 inches). In contrast, many others are ﬂattened, remaining within millimeters of the soil. The liverwort Marchantia (left) is an example.

FIGURE 1.19 The life cycles of bryophytes, like those of all plants, show alternation of generations. However, in bryophytes, the dominant, photosynthetic plant is the haploid gametophyte, which may be either male or female. The gametophytes produce either biﬂagellate sperm or single egg cells by mitosis. The sperm swims to the female gametophyte through a layer of moisture and fertilizes the egg in the plant’s archegonium. The diploid zygote divides by mitosis to produce a small sporophyte, and, within its sporangium, the sporophytes produces haploid spores by meiosis. The spores then disperse and grow to produce new gametophytes.

called antheridia. In females, they are called archegonia ( Figure 1.20). The male gametes are sperm that have two ﬂagella which propel them through their moist habitats to the female gamete (egg or ovum) for fertilization. Dispersal of spores also often depends on moisture; critical structures change shape as they absorb moisture or dry out, propelling spores away from the parent structure. The diminutive size of bryophytes requires us to look carefully in order to really see the beauty and diversity of their Lilliputian world. Let’s leave the similarities which unite them and explore the delicate structures which make each group unique.

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FIGURE 1.20 Bryophytes have reproductive organs which surround developing gametes with sterile protective tissues: male antheridia and female archegonia. Gametes develop by mitosis within haploid gametophyte plants. When mature, biﬂagellate sperm swim through a thin layer of rainwater or dew to fertilize the egg cell within the archegonium.

Vocabulary

• antheridia (singular, antheridium): A haploid male reproductive organ which produces male gametes by mitosis.

• archegonia (singular, archegonium): A haploid female reproductive organ which produces female gametes by mitosis.

• bryophyte: A type of plant that lacks vascular tissues, such as a liverwort, hornwort, or moss.

• gametophyte: A haploid structure which produces gametes by mitosis. This is the gamete-producing phase in the alternation of generations life cycle.

• rhizoids: Fine, hair-like outgrowths –either unicellular or multicellular - which anchor and support nonvascular plants.

• sporangia (singular, sporangium): Asexual reproductive organs which produce spores.

• spore: A haploid reproductive cell found in plants, algae, and some protists; they can fully develop without fusing with another cell.

• sporophyte: A diploid structure which produces spores by meiosis. This is the spore-producing phase in the alternation of generations life cycle.

Summary

• Bryophytes were once considered one of two separate divisions of the kingdom Plantae. • Bryophyta vs. bryophytes: Bryophyta refers to the moss division, and bryophytes refers to the broader group which includes the division Bryophyta, liverworts, and hornworts. • Contrary to previous hypotheses, contemporary botanists believe that bryophytes are most closely related to vascular plants. • Most bryophytes are small and live in moist, terrestrial habitats. • Bryophyte reproductive cycles differ slightly from other plants in that their haploid gametophyte stage is dominant - these include much larger, greener photosynthetic plants. • In bryophytes, gametophytes produce gametes by mitosis within reproductive organs called antheridia (male) and archegonia (female).

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Practice

Use this resource to answer the questions that follow.

• What is a bryophyte anyway? at http://sciweb.nybg.org/science2/hcol/bryo/bryogen.html .

1. What comprises the bryophytes? 2. How many bryophytes exist today? 3. How do bryophytes reproduce? 4. What signiﬁcant feature distinguishes bryophytes from ferns and ﬂowering plants?

1. List three types of bryophytes. 2. How do bryophytes transport water between cells? 3. Why is the size of bryophytes limited? 4. What is a requirement for bryophyte reproduction? Why?

Practice Answers

1. The bryophytes are a group of plants comprising the mosses, liverworts, and hornworts. 2. There are about 25,000 different species of bryophytes in the world today. 3. Bryophytes reproduce by spore production. 4. Bryophytes do not have a true vascular system and are unable to pull water and nutrients up from the ground at any signiﬁcant distance. Lacking this specialized system distinguishes bryophytes from ferns and ﬂowering plants.

Review

1. How are bryophytes and Bryophyta different? How are they related? 2. List three distinguishing characteristics of bryophytes. 3. Describe the role gametophytes play in the reproductive cycle of bryophytes. 4. What kind of habitats are optimal for bryophytes to reproduce in and why?

Review Answers

1. Bryophyta refers to the moss division, while bryophytes encompasses three divisions: Bryophyta (mosses), liverworts, and hornworts. 2. Most bryophytes are minute, occupy niches in moist, shady, terrestrial habitats, and have a dominant gametophyte phase. 3. Gametophytes produce either biﬂagellate sperm or a single egg cell by mitosis. The sperm swims to the female gametophyte and fertilizes the egg in its archegonium. 4. Bryophytes reproduce best in moist habitats so that the sperm can be easily propelled to the female gamete. In addition, dispersal of spores often require moisture to retain their structures.

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1.6 Diversity of Nonvascular Plants - Advanced

• Outline the characteristics that separate the three divisions of bryophytes: liverworts, hornworts, and mosses. • Cover the reproductive cycles of different liverworts.

What are the different Bryophytes? Shown above is a hornwort. Notice the long, bluntly pointed "horns." Both these and other similar features are used to distinguish the bryophytes from each other.

Major Groups of Bryophytes

Although the three groups of bryophytes are each considered separate divisions, they did not arise simultaneously. The order in which the divisions are listed reﬂects their age, beginning with the oldest. We will use that pattern to organize our study of the plant kingdom.

Liverworts (Division Marchantiophyta)

Botanists believe that liverworts are the oldest of the nonvascular plants. Their rhizoids are unicellular, and their photosynthetic tissues take one of two basic forms ( Figure 1.21): over 85% of some 10,000 species are “leafy,” with flattened stems covered with overlapping scales. The remaining species are “ribbon-like,” with a flattened, often branching structure. Branching, flattened species often appear lobed; their resemblance to the lobed human liver earned them their common name. Both types are quite small: individual plants are 2-20 mm wide and less than 10 cm (4 inches) long. However, they often grow in colonies, which can carpet large moist, shady areas. Some of their reproductive structures are delightfully intricate; we will use the familiar genus Marchantia, for which the division is named, as an example. Marchantia has flattened, ribbon-like gametophytes, which are either male or female. They reproduce asexually by building tiny saucer-like "gemmae cups" ( Figure 1.22), which resemble little nests of “eggs.” The gemmae, bits of tissue, can splash out when it rains to form new individuals. Gametophytes eventually grow reproductive organs which resemble miniature umbrellas ( Figure 1.23). These become female archegonia and male antheridia, which reach heights of at least a centimeter above the plant’s

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FIGURE 1.21 Liverworts (division Marchantiophyta) are tiny nonvascular plants whose common name derives from the multi-lobed “liver” shape of the “leaves” of some species (above). Most liverworts are “leafy” rather than ﬂattened and lobed.

FIGURE 1.22 The liverwort Marchantia reproduces asexually with gemmae cups which arise from its ribbon-like gametophytes. Rain- water splashes out bits of tissue called gemmae, which can grow new individuals.

ﬂat surface and produce eggs and sperm respectively. The biﬂagellate sperm must swim from antheridium to archegonium –which, in many species, is on an entirely separate plant. After the egg is fertilized, the diploid zygote divides by mitosis to form a small sporophyte which remains within the female “umbrella” (archegonia). Within the sporophyte’s sporangium, haploid spores are produced through meiosis, and other cells develop into spring-like, water-absorbing elaters which help with dispersal. Once released, the spores develop into new gametophytes. The sporophyte –rarely ever visible –disappears soon after the spores mature. Return to the Life Cycles of Bryophytes Figure (in the Bryophytes: Characteristics (Advanced) concept) to review this miniature but complex life cycle.

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FIGURE 1.23 Marchantia gametophytes develop reproductive organs resembling miniature umbrellas. No more than a centimeter tall, they rise above the ribbon-like plants, producing egg cells in female archegonia (top right) and biﬂagellate sperm cells in male antheridia (bottom right). The sperm must swim through a thin layer of rainwater or dew to reach the egg cell, and the resulting zygote develops into a sporophyte within the female “umbrella.”

Hornworts (Division Anthocerotophyta)

Only about 100 species of Hornworts exist today. Hornworts are named for their long, bluntly-pointed sporophytes which rise above rosette-shaped gametophytes just 1-5 centimeters wide and only a few cells thick ( Figure 1.24).

FIGURE 1.24 Hornworts are named for their elongated, bluntly-pointed sporophytes which rise just millimeters above the surface of the rosette-like gametophytes. The Hornwort Web Portal at http://www3.uakron.edu/b iology/hornworts/hornworts.html is a nice site to explore more about this division.

Hornwort cells have one large chloroplast fused to other organelles, forming a large food-production-and-storage structure similar to those of their algal ancestors. Archegonia and antheridia develop within the ﬂattened gametophyte, and, as in liverworts, biﬂagellate sperm must swim to and fertilize the egg. Within the green, photosynthetic “horn” which develops from the zygote, relatively large spores and multicellular elaters develop. The elaters twist and change shape as they dry out, helping to disperse the spores, which start a new gametophyte generation.

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Hornwort life cycles resemble those of liverworts, although the antheridia and archegonia are embedded in the ﬂattened gametophyte, so they are much less visible. The photosynthetic, horn-like sporophyte is much more visible than its counterpart in liverworts.

Mosses (Division Bryophyta)

About 10,000 species of mosses make up the third division of nonvascular plants. Unlike liverworts, the rhizoids of mosses are multicellular, and their gametophytes differentiate into stem-like, leaf-like, and root-like structures.

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Mosses (division Bryophyta) may be the nonvascular plants most closely related to vascular plants, although, like other nonvascular plants, their gametophytes are dominant. Sporophytes are usually stalked capsules which grow upward from female gametophytes. Mosses grow in dense clumps which retain necessary moisture. Individual gametophytes have wiry, un-branched stalks surrounded by tiny green, photosynthetic “leaves.” The gametophytes are anchored by multicellular rhizoids, which may run horizontally and reproduce asexually ( Figure 1.25). Gametophytes are either male or female, and, when they mature, they form antheridia or archegonia at their tips. Within these organs, either biﬂagellate sperm or eggs develop by mitosis, and the sperm must swim to and fertilize the eggs. The fertilized egg, or the zygote, grows into a sporophyte with a capsule atop a tall stalk. Within the capsule, the sporophyte produces haploid spores by meiosis, which are then released to form new individual gametophytes. Tiny teeth surround the opening of the capsule, and they expand and contract with changes in humidity, helping to disperse the spores.

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FIGURE 1.25 Like other nonvascular plants, mosses have a dominant haploid gametophyte stage. In mosses, the gametophyte has tiny photosynthetic leaf-like structures encircling a central “stem.” Multicellular rhizoids, which have the potential to reproduce asexually, anchor the gametophyte “stem.” The diploid sporophyte grows from the archegonium, producing a stalk and capsule (upper right), in which meiosis takes place. When the capsule matures (lower right), tiny teeth surrounding the opening dry out and help to disperse the haploid spores, which can form new individual gametophytes.

Vocabulary

• antheridia (singular, antheridium): Haploid male reproductive organs, which produce male gametes by mitosis.

• archegonia (singular, archegonium): Haploid female reproductive organ, which produce female gametes by mitosis.

• bryophyte: A type of plant that lacks vascular tissues, such as a liverworts, hornworts, or mosses.

• chloroplast: The organelle that carries out photosynthesis.

• elater: A cell or cell structure which absorbs moisture from its environment, causing it to change shape and help disperse associated spores.

• gametophyte: A haploid structure which produces gametes by mitosis. This is the gamete-producing phase in the alternation of generations life cycle.

• sporangia (singular, sporangium): Asexual reproductive organs which produce spores.

• spore: A haploid reproductive cell found in plants, algae, and some protists; they can fully develop without fusing with another cell.

• sporophyte: A diploid structure which produces spores by meiosis. This is the spore-producing phase in the alternation of generations life cycle.

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Summary

• The three divisions of bryophytes arose at different times: first liverworts, then hornworts, and finally mosses. • Liverworts are the oldest of the nonvascular plants. The majority of these plants are leafy, with flattened stems covered with overlapping scales. • Liverworts reproduce using gametophytes which grow reproductive organs that produce sperm and eggs. The sperm must swim to the egg cell and fertilize it. The egg then forms a sporophyte, which in turn produce spores. These spores are then dispersed and develop into new gametophytes. • Hornworts have one large chloroplast for food production and storage. • Mosses are the nonvascular plants most closely related to vascular plants. • Most nonvascular plants, including liverworts, hornworts, and mosses, have a dominant gametophyte stage.

Practice

Use this resource to answer the questions that follow.

• Introduction to Bryophytes at http://blogs.ubc.ca/biology321/?page_id=3602 .

1. Bryophytes include three lineages. What are they? 2. Complete the following sentences: a. Mosses in the Class Bryopsida are commonly known as the ______or “arthrodontous” mosses. b. Mosses in the class Polytrichopsida are often considered as ______plants. c. The morphology of the species in the class Sphagnopsida, also known commonly as ______, is quite different from that seen in the other classes of the phylum Bryophyta. d. According to an old medical doctrine, this resemblance indicated that ______could cure illnesses of the liver. e. ______is the least diverse phylum of the bryophytes, however its distribution is widespread.

Practice Answers

1. Phylum Bryophyta (Mosses), Phylum Marchantiophyta (Liverworts), and Phylum Anthocerotophyta (Horn- worts). 2. Complete the following sentences: a. Mosses in the Class Bryopsida are commonly known as the “joint-toothed” or “arthrodontous” mosses. b. Mosses in the class Polytrichopsida are often considered as pioneer plants. c. The morphology of the species in the class Sphagnopsida, also known commonly as “Peat moss”, is quite different from that seen in the other classes of the phylum Bryophyta. d. According to an old medical doctrine, this resemblance indicated that liverworts could cure illnesses of the liver. e. Anthocerotophyta is the least diverse phylum of the bryophytes, however its distribution is widespread.

Review

1. What are the two main differences between liverworts and mosses? 2. How does a liverwort reproduce? 3. What is the distinguishing feature of hornwort cells? 4. Where do sporophytes develop in mosses, and how are spores dispersed?

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Review Answers

1. Liverworts have unicellular rhizoids. Mosses have multicellular rhizoids, and their gametophytes differentiate into stem, leaf, or root-like structures. 2. Gametophytes grow reproductive organs (female archegonia and male antheridia), and the biﬂagellate sperm swims from the antheridium to the archegonim to fertilize the egg. The resulting diploid zygote then divides by mitosis to form a sporophyte, which in turn produces haploid spores by meiosis. The spores are then dispersed and eventually develop into new gametophytes. 3. Hornwort cells have one large chloroplast that is fused to other organelles, forming a large food-production- and-storage structure. 4. In mosses, sporophytes develop in a capsule on top of a tall stalk. Within the capsule, the sporophyte produces haploid spores. Tiny teeth surround the opening of the capsule, and they expand and contract with changes in humidity, helping to disperse the spores.

47 1.7. Human Uses of Nonvascular Plants - Advanced www.ck12.org

1.7 Human Uses of Nonvascular Plants - Ad- vanced

• Detail human uses for the versatile, nonvascular moss Sphagnum. • Describe a few uses for liverworts.

How are mosses used? Mosses can be used in gardens, as shown here in the exquisitely exhibited Fletcher Moss Park and Botanical Gardens of Manchester, England. Bryophytes have many other uses, some of which are discussed in this concept.

Human Uses of Bryophytes

Humans have many uses for Bryophytes; two of the more important and widely used resources are described below.

Uses for Sphagnum moss

One of the most economically important genera of nonvascular plants is Sphagnum, a genus of perhaps 300 species of mosses which carpet bogs throughout the northern hemisphere ( Figure 1.26). Sphagnum resists decay and builds up into thick layers beneath bogs, where some people (as in Ireland) harvest or mine it for fuel. Sphagnum gametophytes ( Figure 1.27) grow up to 30 cm (12 inches) long, and their “leaves” contain large cells which hold up to 20 times their own dry weight in water ( Figure 1.27). For this reason, Sphagnum moss has been used as an additive to condition sandy soils, a growing medium for orchids and other epiphytes, a fire extinguisher, and even in diapers. Because its chemistry makes it acidic and resistant to decay, Sphagnum mosses have also been used to dress wounds, raise mushrooms and tarantulas, and filter septic system waste. These same characteristics have preserved bodies for thousands of years, allowing us to study past humans and their cultures (see the Haraldskaerwoman from the Iron Age in the Figure 1.27). A unique use of Sphagnum is as smoking malt to flavor Scotch whisky. In times of famine, it has been used to make bread. As with so many other resources, over-harvesting and habitat destruction have begun to endanger Sphagnum moss reserves. However, countries such as New Zealand have begun to take action to ensure preservation of this useful

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FIGURE 1.26 Sphagnum mosses thrive in bogs (left), where acid prevents them from decaying. “Peat,” which consists of layers upon layers of compressed plants - often predominantly Sphagnum - can be mined (upper right) and used for fuel (lower right).

FIGURE 1.27 Sphagnum moss (A) has physical and chemical properties which make it useful for purposes other than fuel. Large dead cells (B) within its “leaves” hold up to 20 times its weight in water, so it is often used to condition sandy soils or grow epiphytes. Its chemistry is acidic and it resists decay, so it has been used to dress wounds. These same properties have preserved bodies, such as this Iron Age Haraldskaerwoman (C), providing us with insight into past human lives and cultures.

moss. New Zealand has developed sustainable management policies which ensure that the resources remain viable. Peat consists of layers upon layers of compressed plants (often predominantly Sphagnum) and can be mined and used for fuel. But mining of moss peat (as opposed to live moss) can permanently destroy peat bogs, causing extensive environmental damage.

Uses for Liverworts

Humans use liverworts to a much lesser extent. In the past, we have consulted the medieval “Doctrine of Signatures,” which shows that plant characteristics reveal their uses to humans. Because the lobed shape of certain liverworts resembled the human liver, people in the past used them to treat diseases of the liver. At least one species of aquatic

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liverwort is sold for use in home aquariums ( Figure 1.28), but liverworts otherwise have more value for their roles in nature. Both mosses and liverworts retain soil and water, preventing erosion, especially along stream banks. A few species of liverworts form erosion-limiting crusts on soils in deserts and Polar Regions. Mosses can sometimes be unwanted as well - a few species of liverworts and hornworts are weeds in greenhouses and gardens.

FIGURE 1.28 The aquatic liverwort Riccia ﬂutans is often sold for use in freshwater aquariums (left). Other liverworts are useful primarily in their native habitats, holding water and preventing erosion along streams (right).

Vocabulary

• epiphyte: A type of plant (or lichen) that grows on other plants for support.

• gametophyte: A haploid structure which produces gametes by mitosis. This is the gamete-producing phase in the alternation of generations life cycle.

• peat: An accumulation of partially decayed vegetation; one of the most common components is Sphagnum moss.

Summary

• Sphagnum moss is used predominantly for fuel. It is extremely resistant to decay and harsh conditions, so it has many other uses, such as being an additive to condition sandy soils, a ﬁre extinguisher, a treatment for wounds, etc. • Sphagnum habitats have become endangered, so countries such as New Zealand have taken action to prevent further damage by creating policies that limit mining of peat moss. Mining peat moss, as opposed to live moss, can permanently damage or destroy peat bogs. • Both mosses and liverworts retain soil and water, preventing erosion, especially along stream banks.

Practice

Use these resources to answer the questions that follow.

• Human uses of Bryophytes at http://wtnhsplantpg.wikispaces.com/Human+uses+of+Bryophytes . • Uses of Bryophytes at http://www.shvoong.com/exact-sciences/471158-uses-bryophytes/ .

1. What bryophyte can be used as a natural sponge? 2. What bryophyte was used as cleansers to wash hair? 3. How are bryophytes used medically? 4. Give an example of how bryophytes are used to monitor environmental conditions. 5. Why are bryophytes important for hemophiliacs?

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Practice Answers

1. Sphagnum moss can be used as a natural sponge. 2. Camplyopus Introflexus was used as cleansers to wash hair. 3. Bryophytes are used medically to treat liver ailments, ringworm, heart problems, inflammation, fever, urinary and digestive problems, female problems, infections, lung disease, and skin problems (burns and wounds). 4. Answers will vary. Both liverworts and mosses are good indicators of environmental conditions. Certain mosses thrives well in a specific range of pH, thus, their presence can be used as an indicator of soil pH. 5. The most novel use of mosses is the use of transgenic Physcomitrella for producing ’blood-clotting factor IX’ for the treatment of Hemophilia B.

Review

1. List a few uses for Sphagnum moss. 2. What makes Sphagnum moss a good preservative? 3. Which moss do we want to prevent harvesting and why? 4. How are mosses and liverworts both useful in the environment?

Review Answers

1. The uses for Sphagnum moss include, but are not limited to, the following: an additive to sandy soil, a growing medium for epiphytes, a ﬁre extinguisher, a component in diapers, a tool for dressing wounds and preserving bodies, etc. 2. Sphagnum’s chemistry is more acidic which makes it more resistant to decay. 3. Peat moss, which consists of layers upon layers of compressed plants; because harvesting this can permanently destroy peat bogs, we could potentially causes extensive environmental damage. 4. Mosses and liverworts both retain soil and water, which prevents erosion along stream banks.

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1.8 Vascular Plants - Advanced

• Analyze the importance of both “circulatory” tissues and a “skeletal” molecule to the evolution of life. • Visualize the transformation of Earth brought about by these adaptations. • Describe the basic structure of xylem tissue. • Relate the xylem’s structure to the cohesion-tension theory which explains its function. • Describe the basic structure of phloem tissue. • Relate the phloem’s structure to the pressure-ﬂow hypothesis which explains its function. • Discuss the characteristics of the molecule lignin and relate them to its function in xylem tissue.

Do the leaves at the top of the tree need water? Of course they do. Recall that water is a reactant of photosynthesis. Where does that water come from? It comes from the ground and works its way upward, demonstrating one of the evolutionary wonders of vascular plants.

Vascular Plants

About 420 million years ago, two new types of tissue and a new molecule revolutionized the plant kingdom. Essentially, new kinds of plants developed "circulatory systems" - or vascular systems - and "bone" - or tough cell walls - which would allow them to grow large and endure periods of drought, cold, and disease in relatively harsh land environments. With circulatory systems, land plants could provide water from the ground to cells high in the drying air. With a reliable water supply and architectural support, plants could grow taller and branch out, intercepting more sunlight to make more food. With circulatory systems and more food, some cells no longer needed to photosynthesize to obtain energy. Fed by the circulatory system, they could reach into the ground to tap deeper water supplies or burrow into rock to obtain minerals. Fed by circulatory systems, deep roots anchored plants so they could grow even taller. Other cells –also fed by the circulatory systems –could focus on support. Some even used the “bony” molecule to engineer reinforcing tissues which would function long after the cells themselves died. Such plants could become

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dormant in the winter or during drought and then begin new leaf growth high off the ground when favorable conditions returned. The larger size of these ﬁrst vascular plants made them better competitors for sunlight. Their endurance allowed them to colonize vast new areas of land. Although 20,000 species of bryophytes continue to carpet moist, shady areas, the new plants and their descendants have dominated terrestrial ecosystems since their appearance hundreds of millions of years ago.

FIGURE 1.29 Vascular tissue and lignin “skeletons” allowed clubmosses and ferns to soar above other plants and spread across the Earth. A Devonian landscape depicts upward growth and branching that characterized early vascular plants (upper left). A Carboniferous scene illustrates the dominance of ferns (right). The lower left image shows a fossilized fern frond with branching veins of vascular tissue. This is one of the earliest true leaves, known as megaphylls.

Imagine the changes that took place on Earth as plants with these new adaptations diversiﬁed ( Figure 1.29):

• Vibrant green spread from the moist borders of lakes and rivers into all habitable areas of the land. • Terrestrial environments transformed dramatically - from shallow, two-dimensional surfaces to soaring three- dimensional domains. • Roots burrowed into and broke apart rock and sand, beginning the formation of soil. • Death, for these large masses of vegetation, enriched and deepened fragile new soils, emphasizing regeneration and growth rather than loss. • Burgeoning food supplies and complex habitats created by plants promoted increases in the size and diversity of terrestrial animals –as well as fungi, bacteria, protists, and plants themselves.

The two tissues and single, critical molecule have transformed Earth irrevocably. Rather than referring to these new plants as “seedless,” let’s acknowledge their “great inventions” and study them as the vascular pioneers that they were. Certainly, bryophytes were the photosynthetic pioneers onto land, but early vascular plants were the photosynthetic pioneers into air.

Characteristics of Vascular Plants

The new “circulatory system” was vascular tissue. Vascular means “vessel” –in plants, as well as in the cardiovascular systems of animals. Cardio, however, means “heart," and it is the heart muscle that pushes blood through animals. Without muscle tissue, plants have no means of pumping liquids through their vessels in order to supply water and nutrients to the vast numbers of cells required to build large bodies. A circulatory system without muscles must harness natural, physical and chemical processes –and vascular tissue does just that. Vascular tissue has long,

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narrow cells arranged end-to-end, forming tubes or pipes. The plant’s structure exploits the properties and behavior of water molecules in order to move water, nutrients, and food through these microscopic “pipes” against the force of gravity. How does this work? By exploiting the properties of cohesion and tension.

Cohesion and Tension

The cohesion-tension theory is explained as follows:

1. Water evaporates into the air through stomata in the plant’s leave tissues. 2. The surface tension of water remaining in the leaves and leaf cells creates a negative pressure called transpirational pull in the leaf veins and vascular tissue throughout the plant. 3. Osmosis moves water from hypotonic soil solutions into hypertonic plant root cells, creating a positive pressure called root pressure. 4. Water molecules within the vessels pull on one another because water is a polar molecule, and these molecules form hydrogen bonds (cohesion).

Xylem

Cohesion between water molecules allows them to form a continuous column moving upward against the force of gravity. The water molecules are “pulled” upward by transpirational pull and “pushed” from below by root pressure. The water molecules carry with them dissolved ions which supply plant tissues with needed minerals. The speciﬁc tissue which serve this function is the xylem ( Figure 1.30). The cell walls of the xylem are reinforced with lignin, the “bony” molecule mentioned earlier and discussed below. At functional maturity, xylem cells are no longer living, yet they form elongated, hollow tubes to carry water and ions.

FIGURE 1.30 Xylem tissues transport water and ions from roots to stems and leaves against the force of gravity by harnessing physical properties of water, such as evaporation (as transpiration), surface tension, osmosis, and cohesion. The cell walls of xylem vessels, such as these, are reinforced with lignin; at functional maturity, the cells themselves are no longer living.

A related characteristic of vascular plants is the secretion by the epidermis of a well-developed, water-conserving cuticle made of waxes and the complex, waterprooﬁng molecule cutin. The cuticle, which in essence is a protective covering, prevents water loss, repels water and dirt, limits gas exchange to stomata, and reduces infection by viruses, bacteria, and fungi.

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Phloem

A second vascular tissue is specialized to transport sugars throughout the plant using a different mechanism. Like the water-carrying xylem, the food-carrying phloem ( Figure below) consists of elongated cells arranged end-to-end. However, phloem cells are living, and their ends are connected with both sieve plates and many tiny bridges called plasmodesmata. To move sap –solutions of sugars which the plant has photosynthesized –plants create and harness diffusion gradients.

FIGURE 1.31 This microphotograph of the phloem of a tree shows the elongated, living cells that transport sap throughout vascular plants.

The pressure ﬂow hypothesis is explained as follows:

1. High concentrations of sugar at photosynthetic “sources” attract water by osmosis, creating osmotic pressure in the nearby phloem. 2. Low concentrations of sugar at metabolic “sinks” (underground tissues, growing regions) lose water by osmosis, lowering osmotic pressure in the nearby phloem. 3. Sap ﬂows through the phloem from areas of high pressure (“sources”) to areas of low pressure (“sinks”). 4. In earlier plants, sugars which diffuse into the phloem are chained into large molecules which cannot diffuse back out; in later plants, companion cells actively transport (“load”) sugar into the phloem.

Lignin

The xylem and phloem ( Figure 1.33) form the “circulatory system” for plants, but a complex organic molecule provides the “bone.” Lignin ( Figure 1.34) is named for the wood (Latin, lignum) it builds. All plants have cell walls of cellulose and other polysaccharides, but vascular plants strengthen and “waterproof” their xylem cells with lignin. Lignin has at least three properties which greatly beneﬁt the plants which synthesize it:

1. Because lignin is hydrophobic, its presence in xylem greatly improves the efﬁciency of water transportation. 2. Because lignin forms covalent bonds which cross-link polysaccharides in the cell wall, it adds mechanical support –individually to xylem cell walls and cumulatively to the whole plant. 3. Because animals (and many bacteria and fungi) do not have enzymes which can digest lignin, this chemical gives plants increased resistance to herbivores. You can see the effects of lignin’s durability in the leaf shown in the Figure 1.34; the veins, made of lignin-containing xylem, remain long after the leaf tissue itself has decayed.

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FIGURE 1.32 Both vascular tissues which form the “circulatory system” of plants are shown in this cross-section of an onion root. The xylem transports water and ions from roots to leaves, and the phloem carries sap from “sugar sources” to “sugar sinks.”

FIGURE 1.33 The xylem and phloem are the two types of vascular tissues in vascular plants.

These two tissues (xylem and phloem) and one molecule (lignin) gave rise to all modern plants, which are grouped together as vascular plants or tracheophytes (“tube plants”). Formerly one of only two divisions, tracheophytes are now divided informally into those which produce spores (the earlier vascular plants) and those which produce seeds (superdivision Spermatophyta). In this lesson, you will explore the characteristics and diversity of two major groups of early vascular plants –the club mosses (division Lycopodiophyta) and the ferns (division Pteridophyta). Most species of today’s club mosses and ferns are modest in size, but, as the ﬁrst plants to solve the problems of climbing into the air, ancient species grew the ﬁrst tall forests, skyrocketing above the nonvascular bryophytes which had preceded them. Vascular plants have one additional major difference from bryophytes. Recall that in bryophytes, the dominant stage within alternation of generations was the haploid gametophyte –with just one set of chromosomes per cell. In contrast, even the earliest vascular plants have dominant diploid sporophytes –with two sets of chromosomes per cell. This difference may render vascular plants slower to evolve than the bryophytes; it also better protects them from the effects of harmful recessive mutations.

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FIGURE 1.34 The molecule lignin strengthens the cell walls of the xylem by cross-linking polysaccharides, improves water- conducting efﬁciency because it is hydrophobic, and protects plants from herbivores because most animals lack enzymes which can degrade it. This leaf shows the ligniﬁed xylem veins which remain after microorganism decay of the leaf material.

Summary Table

The following table summarizes the major characteristics evolved by vascular plants.

TABLE 1.1:

Structure or Characteristic Components Description Function(s) Vascular Tissue Xylem Elongated, dead cells Transports water from with thickened walls, roots to leaves, according connected end-to-end to cohesion-tension theory Lignin Strong, complex, Improves efﬁciency of hydrophobic molecule water transport, defense against herbivores, support Phloem Elongated, living cells Transports organic connected end-to-end molecules from leaves by sieve plates and to growth or storage plasmodesmata regions according to pressure-ﬂow hypothesis Cuticle Cutin Waxy coating covering Protects tissues from de- stems and leaves hydration, infection, and herbivores; controls gas exchange Life Cycle Changes Gametophyte Reduced, but often still in- Produces eggs and sperm dependent of sporophyte Sporophyte Dominant, begins devel- Photosynthesis, produces opment in female gameto- spores phyte

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TABLE 1.1: (continued)

Structure or Characteristic Components Description Function(s) Organs Roots Microphylls with a single Roots: absorb nutrients Stems vein; and water, anchor plant Leaves Macrophylls with branch- Stems: transport water ing veins and nutrients, support leaves Leaves: photosynthesis Growth form and habitat Tall trees Swamp Forests Sexual reproduction restricts these to moist land

The nearly universal combination of xylem, phloem, lignin, and sporophyte dominance among vascular plants is intriguing. Let’s look more closely at what is known of their evolution.

Vocabulary

• cohesion-tension theory: The theory that describes the movement of water and minerals up through the xylem (due to evaporation of water from leaves and the cohesion and adhesion of water molecules below).

• lignin: A chemical compound which forms the woody part of some plant cell walls.

• osmosis: The diffusion of water molecules across a selectively permeable membrane.

• phloem: Vascular tissue which transports food from leaves to storage or growth areas in other parts of the plant; the phloem includes sieve tube elements and companion cells.

• plasmodesmata (singular, plasmodesma): Microscopic channels which traverse the cell walls of plant cells, enabling transport and communication between them.

• pressure ﬂow hypothesis: Explains the transportation of sap by the phloem according to differences in sugar concentration between sugar sources and sugar sinks.

• root pressure: The osmotic pressure within the cells of a root system; this pressure causes sap to rise through a plant stem to the leaves.

• sieve plates: Pores in plant cell walls that facilitate the transportation of materials between them; they are found at the interface between two sieve tube members.

• stomata (singular, stoma): Openings on the underside of leaves which allow gas exchange and transpiration.

• tracheophyte: A type of plant that has vascular tissues, such as a seed plant or ﬂowering plant.

• transpirational pull: The movement of water and minerals up through the xylem due to evaporation of water from leaves and the cohesion and adhesion of water molecules below.

• vascular plant: A plant with tissues for conducting water and minerals throughout the plant.

• vascular tissue: A type of tissue in plants that transports ﬂuids through the plant; the xylem and phloem are vascular tissues.

• xylem: Vascular tissue which transports water and minerals from the roots to the stems and leaves; the xylem includes tracheids and vessel elements.

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Summary

• Two “circulatory” tissues and a supporting molecule enabled vascular plants to colonize dry land. • The appearance of vascular plants revolutionized terrestrial environments, adding soil and many ecological niches. • The xylem is made up of bundles of elongated, dead cells with thickened, lignified walls, connected end-to- end. • The cohesion-tension theory explains how the xylem, evaporation, and cohesion carry water from roots to leaves. • Phloem tissue consists of bundles of elongated, living cells connected end-to-end by sieve plates. • The pressure-flow hypothesis explains how the phloem creates diffusion gradients to move organic molecules. • The molecule lignin adds support, defense, strength, and efficiency to xylem tissue for water transport. • In vascular plants, the diploid sporophyte generation dominates. • A well-developed cuticle limits water loss and gas exchange and reduces infection and herbivory.

Practice

Use this resource to answer the questions that follow.

• Vascular Plant Facts at http://www.life123.com/parenting/education/botany/vascular-plant-facts.shtml .

1. What feature allows certain plants to live in practically any terrestrial ecosystem on earth? 2. Describe the role of the xylem. 3. Describe the role of the phloem. 4. Describe the vascular tissues within a tree.

Practice Answers

1. The vascular system of plants allows certain plants to live in practically any terrestrial ecosystem on earth. 2. The primary function of the xylem is to supply the plant with water and water-soluble nutrients. 3. The phloem is the tissue responsible for distributing the nutrients acquired through photosynthesis. 4. The xylem is the woody part in a tree, which is somewhat spongy while the wood is still "green." The phloem is the layer of tissue inside the tree’s bark, which encircles the trunk and each of the branches.

Review

1. Name and describe the two tissues and the molecule which revolutionized the Plant Kingdom about 420 million years ago. 2. Discuss the importance of “circulatory” tissues and a “skeletal” molecule to the evolution of life. 3. Summarize the cohesion-tension theory which describes how vascular plants transport water without muscle tissues. 4. Summarize the pressure-ﬂow hypothesis which describes how vascular plants transport sap without muscle tissues.

Review Answers

1. Vascular systems, cell walls, and lignin revolutionized the plant kingdom. 2. Circulatory tissues allow water and nutrients to be transported to far cells, while the skeletal molecule rein- forces the plant and allows it to grow tall.

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3. Water evaporates from the top of a plant. Through cohesion, these water molecules pull on other water molecules, and cause water to be pulled up from the roots. 4. As sugar gets used up in tissues, this causes a pressure difference to be created due to osmosis. The pressure difference causes sap to ﬂow to the area that requires it.

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1.9 Evolution of Vascular Plants - Advanced

• Describe the oldest fossils of vascular plants. • Trace the evolution of clubmosses from their appearance during the Silurian to the present day. • Trace the evolution of ferns from their appearance in the Devonian to the 20,000 species living today. • Compare the importance of Carboniferous horsetail ancestors to that of Equisetum today. • Relate the life cycles of early vascular plants to their success in Carboniferous swamps and their decline in the middle Permian.

Why did vascular plants evolve? Shown above is a cartoon reconstruction of Cooksonia. This is considered one of the ﬁrst vascular plants, but yet there are no leaves, no ﬂowers, and no seeds. It was a very simple plant growing just a few centimeters high. But it did have a xylem, moving the water from its simple root hairs upward.

How Did Vascular Plants Evolve?

The oldest potentially vascular plant fossil dates back to the Silurian period, 425 million years ago. The now-extinct Cooksonia ( Figure 1.35) rose just a few centimeters above the ground, with branching stems capped by sporangia (showing it is a sporophyte) but without roots or leaves. In at least one of the ﬁve species, a dark stripe suggests the remnants of vascular tissue. Fossils of a plant with similar structure but different patterns of branching, Psilophyton (literally “bare plant”), date back to the Devonian period. Some species of Psilophyton grew as tall as 60 cm (2 feet).

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FIGURE 1.35 The oldest vascular plant fossil preserves are of Cooksonia, a branching plant with sporangia at the tips of each branch. Cooksonia fossils measure just centimeters in height and date back to the Silurian period. Psilophyton, a species with similar structure but different branching patterns, grew as tall as 60 cm (2 feet) during the Devonian period.

Clubmosses and Ferns

Although these earliest vascular plants did not have roots or leaves, primitive clubmosses and ferns had appeared by the Late Devonian, forming the world’s first forests. Soils began to form, root systems developed, and, by the end of the Devonian, the earliest seed plants had appeared. This “Devonian explosion” of plant forms coincided with and permitted the evolution of terrestrial arthropods and the movement of vertebrates to land. Clubmosses had (and still have) small, simple, almost scale-like leaves, each with a single lengthwise vein; these earliest “true” (vascular) leaves are known as microphylls. Sporangia of living species form on modified leaves and are often compacted into the familiar “clubs” which top so many of today’s clubmosses. By the Carboniferous, Scale Trees over 1 meter in diameter and 30m tall dominated coal-forming swamps. Long, narrow leaves spiraled around their green trunks, but bark, rather than wood, supported their trunks. Branches ended in cone-like structures which produced spores - strikingly similar to today’s clubmosses ( Figure 1.36). Like the clubmosses, relatives of today’s horsetails reproduced by spores born in “clubs” of modified leaves atop green stems. Leaves, however, were reduced to scaly whorls (the surviving genus Equisetum means “horse bristle”) appearing at nodes along the hollow, unbranched stems. Fossils show that the Carboniferous horsetail Calamites ( Figure 1.37) grew as tall as 20 meters, with true wood in its hollow, bamboo-like stems. Ferns developed the first leaves with branching veins ( megaphylls). Ferns reproduce asexually by sprouting from

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FIGURE 1.36 Among the oldest fossils of clearly vascular plants are those of clubmosses and horsetails. First appearing during the late Silurian, clubmosses grew tall during the Devonian, and, by the Carbonifer- ous, Scale Trees dominated coal-forming swamps. The fossil Scale Tree “cone” and leaf scales (left) show a clear relationship to Lycopodium’s “clubs” and microphylls (center). Scale Trees, however, reached 30m in height, and most of today’s clubmosses reach just centimeters above the ground. Equisetum (right), whose species rarely exceed 1 meter in height, is the only surviving genus of a large group of Carboniferous Horsetails which included the 30m Calamites. Note the similarity of Equisetum’s spore-producing “club” to those of Lycopodium and Scale Trees.

FIGURE 1.37 Among the first trees were Calamites, woody horsetails which grew 30 meters tall; fossils (left) formed when minerals filled the hollow, jointed stems with stone. A fern fossil (middle) shows one of the first megaphylls –frond-like leaves much larger than the scale-like microphylls of clubmosses. One of the first trees was the fern relative, Archaeopteris. The image on the right is a microphotograph that shows petrified wood from this genus (scale = 1 mm).

underground stems ( rhizomes), as well as by spores produced by sporangia on the undersides of their fronds. Archaeopteris, one of the first trees, which flourished and then became extinct during the Devonian, had almost gymnosperm-like wood ( Figure 1.37), frond-like leaves, and deep roots which anchored its 10-meter height. Archaeopteris reproduced by spores, but its 1.5-meter woody trunk, stems with nodes and buds, and branching roots lead taxonomists to consider it as an intermediate form between ferns and gymnosperms. Gymnosperms are seed-bearing plants, which will be discussed in the next lesson. Like all vascular plants, early ferns, clubmosses, and horsetails lived most of their lives as diploid sporophyte plants. However, each of the three groups retained an independent - though greatly reduced –haploid gametophyte stage. Note in the Figure 1.38 that the gametophyte of the fern is a small, flattened, heart-shaped prothallus with

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rhizoids anchoring it to the ground. Archegonia and antheridia produce eggs and motile sperm. Sperm cells swim to and fertilize the eggs, and the dominant, familiar sporophyte plant grows out of the inconspicuous, short-lived gametophyte. This stage of early vascular plants’ life cycles required moist habitats. It is perhaps not surprising, then, that their dominance of swamp forests ended by the middle Permian when changes in climate led to their replacement by gymnosperms, whose gametophytes were (and are) well-protected within pollen (male) and seeds (female).

FIGURE 1.38 In ferns, the sporophyte is the dominant, familiar plant. However, the spores produced by meiosis develop into independent, though inconspicuous, haploid gametophytes (right), complete with bryophyte-like rhizoids. Within the gametophyte, archegonia and antheridia produce sperm and eggs. After sperm swim to and fertilize the egg, a new, familiar sporophyte grows out of the tiny gametophyte. All early vascular plants show similar life cycles; the sperm cells of clubmosses, Lycopodium, Selaginella, the whisk fern Psilotum, and the horsetail Equisetum are shown on the left.

The three major groups of early vascular plants did not disappear entirely, however. Ferns continued to diversify, with a “great radiation” in the late Cretaceous. More than 20,000 species survive today, including two families of tropical tree ferns which grow up to 20 meters tall. Horsetails, now classiﬁed with the ferns, are represented today by only one genus (Equisetum) containing just 15 species, none matching the ancient, tree-like Calamites. Some 1200 species of clubmosses survive –all small. Ironically, these include miniature tree-like plants known as “ground pines” and “ground cedars.” The last section of this lesson will explore the major divisions and diversity of the still-seedless descendants of these ﬁrst, pioneering vascular plants.

Vocabulary

• megaphylls: Larger, true leaves with branching veins, characteristic of ferns and seed plants.

• microphylls: Single-veined "tiny leaves" of club mosses, perhaps similar to the ﬁrst true leaves.

• prothallus: The (usually greatly reduced) gametophyte stage of ferns, club mosses, and horsetails.

• rhizome: A modiﬁed, subterranean stem of a plant that often sends out roots and shoots from its nodes.

Summary

• The oldest vascular plant fossils date back to 420 million years ago and show branching stems but no leaves or roots.

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Practice

Use this resource to answer the questions that follow.

• Seedless Vascular Plants at http://www.shmoop.com/plant-evolution-diversity/seedless-vascular-plants.html .

1. What three important characteristics evolved in seedless vascular plants? 2. What was the main beneﬁt of a vascular system in plants? 3. Why were leaves signiﬁcant to the early plants? 4. Describe ancient lycophytes. Why did these plants die out?

1. Distinguish between xylem and phloem. 2. Distinguish between the shoot and the roots. 3. Describe the roles of the leaf and the stem. 4. What is a rhizome? 5. List three types of seedless vascular plants. 6. What happened to forests of seedless vascular plants?

Practice Answers

1. Vascular systems, roots, and leaves are the three important organs and systems that evolved in seedless vascular plants. 2. The evolution of a vascular system meant that plants could grow taller. Plants that can grow taller than their neighboring plants have the advantage of getting more light and shading out the competition. 3. Leaves gave a bigger areas for photosynthesis, which means more energy capture for a plant, giving it a good chance of survival and reproduction. 4. Ancient lycophytes could be 6 feet wide and over 100 feet tall. The plants dies out when the climate became drier.

Review

1. Name and describe the earliest group of vascular plants. 2. Describe the plants which formed the coal deposits we burn for energy today. 3. Compare and contrast ferns and clubmosses.

Review Answers

1. The ﬁrst vascular plant had branching stems, but no roots or leaves. 2. The coal-forming plants were scale trees, essentially giant clubmosses. 3. Ferns have branching veins, while clubmosses don’t.

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1.10 Early Vascular Plants - Advanced

• Differentiate between the two primary divisions of early vascular plants. • Describe the three major groups of clubmosses.

What happened to the early vascular plants? They grew taller. The ﬁrst vascular plants were just a few centimeters tall. As they evolved, they grew taller. Shown above is Lycopodium, a plant with a horizontally branched, creeping main stem giving rise to occasional long, slender, upright shoots between 8-15cm tall. This is much taller than the ﬁrst vascular plant, Cooksonia.

Diversity of Early Vascular Plants

The plant kingdom includes at least ﬁve divisions (the plant equivalent of animal phyla) of early vascular plants, but three of these include only extinct species characterized from Devonian fossils. Just two major groups of pioneers have adapted successfully to changing environments and competition throughout the millions of years since their appearance: club mosses (division Lycopodiophyta) and ferns (division Pteridophyta, “feather plants”).

Clubmosses: Division Lycopodiophyta

The Lycopodiophyta are the oldest plants. Today, the majority of known genera (well known from coal deposits) are extinct. The division includes one extinct and three living classes; again, we will focus here on the three extant (living) groups: the clubmosses, spikemosses, and quillworts.

Clubmosses: Class Lycopodiopsida

The genus from which both this class and the division take their name, Lycopodium derives its name from roots, branching stems, or spore-bearing "clubs" (depending on the source) thought to resemble a “wolf’s foot” (lyco = wolf; pod = foot) ( Figure 1.39).

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FIGURE 1.39 The oldest division of living vascular plants, Lycopodiophyta takes its name from the clubmoss genus Lycopodium, meaning “wolf’s foot.” This drawing shows roots, branching stems with microphylls, “clubs,” spores, and enlarged, photosynthetic and sporangia-bearing microphylls. Missing is the inconspicuous gametophyte stage with archegonia and antheridia. Various sources cite root shape, branching form, and leaf shape as the origin of the name. Which structure do you think most closely resembles a “wolf’s foot?”

The common name, clubmoss, refers to the “clubs” of modified leaves which bear sporangia ( Figures 1.39 and 1.40) and their misleading resemblance to mosses (division Bryophyta). Unlike mosses, clubmosses have true roots, creeping and erect stems, and an underground gametophyte stage with archegonia, antheridia, sperm, and eggs. Their leaves ( microphylls) are small and scale-like with single veins containing the vascular tissue which separates them from the true mosses. The clubmoss class includes clubmosses, bog mosses, and firmosses ( Figure 1.40), which carpet coniferous woodlands. The leafy green plants are used to make wreaths, and their highly flammable spores are used in fireworks ( Figure 1.41). A chemical isolated from firmosses, long used in Chinese medicine, is being studied as a treatment for Alzheimer’s disease and epilepsy.

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FIGURE 1.40 Clubmosses (top left: “club” and top right: stems and microophylls), bog mosses (bottom left), and ﬁr mosses (bottom right) are the surviving groups of the clubmoss class, for which the oldest division of vascular plants, Lycopodiophyta, is named. Although they resemble mosses (division Bryophyta), they have vascular tissue, roots, and microphylls.

Spikemosses: Class Selaginallopsida

“Resurrection plants” ( Figure 1.42) are among the 700 species of spikemosses. These desert plants dry up during droughts but quickly green again when rains arrive. Most species prefer moist habitats, and many live in the tropics. Close relatives of the clubmosses with which they were formerly classiﬁed, spikemosses differ in that their sporangia, nestled in the axils of modiﬁed leaves, produce two sizes of spores. Megaspores develop female gametophytes, archegonia, and egg cells. Microspores grow male gametophytes, antheridia, and sperm cells. See http://www.unioviedo.es/bos/Asignaturas/Botanica/Imagenes/Selaginella%20denticulata%20%28Lycopodioph yta,%20Selaginellales%29.JPG for a detailed diagram of this class.

Quillworts and Scale Trees: Class Isoetopsida

Named for their long, hollow leaves, the 150 extant species of quillworts ( Figure 1.43) are mostly aquatic or semi- aquatic. Leaves from 2 to 20 cm long arise from a swollen, bulb-like rhizome base. The base houses the sporangia, which produce both megaspores and microspores. Quillworts are probably most famous as the closest surviving relatives of the Carboniferous Scale Trees (see the fossil in the Figure under Clubmosses and Ferns in the Vascular Plants: Evolution (Advanced) concept).

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FIGURE 1.41 Because the spores of clubmosses are highly ﬂammable, they have been used in ﬁreworks and in science labs.

FIGURE 1.42 Not mosses but ancient vascular plants, spikemosses include 700 species in a single genus: Selaginella. Many prefer moist habitats (B), but a few grow in deserts (C); “resurrection plants” survive almost complete drying to revive when water re- turns. Unlike their close relatives the clubmosses, spikemosses produce two types of spores (note the sporangia in A); the larger megaspores produce female gametophytes, and the smaller microspores produce male gametophytes.

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FIGURE 1.43 Mostly aquatic, the quillworts are rare today compared to their close relatives, the clubmosses and spikemosses. They are known primarily because their extinct relatives, the Scale Trees Lepido- dendron, dominated Carboniferous coal swamp forests.

Vocabulary

• gametophyte: A haploid structure which produces gametes by mitosis. This the gamete-producing phase in the alternation of generations life cycle.

• microphylls: Single-veined "tiny leaves" of club mosses, perhaps similar to the ﬁrst true leaves.

• rhizome: A modiﬁed, subterranean stem of a plant that often sends out roots and shoots from its nodes.

• sporangia (singular, sporangium): Asexual reproductive organs which produce spores.

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Summary

• Clubmosses had small, single-veined leaves called microphylls; in the past, they were much taller than today.

Practice

Use this resource to answer the questions that follow.

• The First Vascular Land Plants at http://taggart.glg.msu.edu/isb200/ﬂand.htm .

1. Describe Cooksonia. 2. Describe the Rhynia plant. 3. The competition for light among early plants led to the development of what characteristic? 4. What is an enation? What was the purpose of this tissue? 5. Where did leaves evolve from? 6. What led to the evolution of true roots?

Practice Answers

1. Cooksonia was a very small plant, only a few inches tall. Cooksonia had terminal sporangia, a rhizome, and erect, dichotomous branches. They had neither roots nor leaves. 2. Rhynia consisted of an underground stem with simple, erect, dichotomous and photosynthetic branches bearing terminal, ovoid sporangia. The plant did not have leaves. 3. The competition for light led to an increase in both size and the complexity of branching. 4. Enations were small ﬂaps of photosynthetic tissue that covered the branches of some early plants without leaves. This tissue served to increase the total area of photosynthetic tissue. 5. Lycopods were the ﬁrst plants that evolved leaves, which formed by vascularization of simple enations. 6. Leaves, with the increased loss of water they produced, created selective pressure to improve water absorption, leading to the appearance of true roots.

Review

1. Clarify the similarities and differences between clubmosses, spikemosses, and Bryophyte mosses.

Review Answers

1. Clubmosses and spikemosses have vascular tissue, roots, and microphylls, while Bryophyte mosses do not.

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1.11 Ferns - Advanced

• Compare and contrast the major groups of ferns. • Identify and describe true ferns.

What is a fern? Ferns are plants that are familiar to most people. But what exactly is a fern? Obviously, it has a vascular system for the transport of water and nutrients. But more precisely, a fern is a ﬂowerless plant that has feathery or leafy fronds and reproduces by spores released from the undersides of these fronds.

Ferns: Division Pteridophyta

Ferns include not only the more familiar “true ferns,” after which the division is named, but also much less common plants with the picturesque, common names “horsetails” or “scouring rushes,” “whisk ferns,” “adders’-tongues,” and “moonworts.” The division name (Pterido = feather; phyta = plant) refers to their distinguishing feature: large leaves with branching veins called megaphylls, in contrast to the clubmosses’ microphylls. In the true ferns, these leaves are better known as fronds. Ferns today grow in moist, shady forests, bogs and swamps, rocky crevices, and the tropics, where some are trees and many are epiphytes. As their leaves suggest, ferns are much more closely related to vascular plants than clubmosses are. The division includes one class of extinct trees and four extant classes. We will brieﬂy explore the latter classes below:

Whisk Ferns, Adders’-tongues, and Moonworts (Class Psilotopsida)

The class Psilotopsida (“psilo” = bare) includes a few colorfully named families ( Figure 1.44). The class’ name is taken from the leafless whisk ferns, originally classified with clubmosses but later found to be secondarily “bare.” Adders’-tongues resemble their name, producing single, fleshy leaves from which sporangia-bearing “tongues”

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emerge. Rounded, half-moon–shaped leaves give moonworts their name; an alternative name, based on the stalk of clustered sporangia, is “grapefern.” Many of these species have underground gametophytes which depend on mycorrhizal fungi for their nutrition.

FIGURE 1.44 “Bare ferns” include moonworts or grape- ferns (A), whisk ferns (B), and adders’- tongues (C).

Horsetails and Scouring Rushes (Class Equisetopsida)

The 15 modern species of Equisetum (“horse bristle”) represent a very small fraction of the past diversity of horsetails, which included the Carboniferous tree Calamites, mentioned in the Vascular Plants: Evolution (Advanced) concept. Hollow, photosynthetic stems with whorls of microphylls at each node make up the body of the horsetail, and sporangia are clustered in “clubs” or “cones” very similar to those of clubmosses. Spores have moisture-sensitive elaters, which change shape to propel the spores through the air for dispersal. Fossil species such as Calamites had woody stems which enabled them to grow to heights of 10-30 meters, but only a few modern species exceed 1.5 meters in height. Because some rush-like species contain abrasive silica, they are used to scrub pots –thus their name, scouring rushes.

The 15 extant species of horsetails represent only a tiny fraction of the diversity of their ancestors, which included the giant Carboniferous tree, Calamites (see the Vascular Plants: Evolution (Advanced) concept). Horsetails have hollow, jointed stems with whorls of scaly leaves at each node (A, B). Club-like clusters of sporangia (C,D) form spores with moisture-sensitive elaters (E) which propel the spores into the air.

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Giant Ferns (Class Marattiopsida)

An ancient evolutionary offshoot of the true ferns are the giant ferns, with fronds up to 9 meters in length. Today, only one family survives in this class, but paleobotanists have identiﬁed many families of fossils. The king fern of New Zealand has ﬂeshy rhizomes used by native Maoris for food, earning it a second name: potato fern.

Giant ferns have descended from an ancient offshoot of the true ferns. Some species of Angiopteris have fronds up to 9 meters in length! Many have ﬂeshy rhizomes that humans use for food.

True Ferns (Class Pteridopsida or Polypodiopsida)

The largest and most diverse group of extant ferns are the Polypodiopsida. Polypodiopsida refers to the genus Polypodium, named for the “many feet” of its rhizome and roots ( Figure 1.45). Like Polypodium, most members of the class have rhizomes, ﬁbrous roots, fronds which emerge as ﬁddleheads, sporangia beneath the fronds, and an inconspicuous, independent, heart-shaped, gametophyte: the prothallus. A brief description of some of the major orders will illustrate their diversity:

• Flowering ferns do not have flowers, but their spores develop on separate stalks and are often colorful or showy. • Filmy ferns have fronds only a single cell thick, with no stomata. They thrive only in very wet habitats, such as waterfalls or springs in tropical or temperate rainforests. • Climbing ferns have fronds 3 to 12 meters long which twine around supports in tropical habitats. • Mosquito ferns are aquatic and resemble seed plants in that the gametophytes are male or female and develop entirely within the spores which produce them. Also called duckweed fern and fairy moss, they float on the surface of the water, forming dense mats. Symbiotic relationships with nitrogen-fixing bluegreen algae allow them to grow rapidly. In eutrophic habitats, they can form unwanted blooms. They have long been used to “bio-fertilize” rice paddies in Southeast Asia, where they double as weed-suppressors. Currently, mosquito ferns are involved in the sustainable production of food for livestock and poultry. Some paleoclimatologists suggest that a massive bloom of mosquito ferns during the Eocene absorbed enough carbon dioxide to transform the earth from a “greenhouse” to an “icehouse.” • Up to 20 meters in height, tree ferns populate tropical and subtropical areas –the only surviving group of seedless plants to retain this size and growth form. In some areas, tree ferns are used as building material.

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• More than 80% of today’s fern species are classiﬁed as polypody ferns; the current class name derives from this group name. According to recent genetic analyses, these are the most evolutionarily advanced ferns, and they may have arisen after the appearance of the angiosperms. One of the many species is the bracken fern, whose ﬁddleheads are eaten as a vegetable in Japan; known carcinogens in this fern are blamed for high rates of stomach cancer among the Japanese. Brake ferns are used to absorb arsenic from polluted soils. The polypody family itself contains 30 of the 50 genera and more than 1000 species of small-to-medium-sized ferns. Nearly all members of this family are epiphytes.

FIGURE 1.45 True Ferns form the largest class within the division Pteridophyta. Two names are used: Pteridopsida (“feather plants”) refers to the deeply divided fronds, and the preferred Polypodiopsida refers to the genus Polypodium, named for the “many feet” of its rhizome and roots. Like Polypodium, most members of the class have rhizomes, ﬁbrous roots, sporangia beneath the fronds, and an inconspicuous, independent, heart-shaped, gametophyte: the prothallus. Fronds unfurl as ﬁddleheads (above) which are sometimes cooked as a vegetable.

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True ferns form the largest class in the division Pteridophyta. Illustrating their diversity above are (clockwise from top left): a ﬂowering fern, maidenhair fern (a polypody), a climbing fern, a ﬁlmy fern, a tree fern, and mosquito fern.

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Vocabulary

• fronds: The leaves of ferns; they are large, divided leaves.

• gametophyte: A haploid structure which produces gametes by mitosis. This is the gamete-producing phase in the alternation of generations life cycle.

• megaphylls: Larger, true leaves with branching veins, characteristic of ferns and seed plants.

• paleobotanist: A scientist who studies plant fossils.

• prothallus: The (usually greatly reduced) gametophyte stage of ferns, club mosses, and horsetails.

• rhizome: A modiﬁed subterranean stem of a plant that often sends out roots and shoots from its nodes.

• sporangia (singular, sporangium): Asexual reproductive organs which produce spores.

• vascular plant: A plant with tissues for conducting water and minerals throughout the plant.

Summary

• Ferns brought about the ﬁrst true leaves (megaphylls); today they inhabit moist, shady habitats. • Archaeopteris had fernlike fronds and spores but gymnosperm-like wood and formed the ﬁrst forests. • Horsetail ancestors grew tall in Carboniferous swamps, but only a single genus, Equisetum, survives today. • Clubmosses, ferns, and horsetails require moisture for gametophyte rhizoids and motile sperm; they dominated Carboniferous swamp forests but were replaced by gymnosperms as the climate cooled and dried. • Living relatives of early vascular plants are the clubmosses, which have microphylls, and ferns, many of which have macrophylls. • Clubmosses include the “wolf’s foot” clubmosses, spikemosses, and the semi-aquatic quillworts. • Ferns include whisk ferns, horsetails, giant ferns, and true ferns.

Practice

Use this resource to answer the questions that follow.

• A Brief Introduction to Ferns at http://amerfernsoc.org/lernfrnl.html .

1. How long have ferns been around? When were they the predominant plant? 2. Describe the structure of fronds. 3. What tissues are in the rhizome? 4. What happens to most fern spores? Why?

Practice Answers

1. Ferns have been with us for more than 300 million years. The ferns were at their height during the Carbonif- erous Period (the age of ferns). 2. The frond is divided into two main parts: the stipe (leaf stalk or petiole) and the blade (the leafy expanded portion of the frond).

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3. The rhizome contains the conducting tissues (xylem and phloem) and the strengthening tissues (sclerenchyma fibers). 4. Ferns drop millions, often billions of spores during their lifetime, but very few ever land in a spot suitable for growth. Spores from the parent fall to the ground and, with an enormous amount of luck, they will find suitable moisture and light to begin growth. Since most do not find an area suitable for growth, they perish.

Review

1. Describe mosquito ferns and the unusual relationship they forge with bluegreen algae. How does this help the fern? In what ways have humans beneﬁted from this relationship? In what ways has it harmed us?

Review Answers

1. The mosquito ferns provide a base for the algae to grow on, while the algae helps ﬁx nitrogen for the ferns. This provide valuable nutrients for the fern. Humans use the ferns to suppress weeds and feed livestock and poultry. However, the ferns can form large blooms that are bad for the environment.

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1.12 Seed Plants - Advanced

• Compare a seed to a shelled egg in terms of structures and functions. • Contrast the gametophytes of seed plants to those of early vascular plants. • Describe the general structure and function of a seed plant male gametophyte. • Clarify the origin, structure, and function of a seed plant ovule. • Analyze the parts of a seed, and of the embryo within the seed. • Discuss the functions of seeds.

What is a seed? A seed has many similar deﬁnitions. But they seem to state that a seed is a small object produced by a plant from which a new plant can grow. Do all plants produce seeds? No. The earliest plants to evolve did not have seeds.

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Seed Plants

Having planted seeds since kindergarten or preschool, most of us probably take for granted the ingenuity and adaptive value of a seed. In the previous concepts, you studied ﬁve divisions of plants that do not produce seeds. Perhaps now you are a bit more prepared to appreciate the dramatic innovation that appeared in the middle of the Devonian and today allows seed plants to dominate the Earth. How are seeds different from the spores which preceded them (spores are still used by liverworts, hornworts, mosses, club mosses, and ferns)? You may have planted seeds, but have you ever dissected one to see what is inside? A seed is surprisingly similar to an egg ( Figure below), and the two evolved roughly at the same time - as plants and the animals that fed on them were moving to dry land during the Devonian. Terrestrial vertebrates (reptiles and eventually dinosaurs) which could “jumpstart” their developing embryos with food supplies (yolk and white) and protect both with a tough, waterproof covering (the shell) were able to reproduce more successfully in a greater variety of habitats than their amphibious predecessors - most of whom had to return to water to lay their gelatinous eggs.

Plant seeds (right) closely resemble the eggs of reptiles and birds (left). In an egg, the protective covering is the shell; in a seed, the seed coat plays a similar role. In the egg, the white and the yolk store food for the growing animal; in a seed, the endosperm nourishes the developing embryo. The photo at the upper left shows fossilized dinosaur eggs. At upper right are seeds/kernels of corn. In the same way, terrestrial plants which provided developing sporophytes ( embryos) with stored food, and enclosed both in a tough, protective shell (the seed coat) were able to reproduce more successfully in a greater variety of habitats than their moss and fern predecessors, which launched minute spores to produce fragile, moisture- dependent gametophytes. Seeds are similar to vascular tissue in the extent of their importance to modern plants. Once they had evolved, their descendants diversiﬁed to ﬁll terrestrial niches throughout the Earth. Today, seed plants with vascular tissue dominate the Earth. In the Seed Plant concepts, you will explore the evolution, characteristics, and diversity of seed plants.

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Characteristics of Seed Plants

Like shelled eggs, seeds solved the problem of launching offspring into a dry world. However, by themselves, neither seeds nor eggs were a complete solution to the problems of terrestrial reproduction. Before an embryo could be “launched”, it had to be created: a single, haploid cell from one individual had to ﬁnd and join with a single haploid cell from another individual –without drying out. Recall that sexual reproduction provides variety in offspring, which increases the odds of a species’ surviving environmental change. For the aquatic ancestors of both animals and plants, a watery habitat very much simpliﬁed the sperm-to-egg problem. For reptiles, evolution solved the problem with internal fertilization, leading to elaborate rituals of courtship and mating between males and females. Plants, however, remained rooted in place, and had to solve the problem of “mating” on dry land in other ways.

Beneﬁts of Seeds

Evolution’s solution for plants was twofold:

1. First, miniaturize the haploid gametophyte stage of the old “alternation of generations” life cycle, and retain the tiny gametophytes within and dependent on the sporophyte parents. 2. Second, evolve a means of “ﬂying” a sperm cell from one plant to another. Evolution’s solution was to build the ingenious male microgametophyte that you know as pollen ( Figure below), and then harness natural processes such as wind to provide the energy for transport. Pollen develops from haploid spores produced by meiosis in male sporangia; it consists of several cells or nuclei enclosed in cellulose and a thick, complex, highly resistant organic sealant.

“Flying gametophytes,” better known as pollen, carry sperm cells from male to female, allowing seed plants to reproduce sexually on dry land. The electron microphotographs above show pollen from a variety of angiosperms, but the principle is the same for gymnosperms, cycads, and ginkgos. Of course, pollen and reduced, dependent gametophytes are not the whole story. How does the sperm escape its isolation chamber to fertilize the egg? How does the seed develop? The plot thickens.

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The Ovule

As the male gametophyte became pollen, the female gametophyte –together with parental tissue roughly equivalent to the sporangium of ferns and clubmosses –became an ovule (literally, “small egg”) ( Figure below). Within the ovule, meiosis forms a haploid spore that develops into the female gametophyte –entirely within the ovule. The gametophyte eventually produces an egg cell by mitosis. Parental (sporophyte) tissue further surrounds the ovule with an integument (skin) –except, cleverly, for a tiny opening at one end, the micropyle. Perhaps you can predict at least one function of this opening; we will discuss both, below.

The female gametophyte in seed plants develops entirely within the ovule, shown above. The nucellus is parental tissue roughly equivalent to the sporangium. A single cell (the megasporocyte above) undergoes meiosis and one of the resulting haploid cells develops into the female gametophyte, eventually producing an egg. Parental tissue surrounds the egg cell and nucellus with an integument - except for the micropyle. After fertilization of the egg, the ovule develops into a seed.

The Seed

After wind (initially) carries the male gametophyte/pollen to the female gametophyte, the pollen germinates, growing a tube (often through that convenient micropyle) into the ovule. The pollen tube allows sperm or sperm nuclei to enter the ovule and fertilize the egg, forming the zygote. The ovule then develops into a seed ( Figure below), with three basic parts:

1. The zygote divides and differentiates, forming the embryo –a young plant (sporophyte) of the next generation. While still within the seed, the embryo develops a primordial leaf (the cotyledon), stem (the hypocotyl = “beneath the cotyledons”), and root (the radicle). 2. Stored food to nourish the seed in its eventual home develops from maternal or gametophyte tissue. 3. The ovule integument becomes the protective seed coat.

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This diagram of an avocado seed shows the three basic parts of the seed: seed coat, stored food (endosperm, in the case of this ﬂowering plant), and embryo. While still protected in the seed and nourished by the food supply, the embryo develops primordial leaves (the cotyledons), stem (the hypocotyl) and root (the radicle). The seed serves multiple functions for seed plant species:

1. The pre-developed embryo, nourished by the seed’s stored food supply, has a considerable head start over a simple spore, which is usually haploid and unicellular. 2. Many seeds have specialized adaptations for dispersal –from wings or hooks to sugary fruits, which induce animals to participate in the process. 3. Dormancy mechanisms, which suspend development for planned periods or conditions, facilitate dispersal and timely germination. 4. As products of sexual reproduction, seeds are vehicles of variability among offspring, providing material for natural selection and evolution.

Summary Table

The following table summarizes the characteristics of seed plants, and compares them to the early vascular plants and bryophytes.

TABLE 1.2:

Seed Plants Early Vascular Plants Bryophytes Support Woody xylem Woody xylem Osmotic pressure in large central vacuoles within cells Transport Xylem, Phloem Xylem, Phloem Diffusion, osmosis Tissues/organs Roots, Stems, Leaves, Roots, Stems, Leaves, Rhizoids, Sporangia, Pollen, Ovules, Sporangia, Archegonia, Archegonia, Antheridia Antheridia, Gametophyte Reduced; entirely depen- Reduced, but often still in- Dominant stage dent on sporophyte dependent of sporophyte Sporophyte Dominant, begins devel- Dominant, begins devel- Reduced, begins develop- opment in seed opment in female gameto- ment on female gametophyte phyte

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TABLE 1.2: (continued)

Seed Plants Early Vascular Plants Bryophytes Male gamete Pollen is carried by wind Sperm must swim to egg Sperm must swim to egg Female gamete Egg cell Egg cell Egg cell Dispersal Seed, vegetative repro- Spores, vegetative repro- Spores duction duction Habitat Dry terrestrial Moist shady terrestrial, Moist, shady terrestrial swamps

Vocabulary

• cotyledon: an embryonic leaf

• embryo: A multicellular diploid eukaryote in its earliest stage of development; the developing individual from implantation through the ﬁrst eight weeks after fertilization (in human development).

• gametophyte: The gamete-producing phase in the alternation of generations life cycle; a haploid structure which produces gametes by mitosis.

• hypocotyl: The stem of a germinating seedling, found below the cotyledons (seed leaves) and above the radicle (root).

• micropyle: A small opening in the surface of an ovule through which the pollen tube penetrates.

• ovule: Structure in seed plants which produces the egg cell and develops into a seed.

• pollen: Plant reproductive structure which protects male sex cells during pollination.

• radicle: The primordial root; part of the embryo within a seed.

• seed: An embryonic plant and food supply stored within a protective seed coat.

• seed coat: A tough covering of a seed that protects the embryo and keeps it from drying out, until conditions are favorable for germination.

• sexual reproduction: Reproduction involving the joining of haploid gametes, producing genetically diverse individuals.

• sporophyte: The spore-producing phase in the alternation of generations life cycle; a diploid structure which produces spores by meiosis.

Summary

• Both the seeds of plants and the eggs of reptiles and birds help them to reproduce on dry land.

1. Seeds and eggs contain developing embryos. 2. Both contain food supplies for the developing embryo.

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3. Both contain protective coverings which prevent drying.

• Gametophytes of seed plants develop within and almost entirely dependent on their sporophyte parents. • The male gametophyte is a pollen grain - several cells or nuclei covered by a tough, sealed wall. • An integument of parental tissue, sporangial tissue, and the female gametophyte form an ovule. • After pollination/fertilization, the ovule develops into a seed. • A seed consists of protective seedcoat, an embryonic plant, and a stored food supply. • Within the seed, the embryo develops primordial leaves (cotyledons), stem, and root (radicle). • Seeds provide offspring with a “headstart,” dispersal, and dormancy. • As products of sexual reproduction, seeds ensure variability among offspring.

Practice

Use this resource to answer the questions that follow.

• Seed Plants at http://www.infoplease.com/dk/science/encyclopedia/seed-plants.html .

1. Distinguish between angiosperms and gymnosperms. 2. What is a seed, and what is inside a seed? 3. What ﬁrst develops from a seed? What causes this to occur? 4. What is the role of a fruit?

• http://www.hippocampus.org/Biology Biology for AP* Search: Cones, Flowers, and Seeds ! !

1. Deﬁne the ovule of a plant. 2. How are pollen grains carried to the ovule? 3. Describe the relationship between the ovule and a seed.

Practice Answers

1. Angiosperms are the flowering plants. Their seeds develop inside a female reproductive part of the flower, called the ovary, which usually ripens into a protective fruit. Gymnosperms do not have flowers or ovaries. Their seeds mature inside cones. 2. A seed is the first stage in the life cycle of a plant. Protected inside the tough seed coat is the baby plant, called an embryo. 3. During germination, the seed absorbs water, the embryo starts to use its food store, and a young root, or radicle, begins to grow downward. 4. The role of fruit is to protect the seeds and help disperse them.

Review

1. Describe the parts of a seed, and of the embryo within the seed. 2. Discuss the functions of a seed, and relate the functions to seed structure. 3. What additional changes (beyond the seed itself) did seed plants make over earlier vascular plants? Evaluate these in terms of their importance to the evolution of seeds.

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Review Answers

1. The three major parts of a seed are the seed coat, stored food, and embryo. The three parts of the embryo are the primordial leaf (cotyledon), stem (hypocotyl), and root (radicle). 2. The seed provides food for the pre-developed embryo; helps with dispersion through adaptations such as wings or hooks; provides a mechanism for dormancy, and provides genetic variability. 3. They also have reduced gametophytes and pollen, which are involved in the fertilization process of the seed.

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1.13 Seed Plants Evolution - Advanced

• Explain why biologists consider Ginkgo biloba to be a “living fossil.” • Compare sexual reproduction in cycads and ginkgos to sexual reproduction in early vascular plants. • Compare fertilization in cycads and ginkgos to fertilization in conifers.

Are these seeds? Yes. Many dandelion species produce seeds asexually, where the seeds are produced without pollination, resulting in offspring that are genetically identical to the parent plant. The dandelion seeds are easily seen in here. They easily ﬂow through the air to a new destination. Why would this be beneﬁcial?

How Did Seed Plants Evolve?

An ancient, venerated tree from China may reveal the story of seed plant evolution. Biologists consider Ginkgo biloba a living fossil. What does this mean? What does it tell us about ginkgos, and about the evolution of seed

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Ginkgo biloba holds clues to the evolutionary history of seed plants. Biologists consider Ginkgo a “living fossil” because it alone closely resembles fossils of a group of plants which were widespread and diverse in the past; it is a long-term “survivor”. Clockwise from large photo, upper left: Ginkgo tree; fossil leaves from the Jurassic period; modern leaves, seeds and “fruit” (not a true fruit!); male tree and pollen; female tree and seeds.

A living fossil is a species which exists as both a fossil and as an isolated living species –that is, a species with no close living relatives. Ginkgo biloba appears to be the sole survivor of a large and widespread group of plants, which are common in the fossil record beginning 280 million years ago (with the dinosaurs) but which disappear after the Pliocene. Individual trees today are survivors, as well - deep-rooted and resistant to wind, snow, insects, and disease, some have lived more than 2500 years. We are fortunate that ginkgos survive, because they preserve a means of reproducing which is intermediate between early vascular plants and seed plants, showing a pathway evolution might have taken millions of years ago. Ginkgos are dioecious (di = two; eco = house); that is, an individual tree is either male or female. Let’s look at what happens in each tree separately. Male trees produce “cones” or “clubs” of modified leaves, much like early vascular plants. Each modified leaf bears two sporangia, and inside the sporangia, meiosis produces spores. However, instead of dispersing as in bryophytes and early vascular plants, the spores develop by mitosis into pollen. A pollen grain may look very similar to a spore, but it is actually a superbly packaged “flying gametophyte”, to be carried by the wind to the female tree. Female trees produce two ovules –each a large sporangium surrounded by an integument - at the end of each stalk. Each ovule produces a sticky droplet at its tip (the micropyle) to catch the pollen. When pollen is trapped, the droplet retracts, pulling in the pollen. This event triggers meiosis within the sporangium, and one spore begins to

88 www.ck12.org Chapter 1. Plant Biology - Advanced develop into the female gametophyte - still within the ovule. Eventually, the gametophyte produces an archegonium within an unusual internal "sea", an egg cell, and an extensive stored food supply. However, it is an extremely slow process –as if evolution took its time to work out this “dry-land” form of sexual reproduction. Note that we haven’t yet mentioned fertilization. How do the cells find each other inside the ovule? While the female gametophyte develops, the pollen is changing, as well. Germinating, the pollen grows feeding tubes, which branch throughout the ovule. Eventually, within the tubes, two large sperm - each with a “belt” of thousands of flagella –develop. The tube opens, and the flagella propel the sperm through the “internal sea” created by the female into the archegonium –a short, but critical distance. Finally, a zygote forms and begins to develop; the ovule becomes a seed ( Figure below). The entire process of development and fertilization requires between 4 and 5 months after pollination, and the embryo continues to grow and develop for 9 or 10 additional months.

Ginkgos have large seeds which show clearly the three major parts of all seeds: a tough, protective seedcoat, a well-developed embryo, and a generous supply of stored food. In ﬂowering plants, the stored food supply, called endosperm, develops from double fertilization and is often triploid. In Ginkgo and other gymnosperms, the stored food develops from female gametophyte tissue, and so is haploid.

Cycads

Cycad pollination and fertilization ( Figure below) are very similar to pollination and fertilization in ginkgos, down to the artiﬁcial “inside sea” –although cycad sperm are even larger, and have tens of thousands of cilia/ﬂagella each. Both cycads and ginkgos retain the ancient “sperm swims to egg” script of sexual reproduction; but in the absence of water, they construct a watery, internal medium to ensure that the ancient drama can take place far from the moist shorelines where it evolved.

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Like ginkgos, cycads are dioecious. Males produce “cones” of modified leaves with sporangia which produce pollen. In female “cones”, the modified leaves are even more apparent; recall the modified leaves which formed the “clubs” of clubmosses. In cycads, sporangia tucked in the axils of these leaves develop into ovules, and later seeds. Both cycads and ginkgos retain motile sperm –released by the pollen into an “inside sea” prepared by the ovule so that the sperm can swim to the egg.

Conifers

In conifers ( Figure below), sperm have no motility. In place of sperm motility, a pollen tube grows into the ovule, and like a pipe full of liquid, delivers two non-ciliated sperm directly to the egg. For conifers, this is still a slow process; pines need about three years from pollination to fully develop the seeds.

Like cycads and ginkgos, conifers produce pollen in male cones (left) which is carried by wind to ovules on female cones (right). However, unlike cycads and ginkgo, conifer pollen grows a pollen tube which delivers non-motile sperm directly to the egg cell.

Vocabulary

• dioecious: Having individuals of separate sexes for gamete production.

• fertilization: The joining of gametes during reproduction.

• living fossil: A living species (or clade) of organism which appears to be the same as a species only known from fossils; has no close living relatives.

• micropyle: A small opening in the surface of an ovule through which the pollen tube penetrates.

• ovule: Structure in seed plants which produces the egg cell and develops into a seed.

• pollination: Fertilization in plants; process in which pollen is transferred to female gametes in an ovary.

• sexual reproduction: Reproduction involving the joining of haploid gametes, producing genetically diverse individuals.

Summary

• As the sole survivor of a large group of extinct plants, Ginkgo biloba is a “living fossil.” • Sexual reproduction in Ginkgo and cycads is intermediate between early vascular plants and conifers, involving motile sperm and an “inside sea” within the ovule. • In conifers, a pollen tube transports non-motile sperm directly to the egg cell.

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Practice

Use these resources to answer the questions that follow.

• Evolution of Seed Plants at https://www.boundless.com/biology/seed-plants/evolution-of-seed-plants/intro duction/ . • How did plants come to have seeds? at http://earthsky.org/earth/how-did-the-ﬁrst-plant-seeds-evolve .

1. What is the evolutionary advantage of seeds? 2. How do seeds differ from a fern spore? 3. Why don’t seeds need a constant water supply? 4. What was the role of progymnosperms in seed evolution?

Practice Answers

1. Seed plants have broken free from the need to rely on water for their reproductive needs. They are an evolutionary advantage to dry habitats. 2. Seeds contain a diploid embryo that will germinate into a sporophyte. Seeds also have storage tissue to sustain growth and a protective coat. 3. Seeds have several layers of hardened tissue that prevents desiccation, freeing reproduction from the need for a constant supply of water. 4. Prior to seed evolution, progymnosperms started manufacturing two sets of specialized spores: male spores, and female spores –the living tissues inside these spores produced eggs and sperm. Over time, some progymnosperms evolved into seed ferns.

Review

1. How is the "living fossil" status of Ginkgo related to its classiﬁcation? 2. Describe sexual reproduction in Cycads and Ginkgo. 3. Compare sexual reproduction in Cycads and Ginkgo to sexual reproduction in early vascular plants and conifers.

Review Answers

1. Ginkgo trees have no living close relatives, and have a reproduction method between those of early vascular plants and those of seed plants. 2. In Cycads and Ginkgo, Pollen lands on the micropyle, which pulls it into the ovule. The pollen releases sperm, which swim across an internal "sea" to fertilize the egg cell. 3. Early vascular plants rely on water for fertilization, while the sperm cannot move in conifers.

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1.14 Diversity of Seed Plants - Advanced

• Explain the use of the term “Spermatophyta” in classificiation. • List six divisions of seed plants, differentiating gymnosperms from angiosperms. • Evaluate the importance of seed fern Glossopteris fossils to our understanding of the Earth’s history. • Describe the diversity of cycads. • Relate the classification of Ginkgo to its status as a living fossil. • Discuss the diversity of conifers. • Summarize the potential evolutionary significance of the gnetae.

Could you eat this? This is a baobab fruit split open to show the seeds. The seeds are dark and encased in cubes of dry, white pulp, which can be dissolved in water with sugar or warm milk to make a drink. The Baobab Tree is known as the "tree of life." It can provide shelter, clothing, food, and water for the animal and human inhabitants of the African savannah regions. The cork-like bark and huge stem of the tree are ﬁre resistant and are used for making cloth and rope. The leaves are used as condiments and medicines. The fruit is edible, and full of Vitamin C. The fruit has a velvety shell and is about the size of a coconut. It has a somewhat acidic ﬂavor, described as "somewhere between a grapefruit, pear, and vanilla." The tree can store hundreds of litres of water, which is an adaptation to the harsh drought conditions of its environment. Mature trees are usually hollow, providing living space for many animals.

Diversity of Seed Plants

Six divisions of the plant kingdom form the superdivision Spermatophyta –formally in some classiﬁcation systems, and informally in others. Don’t let the “sperm” in Spermatophyta confuse you; male gametes were named sperma- tozoa, meaning “seed animals” from the Greek word for “seed”. Spermatophytes, therefore, are seed plants. The six divisions which reproduce using seeds and pollen are:

1. Seed ferns (known only from fossils)

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2. Cycads 3. Ginkgos 4. Conifers 5. Gnetae or gnetophytes (there is no common name for them) 6. Flowering plants

You may wonder how the terms gymnosperm and angiosperm ﬁt into this taxonomy. These names refer to the seeds (once again, sperm = “seed”). Angio- means "vessel," and in plants, refers to the ovary, which encloses the seeds in angiosperms. Gymnos means “naked”, so gymnosperms lack a vessel or ovary; their seeds are naked, or uncovered ( Figure below).

In angiosperms, the ovule and seed (blue, green, and yellow) develop within an ovary (pink and red) which eventually forms a fruit. In gymnosperms, the ovule is born “naked” on a modified leaf or scale. Recent evidence continues to support the angiosperm group as evolutionarily unified; angiosperms are the flowering plants, now named Magnoliophyta after the classic member. Because these most recent seed plants dominate our world today, they will be the focus of their own concepts (see the Flowering Plants concepts). Further study of gymnosperms has revealed that they are not a unified group. Cycads, ginkgos, conifers, and gnetae all lack ovaries, so they are all gymnosperms. However, they evolved independently, so we now classify them in separate divisions.

Seed Ferns: Division Pteridospermatophyta

The seed fern division includes fossils only of plants which lived as long ago as the Devonian. The most famous seed fern is Glossopteris, whose multi-continent Permian-to-Triassic fossil distribution ( Figure below) led Eduard Suess to hypothesize the past supercontinent Gondwana, and Alfred Wegener to propose his theory of continental drift. Glossopteris species are softwood, seed-bearing trees or shrubs with tongue-shaped deciduous leaves (glossa = tongue) from 2 to 30 cm (1 to 12 inches) long. Although the seed ferns may have been the earliest seed-bearing plants, current data suggests they are not ancestors of modern seed plants.

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Fossils of Glossopteris (8), an ancient seed fern, are found across the continents of South America (1), Africa (2), India (3), Antarctica (4), and Australia (5), in the pattern shown by the blue-green swath. This distribution is one form of evidence which supports the existence of the past supercontinent Gondwanaland.

Cycads: Division Cycadophyta

Often mistaken for palms or ferns, more than 300 living species of cycads (the Greek “kykas” means palm tree) descend from early seed plants which both preceded and outlasted the time of the dinosaurs. Evergreen with stout trunks, cycads today inhabit many tropical and subtropical habitats. We have already discussed and illustrated their “missing link” reproduction (see the Seed Plants: Evolution (Advanced) concept), in which “cones” of modified leaves bear pollen and ovules, but with sperm still needing to swim through the ovule to the egg. They are gymnosperms, producing seed from “naked” ovules; although the seeds are often brightly colored, they are not fruits. Cycads may have been among the first plants to employ insects as pollinators, and the bright colors of their seeds probably attract birds and monkeys for dispersal. The seeds –and the bats which eat them, considered a delicacy in Guam –are notorious for containing a neurotoxin which causes a disease similar to Parkinson’s and/or Lou Gehrig’s disease (Amyotrophic Lateral Sclerosis, or ALS http://www.alsa.org/als/what.cfm ). Although humans have long soaked seeds to remove the toxin before grinding them for flour, fruit bats concentrate the toxin in their fat; their role as a delicacy may explain the 50- to 100-fold increase in this neural disease among Chamorro residents of Guam. The source of this toxin in cycads reveals another symbiosis: it is produced by Nostoc, a nitrogen-fixing bluegreen bacterium which lives in the roots of cycads, providing them with usable nitrogen compounds. Figure below reviews these relationships.

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“All things are connected”, said Chief Seattle, an 18th century leader of the Suquamish and Duwamish Native American tribes. Perhaps this set of connections will convince you that studying unusual plants and remote cultures provides important and useful information. Bluegreen bacteria (left) ﬁx nitrogen among the roots of cycads –an arrangement which beneﬁts both. However, the bacteria produce a neurotoxin (inset) found in the seeds (center) eaten by fruit bats (right). Fruit bats concentrate the toxin in their fatty tissues, much like peregrine falcons, eagles and osprey concentrated DDT. Chamorro natives of Guam consider fruit bats a delicacy, and their rate of the Parkinson’s/ALS disease is 50 to 100 times worldwide rates.

Ginkgos: Division Ginkgophyta

Ginkgo biloba, the only extant species in its division, was noted earlier for its importance with cycads as an evolutionary intermediate between early vascular plants and seed plants. Long revered in China and other Asian countries with Buddhist and Confucian inﬂuences, some ginkgos planted at temples are up to 1500 years old. Ancient cultivation and naturalization obscure the existance of “wild” individuals. They are now cultivated worldwide, although in the US at least, most cultivars are male clones, because the seeds produced by female trees contain butyric acid, whose odor is disagreeable to many people. Ginkgo has been used for many medicinal purposes, but scientiﬁc data supporting its effect are incomplete.

Ginkgo biloba, the only surviving species in its division, may not be surviving in the wild; centuries or even milennia of cultivation obscure its natural history. Nevertheless, its life cycle, with pollen, motile sperm, and seeds, illuminates an important link in the evolutionary history of seed plants.

Conifers: Division Pinophyta

Over 700 living species make the conifers the largest division of gymnosperms; the division is also the most economically important –for timber and paper. Conifers date back to the Carboniferous, and often dominate the wide variety of habitats in which they are found. All are woody, and most are trees. The tallest (coastal redwood), oldest (bristlecone pine), largest (giant sequoia) and thickest (Montezuma cypress) species of living things today are members of this division. Conifer leaves are often needle- or scale-like, with thick cuticles which permit an evergreen habit. Conifer reproduction often involves the familiar woody female cones, which bear ovules “naked” on woody scales and smaller herbaceous pollen cones. In some families, an aril –a scale which is transformed into a colorful, ﬂeshy berry-like

95 1.14. Diversity of Seed Plants - Advanced www.ck12.org cup, covers each single seed. Conifers depend on wind for pollination, and liquid-ﬁlled pollen tubes carry nonmotile sperm nuclei to the egg cells within ovules. We will brieﬂy survey seven families to illustrate the diversity of conifer species:

Pines

The largest group of conifers, containing as many as 250 species, is the Pine family; mostly in the Northern Hemisphere, they are also probably most familiar to us. Pines are almost all monoecious (“one house”) evergreen trees, with needle-like leaves, small herbaceous pollen cones, and large, woody female cones bearing two seeds on each scale. Pollen and seeds are wind-dispersed, although birds contribute to seed dispersal in some species. Embryos within those seeds have from three to 20 cotyledons! Members of this family include;

• more than 100 species of pines • 50 ﬁrs • 35 spruces • 14 larches or tamaracks • 9 hemlocks • 5 Douglas-ﬁrs • 2-4 Mediterranean/Himalayan cedars

The tamaracks are unusual in that their needles are deciduous, turning gold in autumn before falling ( Figure below).

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The largest and most familiar family of conifers is the Pine family. Among its 250 member species are the pines (A), spruces (dark, B), tamaracks (yellow, B), and ﬁrs (C). All have woody female cones which produce winged seeds (A-1) and herbaceous pollen cones (A-2, with pollen, inset).

Araucaria Family

Araucaria are perhaps best known to North Americans as the fossils of Arizona’s Petriﬁed Forest ( Figure below); these trees, which inhabited the tropical Triassic swamplands, grew up to 60 meters tall. After worldwide distribution of the Araucaria family ended with extinction (together with the dinosaurs), today’s 41 species remain conﬁned to the Southern Hemisphere. By far the hardiest species has been cultivated extensively in the Northern Hemisphere: the poorly named “monkey puzzle” (only slightly more sensibly called “monkey’s despair” in France for its spiky leaf points). Its native habitat is far from any monkeys, above 1000 feet in the Chilean and Argentinian Andes.

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The Petriﬁed Forest in Arizona contains fossils of giant Araucaria trees from the Triassic period (A). Many species of conifers have been cultivated and now grow far outside their ranges. The “monkey puzzle” (B), the hardiest member of the Araucaria family, has been grown for its symmetrical, almost reptilian branches and its large nut-like seeds (E). (C) shows female (seed) cones, and (D) shows pollen cones.

Cypress Family

The cypress family includes over 100 species of hardy shrubs and magniﬁcent trees with needle- or scale- like leaves, vertically shredding bark, and globular cones. Montezuma cypress is one of several “superlatives” within the family: the largest ( Figure below) measures over 11 meters in diameter –the thickest trunk of any living tree. While Montezuma cypress is evergreen, growing along upland rivers and in mountainous regions of Mexico, bald cypress, of humid southeastern US swamps, is a deciduous species of rich riparian woodlands; cypress “knees” and buttresses may help to stabilize the trees in swampy areas. Bald cypress wood is prized for its high resistance to decay. Monterey cypress endures in a still different habit of fog and storms, along the Paciﬁc coast of California. Other notable members of the cypress family are the coastal redwood and giant sequoia, mentioned earlier as superlative species (tallest and largest), cedars (such as arborvitae) and junipers. Many are cultivated for landscaping.

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A number of genera of conifers are known as “cypress” trees. The Montezuma cypress (A) inhabits upland riparian and mountainous habitats of Mexico; the trunk of this individual measures over 11 meters in diameter –the thickest tree known on Earth. Bald cypress (B) and its “knees” (C) inhabit swamps of the Southeastern US. Stiff winds, fog, and cool climate shape Monterey cypress (D) along the Paciﬁc coast.

Yellow-Wood Family

As many as 200 species of evergreen trees and shrubs make up the yellow-wood family. Yet this large family is little known in North America because it, too, is conﬁned to the Southern hemisphere. Over half of the species belong to the genus Podocarpus ( Figure below), whose cones mature into red or purple ﬂeshy, berry-like arils, to enlist birds for seed dispersal.

Umbrella Pines

A single living fossil species, the Japanese umbrella pine ( Figure below), comprises the Umbrella-pine family. Endemic to Japan, this unique conifer is often grown for its evergreen foliage.

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Plum-Yews

About 20 species of mostly Asian, aril-producing trees and shrubs comprise the plum-yew family ( Figure below). Two North American species are the California Torreya and the endangered Florida Torreya.

Several groups of conifers modify cone scales to form arils - colorful, ﬂeshy berry-like cups around each seed; among these are yellow-woods (A), torreyas (C), and yews (D). Japanese umbrella-pines (C) are a single “living fossil” species within their family.

Yews

A dozen species of small trees and shrubs make up the Yew family, but their uses are many: horticulture, landscaping, cancer chemotherapy, and making longbows. Special concern for the possible extinction of certain yew species rests on their value for the chemotherapy drug paclitaxel. Otzi, the “Iceman” mummy from 53 centuries ago, found in 1991 in the Austrian alps, was found with a longbow and ax-handle made of yew wood. Yews are highly toxic except for the arils ( Figure above), sweet ﬂeshy seed coverings which induce seed dispersal by thrushes, waxwings, and other birds, which cannot break down the seed coat to release the poisons inside.

Gnetae: Division Gnetophyta

Although they are probably least well-known among the seed plants, the gnetae are in some ways most similar and perhaps most closely related to the very familiar ﬂowering plants (angiosperms). Unlike the cycads, ginkgos,

100 www.ck12.org Chapter 1. Plant Biology - Advanced and conifers, but like the angiosperms, gnetae produce larger, more efﬁcient vessel elements as well as the simpler tracheids in their woody xylem. Modern plants include three genera of gnetophytes. Gnetum is the genus which gives its name to the division. Most members of this genus are tropical woody vines, but the most familiar species is a medium-sized tree native to Southeast Asia ( Figure below). Evergreen leaves up to 20 cm long are opposite on the branches, and red seeds which develop in “cones” provide food for reptiles, birds, and mammals, including people.

The gnetophytes genus Gnetum includes a number of woody tropical vines and this Southeast Asian tree. The “naked” seeds provide food for reptiles, birds, and mammals. The bizarre Welwitschia mirabilis, the only living species in its genus, grows in the Namib desert of Africa ( Figure below). A long taproot supplies water to two large, strap-like leaves, which grow continuously from a short, thick trunk. Individuals may live hundreds of years, and the two leaves may reach lengths of up to 4 meters, but they often become tattered and split. Welwitschia is the only gymnosperm known to use CAM metabolism –a variation of photosynthesis found in some desert angiosperms (See thePhotosynthesis concepts). Also unique is the production of nectar by both male and female cones (the species is dioecious) and insect pollination.

The long-lived gnetophyte Welwitschia mirabilis is considered one of the most bizarre of all plants, as well as a “living fossil”; unique features include nectar production, insect pollination, and CAM metabolism –uncommon or unknown in other gymnosperms. Like other gnetophytes but unlike other gymnosperms, Welwitschia has vessel elements as well as tracheids in its wood. The genus Ephedra ( Figure below) is known primarily for the stimulant and decongestant effects of ephedrine and pseudoephedrine, chemicals related to amphetamines found in some species. E. sinica has been used for thousands

101 1.14. Diversity of Seed Plants - Advanced www.ck12.org of years in Chinese medicine to treat asthma, allergies, and the common cold. Until evidence showed that ephedrine can cause heart attacks, stroke, and death, ephedrine was used in decongestants and dietary supplements and by athletes as a performance-enhancing drug. After the deaths of Minnesota Viking football player Korey Stringer (2001) and Baltimore Oriole baseball player Steve Bechler (2003) suggested connections to ephedrine, ephedra- containing supplements were banned by the NFL and the US Food and Drug Administration. The International Olympic Committee, the World Anti-doping Agency, and the NBA have also banned the drug. On the positive side, some plants produce compounds that are beneﬁcial to human health. One such instance is found in an Australian rain forest species that produces a substance with anti-carcinogenic (cancer-ﬁghting) properties. See Australian rainforests may provide potential cancer cure at http://www.news-medical.net/news/20100614/Australian -rainforests-may-provide-potential-cancer-cure.aspx .

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A number of species of the gnetophytes Ephedra produce the chemicals ephedrine and pseudoephedrine, which have stimulant and decongestant properties. Because ephedra has been linked to stroke, heart attacks, and death, it is now illegal in the US and in sporting events worldwide.

Flowering Plants (Division Magnoliophyta)

The only group of seed plants that are not gymnosperms is the angiosperm group: plants that surround ovules and seeds with “vessels” or ovaries, which ripen into fruits. Angiosperms are also unique in their production of ﬂowers.

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Fruits, ﬂowers, and the astounding diversity of angiosperms will be discussed in the Flowering Plants concepts.

Vocabulary

• angiosperm: Seed plant in which seeds develop within a vessel, which may later become the fruit.

• aril: A cone scale modiﬁed to form a colorful, ﬂeshy cup around a seed in gymnosperms.

• CAM metabolism: A photosynthetic adaptation to arid conditions in some plants; allows stomata to be closed during the day.

• continental drift: The early 20th century hypothesis that the continents move about relative to each other by appearing to drift across oceans.

• cotyledon: an embryonic leaf

• Gondwana: The southernmost of two supercontinents that later became parts of the Pangaea supercontinent; originally known as Gondwanaland.

• gymnosperm: A type of seed plant that produces bare seeds in cones.

• monoecious: Also called hermaphroditic; individuals capable of producing both eggs and sperm.

• pollinator: The biotic agent that moves pollen from the male anthers of a ﬂower to the female stigma of a ﬂower to accomplish fertilization.

• spermatophyte: A type of plant that reproduces by producing seeds.

• tracheid: An elongated cell in the xylem of vascular plants that serves in the transport of water and mineral salts.

• vessel element: An elongated, water-conducting cell found in xylem; one of the two kinds of tracheary elements.

Summary

• Six divisions of the plant kingdom form the superdivision Spermatophyta (“seed plants”). • Five of these are “gymnosperms” because they bear their seeds “naked” on modiﬁed leaves. • Seed ferns are the oldest group of seed plants, known only from fossils. • Cycads and ginkgos show an intermediate form of seed production, with pollen and motile sperm. • More than 700 species of conifers make up the largest gymnosperm division. • The gnetae may be the gymnosperms most closely related to angiosperms. • Seeds develop within an ovary in the angiosperms, also known as ﬂowering plants.

Practice

Use this resource to answer the questions that follow.

• Seed Bearing Plants at http://plantspages.com/seedbearingplants.htm .

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Practice Answers

Review

1. Differentiate between conifers, gymnosperms, and angiosperms, including their characteristics and classiﬁca- tion. 2. Explain how the fossils of the seed fern, Glossopteris, help us to understand the history of life on Earth. 3. List the divisions of gymnosperms, grouping them according to number of species, economic value, and/or familiarity –as you choose. 4. List the families of conifers, grouping them according to number of species, economic value, and/or familiarity –as you choose.

Review Answers

1. Gymnosperms produce bare seeds in cones, while angiosperms produce seeds which are protected. Conifers are a certain type of gymnosperm, and have needle or scaled-shaped leaves, with woody female cones, and are all gymnosperms. 2. The six divisions are seed ferns, cycads, ginkgos, conifers, gnetophytes, and ﬂowering plants. 3. The families are pines, araucaria, cypress, yellow-wood, umbrella pine, plum yew, and yew.

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1.15 Flowering Plants - Advanced

• Explain the importance of the ﬂower. • Derive the purpose of ﬂowers by examining several examples.

If you were a bird or an insect, would this attract you? Possibly. And that’s the whole point of a ﬂower. To get animals to notice them. This is a ﬂowering Billbergia porteana from the University of Cambridge Botanic Gardens, United Kingdom.

Flowering Plants

Nobody sees a flower - really; it is so small. We haven’t time, and to see takes time - like to have a friend takes time. -Georgia O’Keefe When mud just begins to mix with snow in certain wet woodlands of northeastern North America, a twisted, mottled maroon hood pushes rather rapidly up through the still-frozen ground, reaching a height of 12-15 cm (5-6 inches). Inside the hood is a sphere studded with tiny straw-colored flowers, and the distinct odor of rotten meat. The skunk cabbage ( Figure below) fuels this creation with starch stored over winter in fleshy roots, and also uses this biofuel to heat the interior of the little tent to as much as 20oC (36oF) above the outside air temperature, often melting a little circle of snow around it. Why does the skunk cabbage budget so much ATP for such a strange structure, so early in the year?

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Why do plants spend the energy to produce flowers? Clockwise from upper left: Flowers of Skunk Cabbage, Arizona Queen of the Night, a sister species to Darwin’s Orchid, Common Fig, and Angel’s Trumpet. Far to the southwest in the deserts of Arizona, well after sunset in midsummer, a dried, shrubby, lead-gray cactus opens an exotic array of waxy-white, pointed petals 11 cm (over 4 inches) wide. A multitude of creamy yellow stamens project 2.5 cm above the petals. A 20 cm tube holds sugar-laden nectar and disperses an ethereal vanilla scent. By morning, the Arizona Queen of the Night has closed and withered. High in the Andes of South America, the dramatic Angel’s trumpet hangs up to 50 cm (20 inches) down from its branch, emitting a delicate, lemony fragrance to announce its store of nectar. In Madagascar, a star-shaped white orchid named after Darwin hides nectar at the base of a narrow nectar tube over twice as long as the flower itself. Why do these plants “give away” sugars, advertising them with exotic chemicals? Perhaps most bizarre of all, the common fig hides its tiny white flowers entirely, bearing them on the inside surface of a fleshy green globe. The flowers are never seen, unless the “fruit” is cut open. The globe containing the flowers gradually ripens, filling with sugar and taking on a purple color –becoming a fruit. What purpose does this structure serve?

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Why do plants make ﬂowers?

As you study the structure and evolution of flowers in these concepts, you will learn that flowers coevolved with animals. Flowers contain reproductive organs, and their design entices –or bribes –animals to carry male gametes from one plant to another. Elaborate colors, designs, scents, and nectars attract insects, birds, mammals and even reptiles, which inadvertently pick up some of the strategically placed pollen. As they visit the next flower, some of the pollen brushes off, so that cross-pollination and sexual reproduction can take place. Free nectar to all is not an efficient use of resources, however; pollen can easily be carried to many plants of different species –and wasted. Therefore, many plants “hide” their nectar, protecting it from all but very specific animals who are much more likely to visit the same species of flower again. In the coevolutionary response, animals developed special adaptations allowing them to tap these hidden food supplies –such as the sword-billed hummingbird ( Figure below).

Plants spend energy to build flowers because they entice animals to help carry out plant sexual reproduction. In turn, any animals benefit from plants’ donations of pollen, nectar, and other food supplies. Clockwise from top left: Flies collect pollen from flowers with rotting meat scents. Bats feed on nectar from night-blooming cacti. Fig wasps lay eggs among the internal flowers of the fig, entering through a tiny opening at the top. Hawk moths extend length “tongues” to get nectar from tubular flowers. The sword-billed hummingbird reaches deep into tubular flowers for nectar. So –flowers are the sex organs of angiosperms. They have evolved to work together with animals in mutually beneficial relationships. The warmth and rotten meat smell of skunk cabbage enlists flies to pollinate. Hairy, long- nosed bats with even longer tongues gather nectar –and pollen - from the Arizona Queen of the Night cactus flowers. Figs hide their flowers inside fruits-to-be from all but the fig wasps, who enter through a tiny opening at one end and lay their eggs among the flowers as they pollinate them. The caterpillars develop within the abundant food resources of the flowers, seeds, and “fruit”. Darwin himself predicted the existence of a hawk moth in Madagascar with a “tongue” long enough to feed in the astounding nectary of the orchid named after him –and 50 years after his death, just such a moth was discovered and named after his prediction. Birds, too, are specialized pollinators; the swordbill hummingbird of Ecuador collects nectar from Angel Trumpet flowers, and pollinates them in the process. We humans are not immune to the allure of flowers; we have coevolved with their beauty, utility, and food sources since our beginning. These concepts explore the biology of flowering plants and our relationship to them.

Vocabulary

• angiosperm: Seed plant in which seeds develop within a vessel, which may later become the fruit.

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• cross-pollination: Fertilization in which pollen from one ﬂower pollinates a ﬂower on a different plant.

• ﬂower: Plant reproductive organ often designed to attract pollinators.

• nectar: A sweet, sugary liquid produced by the ﬂowers of many angiosperms to attract animal pollinators.

• petal: A modiﬁed leaf which helps to form the inner whorl of the perianth of a ﬂower.

• pollen: Plant reproductive structure which protects male sex cells during pollination.

• stamen: The male reproductive structure of a ﬂower that consists of a stalk-like ﬁlament and a pollen- producing anther.

Summary

• Skunk cabbage, Darwin’s orchid, and figs produce flowers so unusual that their characteristics “make sense” only when paired with the equally unusual characteristics of their animal pollinators. • Flowers are reproductive organs of flowering plants, which enlist animals to carry out sexual reproduction.

Practice

Use this resource to answer the questions that follow.

• http://www.hippocampus.org/Biology Biology for AP* Search: Cones, Flowers, and Seeds ! !

1. What is a ﬂower? 2. Describe some uses of ﬂowering plants. 3. Distinguish between the stamen and carpel. 4. What are the roles of the stigma and style? 5. What is the endosperm? 6. How do the seed and fruit form?

Practice Answers

Review

1. What is the adaptive value of ﬂowers, which often “cost” a great deal of energy to produce?

Review Answers

1. Flowers encourage animals to visit the plant, which help carry the pollen to another plant in order to reproduce.

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1.16 Characteristics of Angiosperms - Ad- vanced

• Describe the ﬁve basic parts a ﬂower in terms of structure and function. • Compare angiosperm vascular tissue, seed structure, and dispersal to those of gymnosperms.

Characteristics of Angiosperms

In previous Plant concepts, you have studied nine of the ten non-algae divisions of living plants ( Diversity of Living Plants Divisions Table), tracing the evolution of plants as they moved from water to land and then upward, to form the three-dimensional terrestrial world we inhabit today. Yet the last and youngest division, formally known as Magnoliophyta, includes 90% of known extant species. What are the characteristics of these ﬂowering plants ( angiosperms), and how have these features led to their present dominance of the living world? Flowering plants dominate the Earth today. Of nearly 290,000 species of plants, roughly 260,000 are angiosperms.

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TABLE 1.3:

Informal Group Division Name Common Name Number of Living Species Green Algae Chlorophyta green algae 3,800 (chlorophytes) Charophyta green algae (desmids & 4,000 - 6,000 charophytes) Bryophytes Marchantiophyta liverworts 6,000 - 8,000 Anthocerotophyta hornworts 100 - 200 Bryophyta mosses 10,000 Early Vascular Plants Lycopodiophyta club mosses 1,200 Pteridophyta ferns, whisk ferns & 11,000 horsetails Seed Plants Cycadophyta cycads 160 Ginkgophyta ginkgo 1 Pinophyta conifers 630 Gnetophyta gnetophytes 70 Magnoliophyta ﬂowering plants 258,650

Angiosperm Characteristics Related to the Flower

As their most common name suggests, a major innovation of this group is the flower. However, as artist Georgia O’Keefe implied, flowers are complex. They contain a number of innovations which we will attempt to truly “see” by exploring each one individually; afterward, we will combine them into a complete flower.

Pollen

Pollen –the haploid male gametophyte –is reduced to just three cells ( Figure below). This reduction in size and complexity allows much more rapid fertilization and seed development. Recall that in gymnosperms, time from pollination to seed maturity may require up to three years. In ﬂowering plants, the entire process can occur within a single growing season.

In ﬂowering plants, pollen is reduced to three cells, allowing pollination and fertilization to proceed much more quickly than in gymnosperms. Left: lily anthers covered with pollen; right: lily pollen, scanning electron microscope photograph.

Stamens

Stamens ( Figure below), the male reproductive organs which produce the pollen, have just two pairs of pollen sacs ( microsporangia). Filaments are stalks which transport nutrients to the developing pollen and customize height

111 1.16. Characteristics of Angiosperms - Advanced www.ck12.org and arrangement of the anthers, in which meiosis and pollen development take place. Compared to the pollen cones which preceded them, stamens permit a diversity of highly specialized pollination schemes.

In ﬂowering plants, pollen forms in male reproductive organs known as stamens (right). Stamens are built of a stalk or ﬁlament, which supports an anther containing four pollen sacs (left: anther in cross section). Each pollen sac is a sporangium in which meiosis and development of pollen takes place; eventually, these rupture to release the pollen (left).

The Embryo Sac

The embryo sac (female gametophyte –Figure below) within the ovule is reduced to seven cells with eight nuclei, while gymnosperms may have thousands. The egg cell, closest to the micropyle opening, joins with one sperm nucleus to form the zygote; two adjacent cells (the synergies) may help to guide the pollen tube. The polar nuclei fuse with the other sperm nucleus by double fertilization to form the endosperm –a rich food supply for the developing embryo. Like the similar reduction in pollen, this change supports more rapid fertilization and seed development.

In ﬂowering plants, the female gametophyte or embryo sac (right) within the ovule is reduced to seven cells. The egg cell, closest to the micropyle opening, joins with one sperm nucleus to form the zygote. The polar nuclei fuse with the other sperm nucleus to form the endosperm. Antipodal cells have no apparent function.

Carpel

A carpel encloses developing ovules. The carpel is the “angio” or “vessel” from which the name angiosperm is derived. In the simplest form, the female reproductive organ becomes a pistil ( Figure below) with three parts:

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1. a stigma, which lacks epidermis and is often sticky, to catch pollen 2. a style, which supports the stigma and 3. an ovary, which contains the ovule and its egg cell

Like stamens, the pistil permits more specialized pollination schemes and makes possible the avoidance of self- fertilization.

In ﬂowering plants, carpels surround developing ovules. A pistil (left: diagram, right: in a cucumber) may contain one or more carpels, and consists of a stigma for catching pollen, a style to support the stigma, and an ovary in which the ovules (eventually seeds) develop. The ovary may develop into a fruit, which promotes dispersal by animals. A cucumber is actually a fruit.

Perianth

The perianth (“around the flower”) is probably what most people think of first as the most important part of the flower. However, the petals and sepals which make up the perianth are the sterile parts; their only connection to reproduction (albeit an important one) is to lure insects, birds, or bats to ensure that pollen is transferred from one flower to another. The perianth ( Figure below) has two parts:

1. An inner corolla made of petals, often brightly colored to attract pollinators 2. An out calyx made of sepals, often green, which protects developing ﬂower in bud

In some ﬂowers, petals and sepals are identical; botanists refer to these as tepals.

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The colorful corolla of petals and the often-green calyx of sepals make up the perianth. Colors may combine with shape, scent, and nectar to attract pollinators. Sepals protect the ﬂower while in bud.

The Complete Flower

These five separate structures –pollen, stamens, ovule, pistil, and perianth - combine to form what we know as the flower, the organ of sexual reproduction for angiosperms. Figure 1.46 shows their relationships in a complete (or “perfect”) flower. Note that the many stamens form a whorl, the androecium ("man’s house"). The single pistil is of course the “woman’s house” (gynoecium).

FIGURE 1.46 A complete ﬂower includes male reproductive organs (stamens = anther + ﬁla- ment), female reproductive organ (the pistil = stigma + style + ovary), and perianth (sepals and petals).

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Other Characteristics of Flowering Plants

Although flowers and their components are the major innovations of the angiosperms, they are not the only characteristics common to all flowering plants. Vascular tissue, seeds, and seed dispersal –all features of earlier plants –change significantly in angiosperms with the following adaptations:

Vessel Elements

Vessel elements, in addition to tracheids, are present in the xylem of most angiosperms ( Figure below). These cells form vascular tissue which transport water and minerals more efﬁciently; many biologists attribute the success of the angiosperms to vessels formed of vessel elements.

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Angiosperm xylem contains both tracheids and vessel elements; the latter are wider and more efﬁcient in water transport. Gymnosperms have only tracheids.

Endosperm

Endosperm (meaning “within the seed”) provides a highly nutritious food supply for the developing embryo and seedling –or for humans or other animals which eat the seed ( Figure below). Flowering plants form endosperm when one sperm nucleus from pollen unites with two polar nuclei within the embryo sac.

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A drawing of a seed of wheat shows the rich endosperm produced by the fused polar and sperm nuclei.

Fruit

The ovary may ripen into a fruit ( Figure 1.47), allowing a diversity of seed dispersal mechanisms. Mammals and birds eat fruits which are ﬂeshy (for example, cucumbers), sweet (berries), or oily (acorns), and unwittingly disperse the seeds inside them. You have undoubtedly encountered fruits with hooked burs, which catch onto animals’ fur (or socks) for transport. Other fruits develop wings (maple or elm seeds) or parachutes (dandelions), which catch the wind. Certain fruits dry in such a way as to burst open, spreading seeds mechanically.

Vocabulary

• angiosperm: Seed plant in which seeds develop within a vessel, which may later become the fruit.

• anther: The male reproductive structure of a ﬂower; site of meiosis and pollen development.

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FIGURE 1.47 Fruits are ripened ovaries, which are often brightly colored to attract animals to disperse of the seeds. (Clockwise from top left: fruits of dandelion, oak, burdock, tomato, pepper, maple, columbine, and peach). Dandelion and maple fruits are modiﬁed for dispersal by wind. Columbine fruits dry and burst when ripened, spreading fruits mechanically. Burdock has hooked burs, which cling to animal fur. Oak, tomato, pepper, and peach add food supplies which appeal to squirrels and humans, who unwittingly aid in seed dispersal after eating the fruit.

• calyx: The outer (usually green) whorl of the perianth of a ﬂower, comprised of sepals; often functions to protect the developing bud; a cup-like structure that lies just below the radiating arms of sea lilies and feather stars and contains the digestive system.

• carpel: A female reproductive organ in a ﬂower; composed of an ovary, a style, and a stigma.

• corolla: The inner (usually colorful) whorl of the perianth of a ﬂower, comprised of petals; often functions to attract pollinators.

• double fertilization: Angiosperm process in which two sperm nuclei from pollen fertilize two cells in the ovary, resulting in zygote and endopserm.

• embryo sac: Female gametophyte of an angiosperm, consisting of seven cells and eight haploid nuclei contained with the ovule.

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• endosperm: Tissue produced inside the seeds of most ﬂowering plants around the time of fertilization; stored food inside a plant seed.

• ﬁlament: Stalk which supports and supplies nutrients to the anther within a ﬂower.

• ﬂower: Plant reproductive organ often designed to attract pollinators.

• fruit: Plant ovary (female reproductive organ) which may later develop ("ripen") for dispersal.

• microsporangia: A sporangium that produces spores that give rise to male gametophytes.

• ovary: Small, oval-shaped organ that lies on either side of the uterus; the egg-producing organ of the female reproductive system; the part of the pistil which contains the ovules in angiosperms.

• perianth: The outer, sterile parts of the ﬂower: sepals, petals, and/or tepals.

• petal: A modiﬁed leaf which helps to form the inner whorl of the perianth of a ﬂower.

• pistil: The female reproductive organ in ﬂowering plants.

• pollen: Plant reproductive structure which protects male sex cells during pollination.

• sepal: A modiﬁed leaf which helps to form the outer whorl of the perianth of a ﬂower.

• stamen: The male reproductive structure of a ﬂower that consists of a stalk-like ﬁlament and a pollen- producing anther.

• stigma: Part of the female reproductive structures of a ﬂower; top section of a pistil, often "sticky" to catch pollen; a photosensitive structure that orients the movement of the cell towards light; known as an eyespot.

• style: Central, "neck" section of a pistil which supports the stigma.

• tepals: Undifferentiated petals/sepals, having the same color, size and shape.

• vessel element: An elongated, water-conducting cell found in xylem; one of the two kinds of tracheary elements.

Summary

• Flowers, the major innovation of the angiosperms, are organs of sexual reproduction.

1. Angiosperms have reduced pollen to 3 cells, allowing more efﬁcient pollination and fertilization. 2. Stamens produce pollen and allow various pollination schemes. 3. Embryo sacs in the ovules contain just 7 cells and 8 nuclei, allowing faster fertilization. 4. Carpels surround and protect the egg cell, fertilization, and the developing ovule. 5. The perianth surrounds the ﬂower with colorful petals and protective sepals.

• Additional characteristics of angiosperms relate to vascular tissue, seeds, and dispersal.

1. Most angiosperms’ xylem contains vessel elements, which transport water more efﬁciently. 2. Angiosperm seeds contain stored food endosperm, formed from polar and sperm nuclei. 3. Angiosperm ovaries ripen to form fruits, which aid dispersal.

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Practice

Use this resource to answer the questions that follow.

Practice Answers

Review

1. Compare angiosperm vascular tissue, seed structure, and dispersal to those of gymnosperms. 2. Compare the gametophytes of gymnosperms to those of angiosperms.

Review Answers

1. Angiosperm vascular tissue contains vessel elements, which transport water more efﬁciently. The angiosperm seed contains endosperm, which provides food to the developing embryo. Angiosperms also form fruits, which are eaten by animals, helping in their dispersal. 2. Angiosperm gametophytes are much smaller than those of gymnosperms.

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1.17 Evolution of Flowering Plants - Advanced

• Summarize our current understanding of the origin of flowering plants. • Describe the characteristics of the earliest flowers. • Analyze the general trends in flowering plant evolution. • Discuss the co-evolution of flowering plants with insects and other animals. • Explain the ways in which humans have co-evolved with flowering plants.

What were early flowers like? They were probably very simple, with just the basic reproductive structures. These flowers from an avocado tree (Persea americana) shows the characteristics of ancient flowering-plant lineages. Its petals (which are colorful in most flowers) and sepals (which usually have a green outer layer) are combined into one organ.

Evolution of Angiosperms

Charles Darwin considered the relatively rapid evolution of the flowering plants an “abominable mystery”. As you have seen, a flower is complex. It could be considered an organ system, analogous to the organ systems in your body. The four concentric rings of organs ( Figure 1.48) which make up a flower are:

1. Beginning in the center is the “woman’s house” or pistil. 2. Surrounding the pistil is the “man’s house” –a ring of stamens. 3. Outside the stamens is the corolla –a whorl of petals. 4. Outermost is the calyx –a whorl of sepals.

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FIGURE 1.48 The evolution of flowers probably involved the change from a spiral of modified leaves (A) to several whorls of reproductive organs (B). In cross section (C), a flower has four concentric rings of organs: innermost female carpel(s), surrounded by a ring of male stamens (yellow-orange), surrounded by sterile petals (brown) and finally, sepals (green). Botanists think that Amborella (D) and (E) and waterlilies (F) are similar to the earliest flowers. Shared characteristics include: spiraled (rather than whorled) flower parts, variable number of parts; perfect flowers with dominant ovary, with or without undifferentiated tepals.

How did such a complex system of organs arise?

In previous concepts, you have seen the gradual reduction of the gametophyte stage to pollen and ovule. Their further reduction in angiosperms follows that pattern. You have also seen that these gametophytes are born on modified leaves which form cones. Imagine, then, that flowers began as an elongated stem with series of slightly modified leaves –primordial ovaries, stamens, petals, and sepals - spiraling around just as leaves had always done ( Figure 1.48). At first, sepals and petals may have been identical ( tepals), only later differentiating into petals and sepals. Gradually, the space between the nodes which bore leaves (the internodes) shortened ( Figure 1.48), until the organs formed whorls, clustered tightly together –the first true flower. This is how biologists hypothesize that flowers evolved.

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The Earliest Flowering Plant

Archaefructus (“ancient fruit”) has been interpreted as the first flowering plant, with carpels and stamens spread out along the stem. The fossil dates back to the early Cretaceous, 125 million years ago. The earliest flowering plant, Archaefructus ligoningensis, dates to the early Cretaceous, 125 million years ago. The fossil ( Figure above) has been interpreted as a “spread-out” primordial flower, with carpels and stamens (but no petals or sepals) spiraling around an elongate stem. However, chemicals used by plants to defend flowers from insect pests (oleananes) have been found in 250 million-year-old fossils of late Permian Gigantopterids. No fossils of Gigantopterid flowers have been found, but similarities to angiosperm shoots, vessel elements, and leaves suggest to some that ancestors of today’s flowers could have lived much longer ago than the Cretaceous. Gnetophytes also produce vessel elements in their xylem, leading some botanists to suggest that they may be angiosperm ancestors. Although botanists still do not know which gymnosperms gave rise to the flowering plants, most agree that the living angiosperm species which most closely resemble the first flowering plants are Amborella trichopoda and/or the water lilies ( Figure 1.48). Primitive characteristics include:

• Flower parts: variable number, spiraling about stem, separate but in contact with each other • Flowers perfect: both stamens and carpels within a single ﬂower • Perianth: undifferentiated tepals - or none - spiraling around the stem • Flower dominated by the ovary • Fruit: a red berry containing a single seed

Advantages of Flowers

What advantages did flowers confer on the plants which expended the energy to produce them? Biologists believe that even the earliest flowers –perhaps appearing on isolated islands –attracted insects. If those insects, dusted with pollen while foraging for nectar, visited several different plants with the same flowers - dropping off and picking up new pollen grains at each stop, they immensely increased the efficiency of fertilization over the random vagaries of wind. Likewise, fruits produced by ovaries or other parts of the flower attracted birds or mammals with their sugars or burrs. Again, flowering plants enlisted the muscle power of animals –this time to do the work of scouting for new habitats. The co-evolutionary relationship between flowers and insects has produced an astounding diversity of forms –elaborate shapes and patterns and scents and nectaries among flowers, and long, curved beaks, proboscis, tongues, and pollen baskets among animals ( Figure below).

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Coevolution between flowers and animals result corresponding adaptations in each. Nectaries within flowers (A) are deeply hidden. Bees are among the mostly closely co-adapted; pollen baskets on the “bees knees” (C) collect food to carry back to larvae in the hive. B: SEM of butterfly proboscis; D and G: hummingbird moth; E: Gold Dust Day Gecko and Bird-of-paradise flower; F: Hawaiian honeycreeper and Alpine Blue-sow-thistle. As flowers have evolved and diversity has increased, some general trends include a number of changes in flowers which attract specific pollinators:

• Reduction of the number of flower parts • Fusion of flower parts • Precision in design and number of flower parts • Special attractants: scents, heat, nectar glands • Timing mechanisms, both seasonal and diurnal • Specific sexes in a single flower or plant (dioecious or monoecious species)

Polyploidy (multiplying chromosome sets) and gene duplication are common means of evolution among flowering plants: strawberries, for example, are octaploid. And as climates changed, flowering plants continued to coevolve with animals. Specific examples of each of these trends will be discussed in the Flowering Plants: Diversity (Advanced) concept. According to molecular clock analyses, one more powerful selective force began to influence the evolution of flowering plants some 10,000 years ago. First in the Fertile Crescent with wheat, then in the New World with teosinte, and finally about 6,500 years ago wth rice, human agriculture increasingly chooses which characteristics will survive. The result has been dramatic change in amount of endosperm, or size of tubers, or sweetness of fruits

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–as we cultivate our food ( Figure below). Or did the rice, the wheat, the corn, and so many more species choose us –by appealing to our desires - to handle their soil preparation, planting, fertilizing, irrigation, and even pollination? As Michael Pollan reminds us in the The Botany of Desire, the process of cultivation is a two-way street more akin to coevolution than to human dominion over nature. We depend almost entirely on angiosperms for our food –as well as for timber, wood, paper, ﬁbers, medicines, drugs, landscaping, gardening, and the beauty of ﬂowers.

Beginning some 10,000 years ago, humans have been a major force in the evolution of ﬂowering plants –and plants have been a major force in the evolution of humans. Domestic corn (right), for example, has much more endosperm and greatly reduced carpels, compared to wild forms, which are known as teosinte. As we harness fossil fuels to increase industrialization and technology, we have become even more of a force in the evolution –often unwittingly. Our increasing mobility results in the introduction of exotic animals and plants which alter the ecology of habitats, rendering some species invasive and others extinct. As we increase our population and thus our demands on agriculture and land, we are destroying habitats for many species of plants and losing genetic diversity among the survivors. As we ﬁght species we consider to be “weeds” and insect “pests” with chemicals, we destroy pollinators and the species which depend on them.

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Vocabulary

• internode: The section of a plant stem between nodes.

• node: The place on a plant stem where a leaf is attached.

• organ system: A group of organs that act together to carry out complex interrelated functions, with each organ focusing on a subset of the overall task.

• perfect: Refers to a ﬂower which contains both male and female reproductive organs; hermaphroditic.

• petal: A modiﬁed leaf which helps to form the inner whorl of the perianth of a ﬂower.

• pistil: The female reproductive organ in ﬂowering plants.

• polyploidy: The duplication of chromosome sets, often resulting in “instant speciation.”

• sepal: A modiﬁed leaf which helps to form the outer whorl of the perianth of a ﬂower.

• stamen: The male reproductive structure of a ﬂower that consists of a stalk-like ﬁlament and a pollen- producing anther.

• tepals: Undifferentiated petals/sepals, having the same color, size and shape.

Summary

• Flowering plants further reduced the gametophyte stage - to 3-celled pollen and 8-nucleated ovule. • Flowers probably began as a series of modified leaves spiraled around the stem. • The oldest fossil is 125 million years old, but other evidence suggests flowers have lived 250 million years. • The identity of the gymnosperm ancestor to angiosperms is unknown. • Among today’s flowers, Amborella and water lilies most closely resemble the first flowers. • As flowers diversified, general trends included:

1. co-evolution with pollinators, especially insects 2. reduced number of flower parts; fusion to create specific designs 3. attractants: scents, heat, nectar glands 4. timing mechanisms, both seasonal and diurnal, and 5. specific sexes in a single flower or plant

• Polyploidy and gene duplication have been important forces in ﬂowering plant evolution. • Humans, too, have co-evolved with ﬂowering plants –as through agriculture and habitat destruction.

Practice

Use this resource to answer the questions that follow.

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Practice Answers

Review

1. Summarize our current understanding of the origin of flowering plants, including a description of the first flowers.

Review Answers

1. The first fossil of a flowering plant dates from 125 million years ago. The flower was spread out with carpels and stamens, but no petals or sepals. Flowers had the evolutionary advantage of attracting animals, which helped fertilize the plants by carrying pollen to other plants.

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1.18 Diversity of Flowering Plants - Advanced

• Interpret the number of different systems of classiﬁcation used for ﬂowering plants. • Compare and contrast the characteristics of the three major groups of angiosperms.

Angiosperm Diversity

The previous concept (Flowering Plants: Evolution (Advanced)) discussed that the evolutionary origin of flowering plants is an active area of research and discovery. Because taxonomy seeks to reflect evolutionary origin, the classification of angiosperms is also evolving. Botanists identify eight distinct groups of flowering plants, but because the vast majority of species belong to just three of these groups, we will focus our discussion of diversity on the major families in these. Eudicots house 75% of known species; monocots, 23%, and magnoliids, 2%. Until recently, angiosperms were divided into two classes: monocots (“one cotyledon”) and dicots (“two cotyledons”) based on the number of embryonic leaves in the seed. Monocots remain as an evolutionary unit of ancestor and descendants, but modern data has led to the removal of several small groups from the dicots. The remaining species are now called eudicots –(“true” dicots) because they form a coherent evolutionary unit according to current data. Of the former dicots no longer considered dicots, one group –the magnoliids –includes a majority of the distantly related species. The three groups differ not only in number of cotyledons, but also in structure of flower parts, pollen, stems, roots, and leaves. An important difference is the arrangement of vascular tissue in stems; whereas monocots have vascular bundles scattered, dicots arrange them in a ring. In dicots, cambium meristem separates the xylem and phloem, allowing secondary growth in width necessary for large, trunk-supported trees. Monocots seldom sustain such woody growth in diameter, so remain herbaceous. A comparison of the 3 Major Groups of Angiosperms Table summarizes these differences. The table compares their seeds, pollen, flowers, stems, roots, and leaves –characteristics which unite them in evolutionary history, and therefore also in classification

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TABLE 1.4:

Monocots Eudicots Magnoliids Number of Cotyledons 1 2 2

Flower Parts Multiples of 3 Multiples of 4 or 5 3

Pollen: 1 3 1

Number of furrows or pores

Stems: Scattered Concentric Ring Scattered

Arrangement of (at least in part) vascular bundles

Roots Adventitious (Fibrous) Radicle (Taproot) Taproot

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TABLE 1.4: (continued)

Monocots Eudicots Magnoliids Leaves: Venation Parallel Branching Branching

Major Families

(or examples of mag- Orchids (21,950 Daisies (23,600 Magnolia noliids) species) species) Nutmeg Grasses (10,035 Peas (19,400 species) species) Tulip tree

Sedges (4,350 Madders (13,183 Cinnamon species) species) Avocado Arums (4,025 Mints (7,173 species) species) Black pepper

Spurges (5,735 species)

Mallows (4,225 species)

Total Species 70,000 175,000 9,000

Although this introductory lesson cannot begin to do justice to the 260,000 known species of angiosperms, a survey of the “top ten” families - 4 monocot and 6 dicot groups - should give you some insight into the great diversity of ﬂowering plants living today.

Vocabulary

• dicot: A ﬂowering plant with two embryonic seed leaves or cotyledons that usually appear at germination; dicotyledon.

• eudicots: Vascular plants which produce seeds that develop from ﬂowers and are enclosed in fruits.

• monocot: A ﬂowering plant with one embryonic seed leaf or cotyledon that usually appears at germination; monocotyledon.

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Summary

• Because taxonomy seeks to reflect evolutionary origin, the classification of angiosperms is also evolving. • Eudicots make up 75% of known species of angiosperms; monocots, 23%, and magnoliids, 2%. • Eudicots have seeds with two cotyledons, leaves with netted veins, and flower parts in multiples of 4 or 5. • Monocots have seeds with one cotyledon, leaves with parallel veins, and flower parts in multiples of 3.

Practice

Use this resource to answer the questions that follow.

Practice Answers

Review

1. Describe the general trends in diversification of flowers. 2. Compare the coevolution of flowers and insects or other animals to coevolution of humans with flowering plants. 3. Why are there so many different systems for classifying plants? Why are they so often disputed or changing?

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1.19 Eudicots - Advanced

• Identify the major families in each major group of angiosperms.

Eudicotyledons (Eudicots)

Aster, Daisy, or Sunﬂower Family

The most abundant eudicots are the mostly herbaceous Composites, members of the Aster family. They differ from other families primarily in their pseudanths (meaning “false flowers”) ( Figure below) which are not so much false as vast communities of “florets” in a single “head.” Each floret has its own ovary, and produces its own fruit. You have probably enjoyed these fruits as sunflower “seeds”, removing the tough carpel “shell” before you eat the seed. Familiar members of this family are dandelions, chrysanthemums, zinnia, lettuce, artichokes, goldenrod, and ragweed, as well as the asters, daisies, and sunflowers which give the family its several common names. You will recognize among these species which are considered “weeds”, species which are cultivated for gardens and for crops, and species which cause “hay fever”. Botanists estimate that the Aster family has been evolving for the past 50,000 years.

Asters, daisies, and sunflowers form the largest family of Eudicots. “Heads” of many tiny individual flowers (“florets”) and a border of additional florets called “ray flowers” combine to form pseudanths, or false flowers.

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Each floret produces a separate “fruit” containing a single seed; you know the fruits of the sunflower (right) as sunflower “seeds,” but you bite off the tough carpel shell before you eat them.

Pea or Legume Family

Peas, beans, acacias, mimosas, clovers, and peanuts are well-known members of the legumes, which range from annual herbs to trees ( Figure below). Many legumes form root nodules with symbiotic nitrogen-fixing bacteria; this explains why the clover in your yard quickly overtakes grass. The flowers of legumes show fusion of parts: sepals, petals, and stamens partially fuse to form a three-dimensional flower with a lip or cup; often many flowers combine to form a head, as in clover.

Pea family flowers have partially fused sepals, petals, and stamens (upper left) designed to attract specific insect pollinators. In some legumes, many flowers combine to form a rounded head. Members include (clockwise, from top right) peanuts, clover, beans, and Acacia trees.

Madder Family

Ten species of coffee, noni fruits, and gardenias ( Figure 1.49) are among the famous members of the madder family of dicots. Noni fruits are used medicinally, although their benefits have not yet been scientifically verified.

Mint Family

A variety of highly aromatic herbs, shrubs, and vines make up the mint family. Flowers are bilaterally symmetrical (mirror-image halves), with petals fused into upper and lower lips ( Figure 1.50). Leaves are often opposite, and stems are often square. Species include basil, peppermint, spearmint, rosemary, sage, savory, marjoram, oregano, thyme, and lavender.

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FIGURE 1.49 The madder family includes gardenias (upper left), noni fruit (upper right) and coffee (bottom: ﬂower, cherries, and roasted seeds).

FIGURE 1.50 Lavender (left), mint (center), and rosemary (right) are among the many aromatic herbs which make up the mint family. Flowers have petals fused to form two lips, although lobes remain to echo ﬁve ancestral petals; mints are dicots.

Spurge Family

Spurges are herbs, shrubs, or trees with a milky latex sap. The sap of one spurge tree has high economic importance as the primary source of rubber ( Figure 1.51). Many species are succulents, with ﬂeshy water-storing stems and/or leaves. Flowers are greatly reduced and combined into pseudanths (false ﬂowers, as in the daisies) –often a single pistil surrounded by a ring of stamens and then a circle of nectar glands. The familiar holiday poinsettia, manioc or cassava root, and the serious agricultural pest, leafy spurge, are other economically important species within the family.

Mallow Family

The mallow family includes a variety of familiar cultivars ( Figure 1.52). Linden (or basswood) trees are planted along boulevards for shade. Hibiscus is grown for its elaborate ﬂowers, and cotton, kenaf, and okra are useful

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FIGURE 1.51 Spurge flowers (A) are pseudanths (false flowers) made of several reduced flowers, often without a perianth; note the single large pistil surrounded by stamens and nectar glands in this species. The spurge family has milky latex sap –the source of rubber from one of its most famous members, the rubber tree (B). Other famous members are leafy spurge (C), a serious pest in North America; manioc or cassava (D), an important root crop in many tropical countries, and poinsettia (E).

crop plants. Mallow ﬂowers are radially symmetrical, often with nectaries, which secrete sweet liquids to entice pollinators.

Vocabulary

• eudicots: Vascular plants which produce seeds that develop from ﬂowers and are enclosed in fruits.

Summary

• The largest families of ﬂowering plants are the daisies and the orchids.

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FIGURE 1.52 Radially symmetrical ﬂowers characterize the mallow family. Famous members include the Linden Tree (A and B), planted along boulevards as a shade tree, kenaf (C), used for making paper; hibiscus (D), grown for its dramatic blossoms, and cotton (E).

Practice

Use this resource to answer the questions that follow.

Review

1. Compare and contrast the major characteristics of the three major families of angiosperms.

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1.20 Monocots - Advanced

• Name the two largest families of ﬂowering plants. • Discuss the angiosperm family which is most important economically.

Monocotyledons (Monocots)

Orchid family

You may be surprised to learn that orchids are one of the two largest families of flowering plants. Because of classification disputes, some would argue they are the largest family. With or without disputes, there are four species of orchids living today for every one mammal or bird. If human-induced hybrids are included, the 25,000 native species increase five-fold. Orchids are indeed exotic in appearance. The flowers have bilateral (two-sided) symmetry, with a prominent lip and fused stamens and pistil. Like all monocots, they have flower parts in threes, and parallel-veined leaves. Pollen is packaged into a pollinium, and the highly specialized design of each flower attracts a highly specialized pollinator. This specialization is probably a primary reason for the number of species. Most orchids are tropical epiphytes (growing on trees or other plants), but temperate orchids are usually terrestrial, and some absorb nutrients from symbiotic mycorrhizal fungi ( Figure below). The beans of vanilla orchids provide the popular flavoring. Orchids may have coexisted with dinosaurs; the oldest fossil is preserved with a pollinating bee in amber dating from the late Cretaceous, 80 million years ago.

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The orchid family is the largest family of flowering plants. With flower parts in multiples of threes and parallel- veined leaves, they are clearly monocots. Flowers are temperate and terrestrial (A and B), or tropical and epiphytic (D and E), or even nonphotosynthetic (C), obtaining nutrients from symbiotic mycorrhizal fungi. D is a species within the genus Vanilla, from whose fruits we extract the flavoring.

Sedge Family

Sedges are mostly wetland plants. Superﬁcially, they resemble grasses, but their stems are triangular in cross- section. Like grasses, the ﬂowers are reduced, without a perianth; wind pollination makes a perianth unnecessary. Among the 4,000 species, papyrus (of Egyptian papermaking fame) and Chinese water chestnuts (which grow as sweet/starchy bulbs on the roots) are perhaps the best known. However, sedges often dominate their wetland and marsh habitats.

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Mostly wetland species, sedges are somewhat grass-like, but with stems distinctly triangular in cross-section. Among the 4000 species are the Chinese water chestnut (top right and inset) and papyrus, source of the material for Egyptian paper (lower photo and inset).

Grass Family

Grasslands cover an estimated 20% of the vegetated land area of the Earth. Many of the 10,000 species in this family feed the world; carpet our lawns, athletic courts, and pastures; and provide a signiﬁcant amount of construction material in bamboo. Some 70% of crop plants are grasses, and members of this family produce over half of human calories worldwide. The characteristics of grasses reﬂect the co-evolution of grasses with large mammal herbivores in their open, seasonal habitat:

• Grass blades (leaves) grow at the base, rather than the tips, allowing grazing.

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• Silica hardens and sharpens the blades, discouraging some herbivores. • A fringe of hairs where blade meets stem prevents water and insects from entering. • Flowers are greatly reduced; the plants depend on the abundant wind for pollination. • Fruits are grains: seed and carpel fused together –as in rice, corn, and wheat.

Dinosaur coprolites (fossil feces) from 65 million years ago have yielded silica crystals from ancestors of rice and bamboo; grasses apparently reach deep into the past.

Grasses co-evolved with large herbivores on open prairie environments; they rely on wind for pollination, so their ﬂowers (A and B) are greatly reduced. Members of the family provide building material (bamboo, B) and half of human calories worldwide (fruits of C: wheat, D: corn, and E: rice, and stems of F: sugar cane.

Arum Family

The unique flowers of the arum family feature a club-like structure which bears flowers, the spadix, and an often- dramatic leaf-like hood, the spathe. The large variety of useful and ornamental species include the foods taro and breadfruit, ornamentals Anthurium, Elephant ear, and Calla lily, and the tropical/houseplants Philodendron and Caladium. Familiar wildflowers include jack-in-the-pulpit and skunk cabbage. Size ranges from the titan arum to the tiny floating duckweed. Some species have individual plants with both male and female flowers (monoecious); others have male flowers above the female flowers on the spadix. In the latter case, the flowers bloom at different times in order to prevent self-pollination. Many plants in this group produce heat and/or a strong rotten-meat odor in

140 www.ck12.org Chapter 1. Plant Biology - Advanced order to attract their beetle pollinators. Skunk cabbage literally melts its way through the snow as it blooms in early spring.

Arum ﬂowers form on a spadix, often hooded by a colorful, leaf-like spathe. Some produce heat to attract pollinators. The arum family includes ornamentals Elephant-Ear (A), Anthurium (B), and titan arum (C); food sources breadfruit (D) and taro (E); and wildﬂowers Jack-in-the-Pulpite (F), Duckweed (G), and Calla lily (H).

Magnoliids

Magnoliids share characteristics with both monocots and dicots, so are thought to be an evolutionarily distinct group. Many of their characteristics resemble early angiosperms. Although they comprise just 2% of ﬂowering plants, magnoliids include a number of plants familiar for use as ornamentals, foods, or spices. The leaves of bay laurel, the bark of cinnamon, the fruit of black pepper, the seed of nutmeg, and the fruit of avocado are examples of foods and spices from this group. The Magnolia order (one of four) includes a number of shrubs and trees, including the tulip tree.

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Magnollids form just 2% of ﬂowering plants, but many are familiar to us as ornametals (Magnolia, top left), foods (avocado, top right), or spices (cinnamon bark, peppercorns, bottom). Magnoliids have ﬂower parts in spirals –often in multiples of 3 –and single fruits with single seeds.

Vocabulary

• epiphyte: Type of plant (or lichen) that grows on other plants for support.

• grassland: Biome where the area is dominated by grasses.

• monocot: A ﬂowering plant with one embryonic seed leaf or cotyledon that usually appears at germination; monocotyledon.

• pollinium: A coherent mass of pollen grains in a plant.

• wetland: Area that is saturated or covered by water for at least one season of the year; swamps, marshes and/or bogs whose soil is saturated.

Summary

• Magnoliids show a mixture of characteristics, similar to the earliest ﬂowering plants. • The largest families of ﬂowering plants are the daisies and the orchids. • Grasses are economically most important, including wheat, corn, rice, and bamboo.

Practice

Use this resource to answer the questions that follow.

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Review

1. Describe the two largest families of ﬂowering plants, and the single family which is economically most important.

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1.21 Flowering Plant Tissues - Advanced

• Recognize that plants live dynamic lives on a longer, slower time scale than animals. • Explain the meaning of "organs" in plants.

Plant Tissues

The world is green, dominated by plants. Yet compared to our active existence, plants are (literally, for most) rooted to the ground, and their relatively sedate lives often seem uninteresting to us animals, even though we may recognize that we are absolutely dependent upon them for food and many other materials and products. Actually, as Alice Walker’s character Shug in The Color Purple knew, most plants live dramatic lives –only on very different time scales. Plants create structures from majestic to fragile and intricate: Mighty redwoods develop from seeds just as we develop from single cells. Some plants “cheat” at the game of life: strangler ﬁgs scale established tree trunks and eventually kill their “hosts” by robbing them of light. Other plants maintain animal armies: Certain species of Acacia feed and protect platoons of ants for the defense of both parties. Plants may even deceive animals in order to meet their needs: certain orchids design elaborate shapes and scents which imitate those of female insects, attracting males which inadvertently pollinate the orchid in their attempts to “mate” with the lures. All plants are chemical factories, annually transforming an estimated 105 gigatonnes (109 metric tons) of carbon (as CO2) into food for themselves and “welfare” for all other life. Some plants practice chemical warfare; black walnut trees produce and release the chemical juglone, which interferes with other plants’ growth. Many plants forge peaceful alliances with other plants: some algae associate intimately with fungi as Lichens, allowing both to grow on cold, bare rock. As you probably know, a few plants are predaceous; a pitcher plant in Borneo holds 2 liters of water and can drown small rodents. Because of plants’ slower time frames, we do not ordinarily notice their creativity, productivity, peacemaking, or violence, but each talent is amply represented in the Plant Kingdom ( Figure below).

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Although plants appear to lead relatively sedate lives compared to animals, some show dramatic behaviors on a slower time scale. (A) Strangler figs scale the sturdy trunks of existing trees to reach the sunlight in dense tropical rain forests. By overgrowing the leafy canopy, the fig may “starve” and kill its host tree. (B) In a mutualistic relationship, this Central American Acacia tree provides nectarines and food bodies and builds hollow thorns to house armies of ants which protect the tree from other insects and clear seedlings from the area surrounding the tree. (C) Certain orchids, such as this Ophrys apifera, construct flowers which so closely mimic specific female insects’ shape and scents that they trick males into pollinating while attempting to “mate” with the flower. (D) This Borneo species of pitcher plant rivals animal predators by drowning small rodents in up to two liters of trapped water. Without nerves or muscles, how do plants carry out these diverse plots? What anatomy and physiology allow them to accomplish such activities? The Plants concepts will explore plant structures and their functions, emphasizing two major biological principles:

1. Life is built of organized levels, each level comprised of smaller components working together to accomplish a larger task more efﬁciently.

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2. At each level, the structure or anatomy directly reﬂects the function or physiology it is designed to carry out.

Organs in Plants?

Your body includes organ systems (the digestive system, for example) made of individual organs (such as the stomach, liver, and pancreas) which work together to carry out a certain function (in this case, breaking down and absorbing food). This is discussed in Concept Anatomy and Physiology (Advanced), but briefly the organs, in turn, are made of different kinds of tissues –groups of cells which work together to perform a specific job; for example, your stomach is made of muscle tissue to facilitate movement and glandular tissue to secrete enzymes for chemical breakdown of food molecules. These tissues, in turn, are made of cells specialized in shape, size, and component organelles, such as mitochondria (for energy) and microtubules (for movement). Plants, too, are made of organs, which in turn are made of tissues. Plant tissues, like ours, are constructed of specialized cells, which in turn contain specific organelles. It is these cells, tissues, and organs, which carry out the dramatic (if slow-motion) lives of plants. In the Plants concepts, we will review the structure and function of plant cells and then three basic types of tissues which orchestrate plant growth and life. Additional concepts will focus on the three basic organs of plants: roots, stems, and leaves.

Vocabulary

• organ: A structure composed of two or more tissues that work together for a common purpose.

• organ system: A group of organs that act together to carry out complex interrelated functions, with each organ focusing on a subset of the overall task.

• tissue: An aggregation of similar cells that work together to carry out a speciﬁc function within the organism/body.

Summary

Practice

Use this resource to answer the questions that follow.

• http://www.hippocampus.org/HippoCampus/Biology?loadLeftClass=Course&loadLeftId=37&loadTopicId=38 72

1. What are the three distinct types of tissues found in plants? 2. What is the role of dermal tissue? 3. Describe the epidermis of a plant. 4. What is bark? 5. What is vascular tissue? 6. What are the two types of vascular tissue, and what are their roles?

Practice Answers

1. Dermal, vascular, and ground. 2. Dermal tissue protects the plant from pathogens and water loss. 3. It is a ﬂexible layer of cells, often coated with a waxy cuticle.

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4. It is called periderm, and it helps protect the outside of the plant. 5. The vascular tissue helps transport materials inside the plant. 6. The xylem carries water and nutrients from the roots of the plant, while the phloem carries sugars, cell secretions, and waste.

Review

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1.22 Plant Cells - Advanced

• Review the hierarchical organization of plants from atom to organism. • Relate the structures unique to plant cells to their importance to photosynthesis. • Compare and contrast the three basic types of plant cells.

A generalized plant cell shows many similarities to other eukaryotic cells. Major differences are marked with an asterisk (*).

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Plant Cells

Unique Structures, and Three Basic Types

Because plants live by making food from sunlight, air (or the CO2 in the air), and water, the structure of plant cells ( Figure below) differs from that of other eukaryotic cells:

FIGURE 1.53 Plant cells have all the same structures as animal cells, plus some additional structures. Can you identify the unique plant structures in the diagram? For a more detailed interactive animation, please see http://www.cellsaliv e.com/cells/cell_model.htm .

Major differences between a plant cell and animal cell are the large central vacuole, a cell wall, and chloroplasts:

1. A large central vacuole may occupy from 30 to 90% of a plant cell’s volume. Surrounded by its own + semipermeable membrane, the vacuole holds water, ions such as K or Cl, enzymes, pigments, and even toxins. A primary role of the vacuole is to maintain osmotic pressure against the cell wall, giving the cell shape and helping to support the plant. The vacuole also pushes the chloroplasts outward toward the cell membrane, increasing their exposure to light. Toxins which might interfere with cellular metabolism may be removed to the vacuole, and in some cases the toxins can help protect the plant from predators. Excess cytoplasmic protons pumped into the vacuole can regulate enzyme activity, at the same time serving a waste disposal function. Pigments stored in the the vacuoles of fruits and ﬂowers provide coloration.

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2. A cell wall outside the cell membrane shapes, supports, and protects the cell. Composed of cellulose, other polysaccharides, and glycoproteins, the cell wall also acts as a filter, preventing large, potentially damaging molecules from reaching the cell membrane. Semi-rigid, the cell wall not only shapes the cell and supports the plant, but also limits osmosis so that excess water cannot cause the cell to burst. An outer, gelatinous layer, the middle lamella, both joins and separates adjacent plant cells. 3. After growth has stopped, many plant cells add a secondary cell wall, a thicker, more rigid layer of cellulose fibers, between the primary cell wall and the cell membrane. Additional molecules such as lignin and suberin may waterproof and strengthen secondary cell walls to make wood and cork, respectively. Pits in the secondary wall allow cells to communicate via the relatively free flow of small molecules and ions through plasmodesmata. Plasmodesmata connect adjacent cells’ cytoplasms. 4. Plastids ( Figure below), closely related DNA-containing organelles, include chloroplasts, chromoplasts, and a diverse group of leucoplasts. As you learned in earlier chapters, chloroplasts (“green bodies” –Figure below) contain the green pigment chlorophyll, and carry out photosynthesis. Chromoplasts (“color bodies”) make and store plant pigments. Various types of leucoplasts (“white bodies”) store starch or fat, detect gravity, and modify and store protein.

Plastids include chloroplasts (left) and leucoplasts (right). Chloroplasts carry out photosynthesis, and certain leucoplasts store starch, as in these potato cells. Three basic types of cells build most plants. 1. Parenchymal cells ( Figure below) most closely resemble the generalized plant cell discussed above. They make up the bulk of plant tissue. Thin-walled (no secondary walls), roughly cube-shaped, and often loosely packed, they are the least specialized of the three basic plant cell types. Parenchymal cells retain the ability to divide to form other types of cells, much as stem cells do in our own bodies. Their primary functions are metabolism, including photosynthesis and cellular respiration, and storage.The ﬂesh of fruits and vegetables such as potatoes are made of parenchymal cells.

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Parenchymal cells (stained red) store starch in this buttercup root cross-section. Parenchymal cells are typically unspecialized with thin walls. 2. If you have ever pulled the “strings” from a stalk of celery, you are familiar with collenchymal cells ( Figure below). These cells are elongated, with irregularly thickened primary walls, and they provide support, especially for growing shoots. Mechanical stress (such as wind) can cause the plant to further thicken the walls, adding to the support.

Collenchyma tissue (left) forms the “strings” which support celery stalks (right). Typically, collenchyma has thicker cell walls than parenchyma. 3. Rope made of hemp, jute, or ﬂax consists largely of the third type of cell, sclerenchyma ( Figure below). Whereas collenchymal cells support growing tissues, sclerenchymal cells strengthen and support parts of the plant which have completed elongation. Thick secondary walls made of cellulose and lignin can make up as much as 90% of the cell’s volume; think of sclerenchyma as the plant’s skeleton. Two types of sclerenchymal cells include ﬁbers and sclereids.

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Fibers - long and strong - have walls made of cellulose and sometimes lignin; we use ﬁbrous plants to make ropes and textiles. Sclereids are relatively short, and often grouped into bundles. You may be familiar with them as the gritty texture of pears or nutshells and pits of cherries or plums.

A cross-section of a ﬂax stem reveals sclerenchymal cells which form ﬁbers used to make rope and textiles such as linen. Sclerenchyma has the thickest cell walls of the three basic types of cells. As for all animals, your body is made of four types of tissue: epidermal (covering), muscle, nerve, and connective tissues. Plants, too, are built of tissues, but not surprisingly, their very different lifestyles derive from different kinds of tissues. Specialized cells derived from the three basic cell types discussed above combine to form three basic types of plant tissues: dermal tissue or covering tissue, ground tissue or body tissue, and vascular tissue or conducting tissue. We will look at these “fabrics” which build plants in the concepts that follow: Plants: Dermal Tissue (Advanced), Plants: Ground Tissue (Advanced) and Plants: Vascular Tissue (Advanced).

Vocabulary

• cell wall: Rigid layer that surrounds the plasma membrane of prokaryotic cells and plant cells; helps support and protect the cell.

• central vacuole: Large saclike organelle in plant cells; stores substances such as water; helps keep plant tissues rigid.

• chloroplast: The organelle of photosynthesis; site of photosynthesis.

• chromoplast: Plastid responsible for pigment synthesis and storage in speciﬁc photosynthetic eukaryotes.

• collenchymal cells: One of the three major plant cell types; irregularly thickened walls and elongated cells arranged in strands help to support growing parts of the plant.

• dermal tissue: One of the three basic types of plant tissues; covers and protects the plant.

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• ground tissue: One of the three basic types of plant tissues; functions in photosynthesis, food storage, and support.

• leucoplast: Non-pigmented plastid specialized for bulk storage of starch, lipid or protein; located in roots and non-photosynthetic tissues of plants.

• lignin: A chemical compound which forms the woody part of some plant cell walls.

• middle lamella: The outermost layer of a plant’s cell wall; cements adjacent cells together.

• parenchymal cells: One of the three major plant cell types; thin-walled, relatively unspecialized, form the bulk of plant tissue and perform photosynthesis, cellular respiration, and storage functions.

• plasmodesmata (singular, plasmodesma): Microscopic channels which traverse the cell walls of plant cells; enables transport and communication between them.

• plastid: Organelle found in the cells of plants and algae; the site of manufacture and storage of important chemical compounds used by the cell; often contain pigments.

• sclerenchymal cells: One of the three major plant cell types; greatly thickened, rigid secondary walls containing lignin and elongated, often-dead (at maturity) cells provide strong support.

• secondary cell wall: An innermost layer present in some plant cells, thicker and more rigid than the primary cell wall, often containing lignin.

• suberin: A waxy chemical compound which waterproofs some plant cell walls, as in cork.

• vascular tissue: A type of tissue in plants that transports ﬂuids through the plant; includes xylem and phloem.

Summary

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Practice

Use this resource to answer the questions that follow.

Review

154 www.ck12.org Chapter 1. Plant Biology - Advanced

1.23 Dermal Tissue of Plants - Advanced

• Name the three basic types of plant tissues. • Relate the structure of epidermis to its protective function. • Describe variations in epidermal tissues, and relate them to their adaptive importance.

Dermal Tissue

Plant “Skin”

Your skin covers your body, protecting it from abrasion, water loss, and infection and secreting conditioning substances. The epidermis of a plant serves similar functions. Usually a single layer of closely-packed cells,

155 1.23. Dermal Tissue of Plants - Advanced www.ck12.org epidermal tissue covers leaves and young stems and roots of vascular plants. Most epidermal cells secrete waxy substances which form a cuticle, or waterproof covering, over the aerial surfaces of the plant, such as the stems and leaves. The cuticle also resists viral, bacterial, and fungal invasion. Epidermal tissue includes several types of specialized cells. Pavement cells, large, irregularly shaped parenchymal cells which lack chloroplasts, make up the majority of the epidermis. Within the epidermis, thousands of pairs of bean-shaped schlerenchymal guard cells ( Figure below) swell and shrink by osmosis to open and close stomata, tiny pores which control the exchange of oxygen and carbon dioxide gases and the release of water vapor. The lower surfaces of some leaves contain as many as 100,000 stomata per square centimeter.

The epidermis of Arabidopsis shows both pavement cells ( A) and stomata made of sclerenchymal guard cells ( B), which control water loss and gas exchange. Like your skin, plant epidermis may also bear hairs, or trichomes ( Figure below), which may be unicellular or multicellular. Hairs vary in size, texture, density, and purpose. Some functions include:

• Protecting trichome leaves and stems from herbivores • Insulating plant tissues against frost • Reducing evaporation due to wind • Reﬂecting solar radiation • Collecting moisture in humid environments

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Dermal tissue may include hairs or trichomes, which have many different forms and functions. The dense white hairs covering the desert Kalanchoe (left) may reﬂect solar radiation, reduce water loss from wind, and protect the plant from herbivores. If you have come in contact with the syringe-like hairs of the nettle (right), you know they can inject a cocktail of painful toxins, protecting the plant from many enemies. Root epidermis contains very different trichomes known, simply, as root hairs ( Figure below). Root hairs are simple outgrowths of single epidermal cells which function to increase surface area for absorption of water and minerals. Root hairs are very small and delicate, and live only 2 or 3 weeks.

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The epidermal cells of root tips produce microscopic extensions known as root hairs, which increase surface area for more efﬁcient absorption of water and minerals. In older woody plants, the epidermis of stems and roots is often replaced by a combination of ground and vascular tissues, which you know as bark. Bark, also known to botanists as periderm, will be discussed in more detail in the concept on stems.

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Scanning electron microscope image of Nicotiana alata (a species of tobacco) upper leaf surface, showing tricomes and a few stomates.

Vocabulary

• bark: A protective, multi-layered outer covering of the stems and roots of many older woody plants, which replaces the epidermis.

• cuticle: A thick organic layer surrounding the outer surface of nematodes and arthropods; a waxy waterproof covering over the aerial surfaces of a plant.

• epidermis: The outermost layer of skin, composed of epithelial cells; the outermost layer of a plant.

• guard cells: Pairs of specialized epidermal cells which regulate water and gas exchange in leaves and stems by opening and closing stomata.

• root hairs: Tiny projections of the epidermis which increase surface area for absorption.

• stomata (singular, stoma): Openings on the underside of a leaf which allow gas exchange and transpiration.

• trichomes: Epidermal hairs, which protect or defend leaves, stems, or ﬂowers.

Summary

Practice

Use this resource to answer the questions that follow.

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Review

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1.24 Ground Tissue of Plants - Advanced

• Relate the structure of ground tissues to their functions in photosynthesis, storage, support, and regeneration.

What’s so special about the leaf? The leaf is the site of photosynthesis. So without the leaf, life as we know it probably would not exist. This is Allestree Park Lake in the United Kingdom. With the lake being surrounded by trees, Autumn brings not only one of the most colorful times of the year to the lake but also one of the most important. As the leaves fall, many ﬁnd their way into the lake, forming a black, silt bottom that is enriched with insect larvae, which feed many of the ﬁsh that live in the lake.

Ground Tissue

Body-building and Metabolism

While epidermal tissue mediates most of the interactions between a plant and its environment, ground tissue conducts the basic functions of photosynthesis, food storage, and support. Ground tissue is the least differentiated of the three major tissues; parenchymal cells are most common, so that regeneration is yet another function of ground tissue. Thickened, live collenchyma and strong, no-longer-living sclerenchyma also contribute to the ground tissue “bodies” of roots, stems, and leaves. As an introduction, we will look more closely at the ground tissue of leaves. Ground tissue makes up most of the interior of leaves, between the two layers of epidermis. Here, two types of parenchymal cells form the two layers of the mesophyll (“middle leaf”):

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A diagramatic leaf cross-section shows all three basic types of plant tissues. Upper and lower epidermis, with cuticle and guard cells, illustrate protective dermal tissues. The mesophyll, including palisade and spongy layers, is the primary photosynthetic ground tissue. Xylem and phloem are vascular tissues, which transport water and food, respectively.

1. Across the top of the leaf (but beneath the epidermis), one or two rows of columnar cells densely packed with chloroplasts form the palisade layer (“palisade” describes its resemblance to a wall of wooden stakes, built for defense). Arranged for optimal light absorption and spaced to allow CO2 diffusion and water distribution, the palisade cells are specialized to perform photosynthesis. 2. Beneath the palisade layer, rounded, loosely packed cells containing fewer chloroplasts make up the spongy layer of the mesophyll. Like palisade layer cells, spongy layer cells carry out photosynthesis, but the air spaces between them and the stomata beneath them are equally important –for gas exchange and evaporative cooling.

Vocabulary

• ground tissue: One of the three basic types of plant tissues; functions in photosynthesis, food storage, and support.

• mesophyll: The ground tissue which makes up most of the photosynthetic interior of leaves; "middle leaf."

• palisade cells: Cells found within the mesophyll in leaves; contain the largest number of chloroplasts per cell, which makes them the primary site of photosynthesis.

• palisade layer: One or two rows of photosynthetic columnar cells in the mesophyll.

• spongy layer: A layer of loosely packed and irregularly shaped chlorophyll-bearing cells; ﬁlls the part of a leaf between the palisade layer and the lower epidermis.

Summary

Practice

Use this resource to answer the questions that follow.

Review

162 www.ck12.org Chapter 1. Plant Biology - Advanced

1.25 Vascular Tissue of Plants - Advanced

• Explain how the structure of xylem facilitates transport of water and ions from roots to leaves. • Analyze the structure of phloem and relate it to transport of sap from sugar sources to sugar sinks.

How does water move from the roots to the leaves? By traveling up a stem. But how does this happen? Water travels upwards through xylem, a type of plant vascular tissue.

Vascular Tissue

Transport

Epidermal tissue separates the plant body from its environment, and ground tissue builds the bodies and carries out metabolic reactions for the plant. However, it is vascular tissue which has allowed vascular plants to dominate terrestrial communities worldwide. What is vascular tissue, and why is it so crucial for plant success? Your body was able to grow from a single cell to perhaps 100 trillion because 21 days after fertilization, a tiny heart began to pump blood throughout your tiny self –and it hasn’t stopped since. The blood it pumps carries water, oxygen and nutrients to each one of your trillions of cells, and removes CO2 and other wastes. Your heart, your blood, and your blood vessels form your cardiovascular system (cardio = heart; vascular = vessels). Of course plants don’t have hearts, but they do have vessels which transport water, minerals, and nutrients through the plant. These vessels are the vascular tissue. Your blood vessels include both arteries and veins –arteries specialized to carry blood away from the heart, and veins specialized to carry blood at lower pressures back to the heart. Plants’ vascular tissue also includes two types of vessels. Xylem carries water and minerals up from the roots, where these valuable substances are absorbed, to stems and leaves which may be hundreds of meters above the ground. Phloem carries food from the leaves which produce it to storage areas (often down to the roots) or growth areas in other parts of the plant. Unlike your arteries and veins, xylem and phloem are packaged together in vascular bundles scattered throughout the ground tissue. If you visualize a handful of straws, you will have a good (although greatly magniﬁed!) image of vascular tissue. Both

163 1.25. Vascular Tissue of Plants - Advanced www.ck12.org xylem and phloem are made of elongated cells placed end-to-end like sections of a pipe. Figure below shows the relationship between vascular tissue and other tissues in a cross-section of a leaf; you are probably familiar with these as the aptly named leaf "veins"!

Compare two familiar “naked eye” views of vascular tissue to a microphotograph. (left) A cross section of a celery stalk shows vascular bundles, which include both phloem and xylem. (center) A cross section through the stem of a pumpkin reveals water and sap escaping from the xylem and phloem, respectively. (right) A microscopic view of a single vascular bundle shows the individual cells of vascular tissue. The word xylem means “wood”, although xylem is found not just in wood, but throughout the plant. Two types of sclerenchymal cells make up the majority of the xylem tissue. Tracheids are long, relatively thin, and tapered; their narrow diameter allows them to hold water by adhesion (that is, the cell walls and water molecules attract each other) against the force of gravity. The secondary walls of tracheids contain lignin, allowing them to support the mass of the plant, as well. The wood of conifers (known as a "softwood") is formed primarily of tracheids. In contrast to tracheids, vessel elements are wider and shorter. Like tracheids, their cell walls are thickened with lignin, sometimes in spiraling patterns, and various types of perforated plates connect them. The design of vessel elements allows them to carry water more efficiently, so they have allowed flowering plants to dominate conifers. Vessels, also known as “pores” because they form large “pipes” in the wood, make up most of the hardwood characteristic of woody flowering plants. Compare the two types of xylem cells in Figure below.

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SEM microphotographs of woods showing the presence of pores (vessel elements) in hardwoods (Oak, top) and absence in softwoods (Pine, bottom). Both woods are made primarily of xylem, but relatively narrow tracheids dominate in softwood (conifers), while vessel elements dominate in hardwood (ﬂowering trees). Tracheids and vessel elements lose their cytoplasm as they mature; they are actually dead cells when they become fully functional. Their thick secondary walls contain “pits” which allow water and ions to enter through the thinner primary walls, as opposed to plasmodesmota. Two kinds of force move water and minerals up through the xylem:

1. Evaporation of water from the leaf interior through the stomata ( transpiration) creates a meniscus, and corresponding negative pressure, in the xylem, causing water to “climb” up the tracheids and vessels through adhesion (the sides of the vessels and water molecules attract each other) and cohesion (one water molecule tugs on another). This transpirational pull can create enough force to provide water to leaves 100 meters above ground, as in the coastal redwood. 2. If the concentration of solutes outside a root is less than the concentration inside the root (a usual situation), water will enter the root by osmosis, creating a positive pressure in the xylem which can push water up the tree. Root pressure can raise water 20 meters above ground.

The word phloem means “bark”, although phloem is only the innermost layer of bark. Perhaps that can help you to remember that phloem faces outward, toward the surface of a stem (or trunk), while xylem faces inward. In leaves, xylem faces the upper surface, and phloem faces the lower surface. Two types of cells make up the majority of phloem tissue. Sieve tube cells are specialized to transport sap, a watery solution of sugar, hormones, and minerals. Elongated and lacking a nucleus and vacuoles, they are joined end-to-end by sieve plates, where plasmodesmata connect the cytoplasm of one to the cytoplasm of the next. Plasmodesmata also connect the sieve tube cells to companion cells, much smaller adjacent cells which maintain high metabolic rates in order to nourish and support the sieve tube cells, and “load” them with sap. Both types of cells are visible in Figure below. Sap ﬂows through the sieve tube cells from photosynthetic regions (leaves) to roots, tubers, or fruits. In spring, as you may know from experience with maple trees, the sap ﬂows from sugar storage areas in the roots upward to developing buds and leaves.

Aphids share our interest in the sugary sap plants transport through phloem tissue (left, arrow). Look carefully at the microphotograph to see both sieve tube (larger lighter) and companion (smaller pink) cells. (Center) An aphid pierces the epidermis of a plant stem with its tubular mouth in order to drink the sap carried by phloem. The phloem is under positive pressure, allowing the sap to surge into the aphid. (Right) We “tap” maple trees in the spring to catch the sap moving upward through the xylem from roots to buds. Roughly 50 liters of sap are boiled to produce 1 liter of syrup. In conclusion, plants’ vascular tissue makes use of physical and chemical forces to create a great deal of activity inside the calm-looking exterior of a plant –without heart or muscle!

Tissue Summary Table

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TABLE 1.5: Three Basic Types of Plant Tissues

Tissue Type Specialized Cells or Structure Function Structures Dermal: Epidermis Pavement cells Single layer, closely- Protect from abrasion, packed, covered with water loss, infection waxy cuticle Dermal: Epidermis Guard Cells Paired; change shape with Gas exchange, transpira- humidity tion Dermal: Epidermis Trichomes Unicellular or multicellu- Protect against sun, frost, lar hairs wind; insulate, conserve water Dermal: Epidermis Root hairs Simple outgrowths of sin- Absorption of nutrients gle epidermal cells and water Dermal: Epidermis Bark Corky replacement of epi- Protection dermis of roots and stems in some plants Ground Parenchyma Undifferentiated Regeneration Ground Collenchyma Thickened cell walls Support, food storage Ground Sclerenchyma Thickened cell walls, cells Support dead Ground Palisade cells Columnar, many chloro- Photosynthesis plasts Ground Spongy layer mesophyll Rounded, some chloro- Photosynthesis, gas ex- cells plasts, widely spaced change Vascular: Xylem Tracheids long, relatively thin, and Water transport; support tapered; secondary walls of tracheids contain lignin Vascular: Xylem Vessel elements are wider and shorter; More efﬁcient water cell walls thickened with transport lignin, sometimes in spiraling patterns; cells connected by perforated plates Vascular: Phloem Sieve tubes Elongated; lack nucleus Transport sap, a watery and vacuoles; joined end- solution of sugar, hor- to-end by sieve plates; mones, and minerals plasmodesmata connect cells’ cytoplasm Vascular: Phloem Companion cells much smaller adjacent Nourish and support sieve cells which maintain high tube cells; “load” them metabolic rates with sap

Vocabulary

• companion cells: Cells in the phloem (of a plant) which support and load the conducting sieve tube elements cells.

• phloem: Vascular tissue which transports food from leaves to storage or growth areas in other parts of the plant; includes sieve tube elements and companion cells.

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• root pressure: The osmotic pressure within the cells of a root system; causes sap to rise through a plant stem to the leaves.

• sap: A watery solution of sugars, hormones, and minerals found in phloem.

• sieve tube cells: Conducting cells in phloem, connected by sieve plates and plasmodesmata; also known as sieve tube elements.

• tracheid: An elongated cell in the xylem of vascular plants that serves in the transport of water and mineral salts.

• transpiration: A process by which plants lose water; occurs when stomata in leaves open to take in carbon dioxide for photosynthesis and lose water to the atmosphere in the process.

• vascular plant: Plant with tissues for conducting water and minerals throughout the plant.

• vascular tissue: A type of tissue in plants that transports ﬂuids through the plant; includes xylem and phloem.

• vessel elements: An elongated, water-conducting cell found in xylem; one of the two kinds of tracheary elements.

• xylem: Vascular tissue which transports water and minerals from the roots to stems and leaves; includes tracheids and vessel elements.

Summary

Practice

Use this resource to answer the questions that follow.

Review

168 www.ck12.org Chapter 1. Plant Biology - Advanced

1.26 Growth of Plants - Advanced

• Compare indeterminate plant growth to determinate growth in most animals. • Compare plant meristems to human stem cells. • Explain how apical and lateral meristems allow for primary and secondary growth in plants.

Plant Growth

Like you, plants grow through cell growth and division (mitosis) and develop through cell differentiation or specialization. Unlike you, most plants continue to grow throughout their lives, although individual organs may grow only to a certain size. Of course, their lives may be short so the growth is limited: plants which complete their growth from seed to ﬂower and seed in a single season are annual plants. Biennial plants grow during their ﬁrst year and

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flower and seed their second year. Perennials ( perennial plants) have the most potential for growth, continuing from season to season indefinitely; some prairie buffalo grass plants are thought to be 10,000 years old, having developed from seeds during the last Ice Age! Buffalo grass stores its growth below ground, in an extensive root system hidden from our view and appreciation. Trees, however, add growth both in width and height above ground, where we can marvel at their majesty. How can trees, covered in tough, protective bark, continue to grow? The key to plant growth is meristem, a perpetually embryonic tissue which can continue to divide and differentiate (specialize) into mature cells with specific functions. Plant meristem tissues are similar to human stem cells, which are very much in the news because they have the potential to grow new tissues and organs –“replacement parts” to treat serious injuries and illnesses. In both plants and animals, differentiated/specialized cells usually lack the potential to divide and reproduce. Only meristem in plants and a few kinds of “adult” stem cells in animals retain the ability to divide and differentiate, so these tissues must produce any new growth or repair. Meristem cells are small and thin-walled, and lack vacuoles and plastids. The location and types of meristem in a particular plant determine its potential for growth. Apical meristems ( Figure below) are found at the apex, or tip, of roots and buds, allowing roots and stems to grow in length, and leaves and flowers to differentiate. Roots and stems grow in length because the meristem adds tissue “behind” it, constantly propelling itself further into the ground (for roots) or air (for stems). Often, the apical meristem of a single branch will become dominant, suppressing the growth of meristems on other branches and leading to the development of a single trunk. In grasses, meristems at the base of the leaf blades allows regrowth after grazing by herbivores –or mowing by lawnmowers.

Microphotographs of the root tip of an onion show rapidly dividing apical meristem tissue just behind the root cap (left). Enlarged (right), the apical meristem shows mitotic cells. Apical meristems differentiate into the three basic types of meristem tissue which correspond to the three types of tissue we discussed earlier in this lesson: protoderm produces new epidermis, ground meristem produces ground tissue, and procambium produces new xylem and phloem. These three types of meristem are considered primary meristem because they allow growth in length or height, which is known as primary growth. Secondary meristems or lateral meristems allow growth in diameter (secondary growth) in woody plants; herbaceous plants do not have secondary growth. The two types of secondary meristem are both named cambium (meaning “exchange” or “change”). Vascular cambium produces secondary xylem (toward the center of the stem or root) and phloem (toward the outside of the stem or root), adding growth to the diameter of the plant. This process produces wood, and builds the sturdy trunks of trees. Cork cambium lies between the epidermis and the phloem, and replaces the epidermis of roots and stems with bark, one layer of which is cork. Figure below shows the relationships between primary and secondary growth.

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Woody plants grow in two ways. Primary growth adds length or height, mediated by apical meristem tissue at the tips of roots and shoots –which is difﬁcult to show clearly in cross-sectional diagrams. Secondary growth adds to the diameter of a stem or root; vascular cambium adds xylem (inward) and phloem (outward), and cork cambium replaces epidermis with bark.

Vocabulary

• annual plant: A plant that performs the entire life cycle from seed to ﬂower to seed within a single growing season; annuals.

• apical meristem: Embryonic plant tissues which allow growth in length or height.

• biennial plant: A plant which requires two years to complete its life cycle; biennials.

• cambium: Secondary meristem responsible for growth in diameter in woody plants; includes vascular cambium which produces secondary xylem and phloem, and cork cambium, which produces bark.

• cork cambium: Undifferentiated, rapidly dividing cells which replace sloughed epidermis and cortex.

• lateral meristem: Embryonic plant tissues which allow for growth in diameter; also known as secondary meristem.

• meristem: Embryonic plant tissue which can continue to divide and differentiate for growth and development.

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• perennial plant: A plant that persists for many growing seasons; perennials.

• primary growth: Growth that results in the lengthening of the stem and roots (of a plant).

• primary meristem: Embryonic plant tissues which allow growth in length or height.

• secondary growth: Growth that results from cell division in the cambia or lateral meristems (of a plant); causes the stems and roots to thicken.

• secondary meristem: Embryonic plant tissues which allow for growth in diameter; also known as lateral meristem.

• vascular cambium: A lateral meristem in the vascular tissue of plants; a cylinder of unspecialized cells that give rise to cells that differentiate and specialize to form the secondary vascular tissues.

Summary

Practice

Use this resource to answer the questions that follow.

• A Plant Story at http://www.youtube.com/watch?v=CXbhdBzx1Ag&feature=related

1. What is seed germination? 2. Describe root and plant growth. 3. Describe the process of photosynthesis.

Practice Answers

1. Seed germination is the process in which seeds grow into plants. 2. Roots grow downwards, while plants grow upwards. 3. During photosynthesis, plants take in Carbon Dioxide and utilize sunlight to create nutrients. This results in the release of Oxygen.

Review

172 www.ck12.org Chapter 1. Plant Biology - Advanced

1.27 Roots - Advanced

• Contrast the environments and functions of roots and shoots. • Understand that both roots and shoots are built of the same basic three tissues.

Roots

You may swim, snorkel, or ski in lakes or ocean; hang-glide, or parachute through the air; explore caves for recreation, or even earn a living working in mines, but you undoubtedly live most of your life on “good old terra ﬁrma” - “rooted to the ground” (but not literally, of course!). We humans are terrestrial beings. Although plants are also primarily terrestrial beings, they live true double lives: at the same time, most individuals are part aerial (the shoot) and part subterranean (the root)(Figure below). The two parts have different but equally essential functions and dramatically different environments, so their structures necessarily differ in order to carry out tasks and adapt to surroundings.

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Plants live double lives. The shoot grows into the air to make food and reproduce, while the root grows downward into a completely different environment in order to absorb water and minerals and anchoring the plant. The aerial shoot, which produces stems and leaves and perhaps ﬂowers, fruits, and seeds, functions to gather light and CO2 for food-making, and often to reproduce and disperse new individuals. To compete for sunlight and CO2, shoots reach as high as 115 meters into the air - an environment which requires major engineering for seasonal changes, support, carrying water, and protecting water supply. In contrast, the subterranean root must concentrate water and minerals from the soil, and often assumes responsibilities for support and over-wintering. To absorb and

174 www.ck12.org Chapter 1. Plant Biology - Advanced concentrate scarce water and mineral molecules, roots must be open rather than sealed. Variations in soil texture, water availability, and mineral content require physical and chemical adaptations very different from those of shoots. Although their functions and environments differ, both shoots and roots grow from the basic “body” plan of three tissue types, via primary meristems and (for woody plants) secondary meristems. The tissues vary, however, to suit the functions and environments of roots vs. shoots. This lesson focuses on roots: their roles in absorption, support, and over-wintering, and their adaptations to their underground environment. We will look at types of roots and root structures, and at how both serve speciﬁc root functions. Finally, we will explore how roots grow.

Vocabulary

• primary meristem: Embryonic plant tissues which allow growth in length or height.

• root: Below-ground (usually) part of a plant specialized for absorption of water and mineral ions.

• secondary meristem: Embryonic plant tissues which allow for growth in diameter.

• shoot: Above-ground part of the plant (stems, leaves, and often ﬂowers, fruits, and seeds) specialized for photosynthesis and reproduction.

Summary

Practice

Use this resource to answer the questions that follow.

• http://www.hippocampus.org/HippoCampus/Biology?loadLeftClass=Course&loadLeftId=37&loadTopicId=38 74

1. Which organs comprise the plant’s vegetative system? 2. Describe the roles of the organs of the vegetative system. 3. Compare primary roots to lateral roots. 4. What is the main role of roots? 5. What are additional roles of roots?

Practice Answers

1. The vegetative system includes the roots, stems and leaves. 2. The vegetative system is responsible for growth, development, food manufacturing, storage, protection, and support. 3. Primary roots extend primarily downwards, while lateral roots spread out to gather additional nutrients from a larger area. 4. Roots absorb water and nutrients from the soil. 5. Additionally, roots also anchor a plant and can act as food storage organs.

Review

175 1.28. Root Types - Advanced www.ck12.org

1.28 Root Types - Advanced

• Compare and contrast taproots and ﬁbrous roots in structure and function. • Identify and describe specialized roots.

Types of Roots

Roots are basic organs of all plants; they vary because their functions and environments vary. Most vascular plants have both primary, original roots which grown downward and secondary roots which branch laterally; together, they can be considered organ systems. Two basic types of root systems are taproot and ﬁbrous root systems.

Taproot Systems

Taproot systems ( Figure below) feature a single, dominant primary root which grows straight down into the soil; much smaller, ﬁbrous secondary roots run outward. The large primary root can reach deep into the soil for water

176 www.ck12.org Chapter 1. Plant Biology - Advanced and store food, adaptations which help them to survive drought and seasonal variations in temperature and sunlight. Carrots are perhaps the most familiar taproots, although dandelions better demonstrate how taproots help survival despite our determined attempts to eliminate them. We exploit the food supplies of taproots of beets, radishes, parsnips, and turnips as well as carrots. Trees often begin life with tap roots, but branch extensively to gather water and nutrients from a wider area as they age. Long taproots, just like many other plant structural features, evolved over many generations in response to changes in the environment.

Taproot systems feature a dominant primary root which reaches deep into the ground to “tap” water during drought and stores food for over-wintering. We exploit the stored food in taproot vegetables such as carrots (left), beets, radishes, parsnips, and turnips. The taproots of dandelions (right) anchor securely into the soil and allow regrowth, even after we remove the tops.

Fibrous Root Systems

Fibrous root systems ( Figure below) are much more diffuse, with many smaller branching roots growing from the stem. A fibrous root is a collection of short, threadlike divisions whose increased surface area facilitates absorption of water and mineral ions. Fiber length and horizontal spread vary according to the species’ environment and competition. Whereas taproots can grow as deep as 60 meters, fibrous roots are generally much shorter and more numerous. Note in ( Figure below) that two prairie plants, lead plant and compass plant, have fibrous roots extending 1 15 feet (5 meters) deep. Mature trees have mostly horizontal fibrous root systems limited to the top 50 cm (2 2 feet) of soil.

Grasses illustrate ﬁbrous root systems (left), specialized for absorption of water and mineral ions. Fiber length and horizontal spread vary according to the species’ environment (right) and competition, showing that dry environments are not limited to taproot-bearing plants.

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Specialized Root Systems

Within these two basic types, roots may specialize, with unusual structures serving as adaptations to unusual environments or functions, as shown in Figure below.

Specialized roots include the aerial roots of orchids and other epiphytes (A), stilt roots of mangroves (B), buttress roots of many tropical rainforest trees such as this kapok (C), and starch-storing tuberous roots, illustrated here by cassava (D), which is extensively cultivated for food in Africa and South America.

• Aerial roots live entirely above the ground, serving epiphytes which live on other plants, and vines such as ivy. • Stilt roots grow downward from lateral branches and anchor plants such as mangroves in very soft, wet soils. • Propagative roots produce shoots which form new plants –a form of asexual reproduction. • Buttress roots support many tropical rainforest trees in the thin soils characteristic of that biome. • Tuberous roots are lateral roots of perennial plants, modiﬁed to store food for survival over the winter.

Root depth depends on soil type, distribution of water and minerals seasonally and throughout the soil, and the form of the plant. Shallow roots predominate in the permafrost soils of the tundra and thin, rocky soils of the boreal forest, and the deepest roots –down to 60 meters - are found in deserts and the rich soils of temperate rain forests. Anchoring roots can grow as deep as a tree is tall.

Vocabulary

• aerial roots: Roots which live entirely above the ground.

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• buttress root: A tree root that extends above ground as a platelike outgrowth of the trunk supporting the tree.

• epiphyte: Type of plant (or lichen) that grows on other plants for support.

• ﬁbrous roots: Many small branching roots growing from the stem; provides increased surface area which facilitates absorption of water and mineral ions.

• propagative roots: Roots that form buds that develop into aboveground shoots, termed suckers, which form new plants.

• root: Below-ground (usually) part of a plant specialized for absorption of water and mineral ions.

• stilt roots: Roots that grow down from lateral branches, branching into additional roots in the soil.

• taproot: A single, dominant primary root which grows straight down into the soil; accesses deep water and nutrients and stores food for over-wintering.

• tuberous root: A root that swells for food or water storage; distinct from a taproot.

Summary

Practice

Use this resource to answer the questions that follow.

Review

179 1.29. Root Structure - Advanced www.ck12.org

1.29 Root Structure - Advanced

• Explore and analyze the structures, cell, and tissues, which make up root organs. • Relate the structures which make up root organs to their absorption, storage, and support functions.

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181 1.29. Root Structure - Advanced www.ck12.org

From The New Student’s Reference Work, 1914.

Root Structures: Anatomy

We’ve described several types of roots based on the structures visible to the naked eye (though usually underground). Let’s look now at the structures common to most roots, revealed by microscopic analysis. These will include variations in the organelles, cells, and tissues you studied in the last lesson; you may want to refer to its text and illustrations. As we look at the structures, consider how each form might suggest its function. We will discuss the functions of individual structures in the next section, but you should be able to predict at least some of them, based on shape, size, and location. A longitudinal section of a root ( Figure below) reveals that each root tip is covered with a root cap of soft, thin- walled parenchymal cells. Some of these cells, called statocytes, contain statoliths, relatively mobile plastids ﬁlled with starch ( Figure below). By studying Figure below and Figure below, can you infer the function of these specialized cells? Dead cells and a gelatinous substance (for what function?) surround the living root cap. See the sidebar to check your answers.

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(left) A stained root tip, magnified 10X, shows the root cap, including statocyte (note internal statoliths) (2) and other parenchymal (3) and sloughed dead (4) cells; the primary or apical meristem (1), and the zone of elongation (5). (right) Statocytes are cells containing starch-filled leucoplasts known as statoliths (brown organelles). Study the above plant cell diagrams and refer to Figure above for their location in the root tip. Can you infer their function from their structure? Beneath the root cap is the primary meristem or apical meristem, a mass of undifferentiated tissue in which dividing cells are common; you probably used prepared slides of onion root tips to find the stages of mitosis in this region. Behind (or above) the meristem are cells which become more elongated, the further they are from the meristem. Can you also infer the function of the meristem? Behind the root tip, the entire root is covered with epidermis, a single layer of closely packed cells which often have thin projections known as root hairs. Based on the structure and location (see Figure below) of this tissue, you can probably predict its function!

183 1.29. Root Structure - Advanced www.ck12.org

Diagramatic cross- and longitudinal sections of a plant root show the arrangement of the three basic tissue systems (epidermis, ground, and vascular) in root organs. How does each type of tissue serve the root’s functions of absorption and support? Beneath the epidermis is ground tissue, consisting of loosely packed parenchyma, which may be ﬁlled with starch. A cylinder of vascular tissue resembling a bundle of straws (thick-walled xylem “straws” to the inside, living phloem “straws” to the outside, and a center of parenchymal pith) forms the core of the root, and a waxy layer of suberin separates it from the ground tissue. Why do you suppose the waxy layer is interior in a root, although it covers the outer epidermis in stems and leaves? A layer of actively dividing cells, a secondary meristem called the vascular cambium, separates the thick-walled xylem vessels and the living phloem cells. In older roots, another meristem forms outside the phloem, and ground tissue and epidermis are sloughed off as a tough layer of bark-like cells appear. The root structures we’ve described and depicted mean little unless we know their functions, as well. If you’ve made an effort to predict the functions based on form and location, you are ready to learn how these structures help roots work.

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Vocabulary

• apical meristem: Embryonic plant tissues which allow growth in length or height.

• epidermis: The outermost layer of skin, composed of epithelial cells; the outermost layer of a plant.

• ground tissue: One of the three basic types of plant tissues; functions in photosynthesis, food storage, and support.

• phloem: Vascular tissue which transports food from leaves to storage or growth areas in other parts of the plant; includes sieve tube elements and companion cells.

• primary meristem: Embryonic plant tissues which allow growth in length or height.

• root cap: A group of soft, thin-walled parenchymal cells, which cover the apical meristem in a root tip.

• root hairs: Tiny projections of the epidermis which increase surface area for absorption.

• secondary meristem: Embryonic plant tissues which allow for growth in diameter; also known as lateral meristem.

• statoliths: Gravity-sensing structures typically found in rhopalia; plastids ﬁlled with starch which enable their cells (statocytes) to detect gravity.

• suberin: A waxy chemical compound which waterproofs some plant cell walls, as in cork.

• vascular tissue: A type of tissue in plants that transports ﬂuids through the plant; includes xylem and phloem.

• xylem: Vascular tissue which transports water and minerals from the roots to stems and leaves; includes tracheids and vessel elements.

Summary

Practice

Use this resource to answer the questions that follow.

Review

185 1.30. Root Function - Advanced www.ck12.org

1.30 Root Function - Advanced

• Relate the structures which make up root organs to their absorption, storage, and support functions.

What is a carrot? Root vegetables, such as the carrot, are generally storage organs, enlarged to store energy in the form of carbohydrates.

Root Functions: Physiology

As you may have guessed, some of the plant structures described in other concepts function for growth; these will be discussed in the Roots: Growth (Advanced) concept. First, let’s focus on structures which help roots perform their primary functions:

• absorption of water and minerals • anchoring and support of the plant • storage of food, as for over-wintering

Refer to the Figures in the Roots: Structure (Advanced) concept to help you visualize the structures which facilitate these functions.

Absorption of Water and Minerals

As a single layer of thin cells, the epidermis is ideally suited to absorb water and ions from the soil. Tiny root hair extensions of these cells add surface area, facilitating absorption. Because solutes are usually more concentrated inside the root cells than outside, water ﬂows in by osmosis; for this reason, most plants cannot survive in salt water. Mineral ions, however, cannot enter by diffusion because they are usually less concentrated outside the root than

186 www.ck12.org Chapter 1. Plant Biology - Advanced inside; they must be actively transported into the root cells. Active transport of ions into the root has the additional advantage of encouraging more absorption of water by osmosis. Water and ions eventually enter the vascular bundle, where xylem carries them upward through the stems to the leaves. Xylem cannot actively “carry” water; its cells are dead. Transpirational pull, adhesion, and the cohesion of water molecules contribute to the upward movement of water through the vascular tissue. Are root hairs adequate structures for absorption of soil nutrients? Perhaps not. The roots of many plants form associations with certain species of fungus, often to the beneﬁt of both parties. Known as mycorrhizae (myco- = fungus; rhiza = root), these associations provide the fungus with a source of carbohydrates, and the plant roots with greatly increased absorption powers. Some fungus species grow over plant roots and between cells, and others actually enter the root cells ( Figure below). Some botanists estimate that as many as 95% of all plant families have mycorrhizal associations. Based on fossil evidence, many hypothesize that plant-fungal relationships preceded and were vital to the evolution of land plants.

By some estimates, nearly 95% of all plant families form relationships between their roots and species of soil fungi, benefiting from the fungal powers of absorption. These mycorrhizal associations involve penetration of root tissue by the fungal partner. In the flax root cells above, the fungus has penetrated individual cell walls, as well. Another type of association involving roots contributes to a very different process of nutrient absorption. In some families of plants such as legumes (the Pea Family), root tissue and certain bacteria together form root nodules, special root structure which house the bacteria ( Figure below). Bacteria in the nodules chemically transform (“fix”) nitrogen from the atmosphere into a form which can be taken in and used by the plants to build amino acids and protein, hence this process is called nitrogen fixation. Such associations give plants a significant competitive edge in low-nitrogen environments; this explains why clover, which has root nodules, grows faster than grasses, which do not, in unfertilized lawns.

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In some plants such as members of the pea family, roots together with certain bacteria develop root nodules in which atmospheric nitrogen is “ﬁxed” so that the plant can absorb and use it.

Anchoring and Support of the Plant

The architecture of root systems (taproots and ﬁbrous systems, each with primary and secondary roots) provides most of the anchoring and support for the plant, and as you learned in the section on types of roots, some roots are specialized for support (as in mangroves and kapok trees). However, the structure of certain cells and tissues also contributes to these functions. In many plants, woody secondary xylem tissue adds strength to individual roots. The bark-like secondary covering which replaces the epidermis in older roots adds toughness to the surface of the roots and improves their ability to anchor plants.

Food Storage, as for Over-wintering

The ground tissue in roots is often specialized for food storage ( Figure below). Sugars synthesized in the leaves flow down to the roots through the vascular phloem cells to the thin-walled parenchymal cells of the ground tissue, which store the sugars as starch. Many biennial plants, such as carrots and parsley, spend their first summer photosynthesizing and growing, store the food they have produced in the roots over the first winter, and then use those sugars to produce flowers and seed during their second summer. Some perennial plants store food in the roots so that they can bloom quickly in the early spring, before annuals have had a chance to grow.

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The ability of roots to store food througout the winter in ground tissue parenchyma allows biennials and perennials to “bank” one year’s food production for use in the next. (left) Parsley produces leaves during the first summer, and stores the sugars they produces as starch in a tap root over winter. The following summer, the stored food energy is used to produce flowers and seeds. (right) Bloodroot is an early-blooming perennial. Leaves grow large later, in the summer; the sugar they produce is stored in a thick root over the winter and used to grow flowers which often bloom before the snow is gone.

Vocabulary

• epidermis: The outermost layer of skin, composed of epithelial cells; the outermost layer of a plant.

• mycorrhiza (plural, mycorrhizae): A symbiotic association between a fungus and the roots of a plant.

• phloem: Vascular tissue which transports food from leaves to storage or growth areas in other parts of the plant; includes sieve tube elements and companion cells.

• root nodules: A structure formed by roots and speciﬁc bacteria; site of nitrogen ﬁxation.

Summary

Practice

Use this resource to answer the questions that follow.

Review

189 1.31. Root Growth - Advanced www.ck12.org

1.31 Root Growth - Advanced

• Compare and contrast the embryonic tissues which accomplish primary and secondary growth in roots.

Root Growth

Roots demonstrate both primary growth (in length or depth) and secondary growth (in width or diameter). As you probably predicted, the actively dividing cells, or meristems, control plant growth. Primary (apical) meristems, the dividing tissue at the tips of the roots, grow longer roots, and secondary meristems, the dividing tissue seen in the cross-sections, grow thicker roots. Refer to Figures in the Roots: Structure (Advanced) concept to visualize how the structures accomplish the process of growth. The root cap secretes the lubricating gel coating and protects the apical meristem from mechanical damage as its cells divide. As the root cap cells die, division in the meristem cells replaces them. Meristem cell division also produces the cells behind the meristem, and these gradually elongate to add new root tissues –and form longer roots which reach deeper into the soil. Statoliths in the statocyte cells in the root cap detect gravity (did you predict this?) and control the orientation of the root as it grows. Secondary meristems, the vascular cambium between xylem and phloem and the cork cambium external to the phloem, add girth to many types of roots ( Figure 1.54). As cells in the vascular cambium divide, those toward the interior differentiate to become secondary xylem, and those toward the surface become secondary phloem. Both add to the diameter of the root, and eventually the epidermis and ground tissue are sloughed off. At this point, the cork cambium begins to divide; it, too, adds cells both medially and laterally. Outer lateral cells become cork, often ﬁlled with waxy suberin. Older roots thus thickened by secondary growth no longer contribute signiﬁcantly to absorption, but they continue to transport water and minerals through their xylem and sugary sap through their phloem. Their secondary growth may also improve their ability to anchor and support the plant, especially if the additional xylem is woody, or increase their ability to store food, as in sweet potatoes, which are tuberous roots thickened by secondary growth.

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FIGURE 1.54 Secondary growth in roots, as in Tax- odium distichum, can produce thicker woody tissues for stronger anchoring and support (left), or additional space for food storage, as in the tuberous roots of sweet potatoes (right).

Structure-Function Relationship: Roots

The following table summarizes the structures which make up roots and their functions. As is so often the case throughout the living world, form clearly determines function. Refer to the Root Structure: Form Reﬂects Function Table to review this relationship for each root structure.

TABLE 1.6: Root Structure: Form Reﬂects Function

Name of Structure Form Function Apical meristem Undifferentiated, rapidly dividing Growth in length/depth cells near the tip of the root Epidermis Single layer of thin cells covering Absorption of water and ions, pro- the root, often with root hairs tection Parenchyma Thin-walled, loosely packed cells in Storage of starch and/or water ground tissue and root caps Phloem Elongated cells, living but without Transport of sugars in sap nucleus or ER, connected end to end by sieve plates and plasmodesmata Root Cap Layer of parenchyma covering root Lubrication and protection during tip, covered with gel growth Root hairs Thin projects from the epidermis Increasing surface area for absorp- into the soil tion of water and minerals Statocyte Cell containing statoliths located in Detection of root orientation root cap parenchyma Statolith Organelle containing starch, free to Moving within the cell in the direc- move about within the cytoplasm of tion of “down” a statocyte Suberin Waxy substance secreted by epider- Waterprooﬁng mis and cork cambium Vascular cambium Undifferentiated, rapidly dividing Growth in diameter cells between the xylem and phloem Xylem Elongated cells with thickened, pit- Absorbing and transporting water ted walls, connected end to end and ions

Vocabulary 191 • cork cambium: Undifferentiated, rapidly dividing cells which replace sloughed epidermis and cortex.

Summary

Practice

Use this resource to answer the questions that follow.

Review 1.32. Stems - Advanced www.ck12.org

1.32 Stems - Advanced

• Deﬁne the structure and function of plant stems.

Stems

The oldest known individual living organism was a Nevada (USA) Bristlecone Pine tree named Prometheus, un- fortunately cut down (in its prime?) in 1964. An upper section of Prometheus’ trunk boasted 4844 rings, leading botanists to estimate its age at over 5000 years. Its oldest surviving relative is a 4839-year-old California youngster of the same species, the aptly named Pinus longaeva. This living individual germinated in 2832 BC, and recently bears the less fitting name Methuselah, after the Biblical man reputed to have lived (only!) 969 years. How does this species muster record longevity? Lest age not sufficiently impress you, the tallest known organism on Earth, identified in 2006, is a Coastal Redwood in Northern California, measuring 115.55 meters tall. That’s 379.1 feet of plant reaching into the sky, which perhaps explains why it has been named Hyperion, after a Greek god of the sun. How did it grow so tall? One more record: the largest known individual living organism in the world, measured by volume and mass, is yet again a tree, this one a Giant Sequoia in California named “General Sherman” back in 1879 (or briefly at the end of the century, “Karl Marx”, by a short-lived nearby utopian society). General Sherman’s maximum diameter is over 11 meters (36.5 feet), its trunk volume is 1487 cubic meters, and its trunk alone is estimated to weigh more than 2000 tons. This tree, however, doesn’t measure up to a Coastal Redwood felled by a storm in 1905 –which boasted a trunk volume of over 2500 m3 and a mass of more than 3600 tons. The blue whale (record weight 210 tons) pales in comparison, despite the support of its watery environment.

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The oldest, tallest, and largest (by mass and volume) individual organisms living on Earth are trees. Bristlecone Pines (top left) have been aged at over 5000 years. Coastal Redwoods (right) are tallest at up to 115 meters. The General Sherman Giant Sequoia (lower left) has a volume of 1487 cubic meters and weighs more than 2000 tons, although a Coastal Redwood which fell in 1905 far surpassed it in both volume (over 2500 m3) and mass (over 3600 tons). It is no accident that the record-holders of the living world are trees, and their trunks –basically stems –are major reasons for their success (http://www.arboretum.harvard.edu/programs/tree_basics.swf ). The history of the word “stem” can be traced back to roots (!) originally meaning “stand”, and “standing” is indeed the importance of stems to trees as well as other plants. In nearly all terrestrial plants, stems are the organs which “stand” and spread leaves, ﬂowers, and fruits in their non-supportive, airy environment. Having distanced these aerial plant parts from their source of water and minerals, stems must transport these vital substances up from the soil - without pumps, buckets, or muscles. Stems must also ferry food away from the leafy factories which produce it to grow and maintain roots, ﬂowers, seeds and fruits as well as new leaves –and new stems! For new growth, stems not only provide materials, but also house the stem cells, which can divide to form new tissues and organs. In many plants, stems perform the additional function of storing food or water supplies against drought or famine. The Stem concepts will explore the structures which build stems and the ways in which these structures carry out these stems’ functions (http://www.b

193 1.32. Stems - Advanced www.ck12.org cb.uwc.ac.za/ecotree/index.htm#top ).

Vocabulary

• Pinus longaeva: The Great Basin bristlecone pine, a long-living species of tree found in the higher mountains of the southwest United States.

• stem: The above-ground part of a plant which bears leaves, ﬂowers, fruits, or cones.

Summary

Practice

Use this resource to answer the questions that follow.

• http://www.hippocampus.org/HippoCampus/Biology?loadLeftClass=Course&loadLeftId=37&loadTopicId=38 74

1. Which organs comprise the plant’s vegetative system? 2. Where are stems located? 3. What are the roles of stems?

Practice Answers

1. A plant’s vegetative system consists of roots, stems, and leaves. 2. Stems are located above ground. 3. Stems link the roots and leaves, elevate and support the plant, and store food and water.

Review

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1.33 Stem Types - Advanced

Stem Diversity

In most vascular plants, stems ( Figure below) are above-ground parts which bear leaves, ﬂowers, fruits, or cones at nodes. The buds which form at nodes can also produce additional stems. Internodes are lengths of stem which separate the nodes and the structures they bear. The next times you eat a stalk of asparagus, note the terminal bud (at the tip), nodes and internodes of this tasty stem vegetable. We’ve listed the major functions for stems above, but there are many types of stems which are adapted to varying environments and special functions. The aforementioned trunks, for example, add extensive wood and bark to form a primary support for trees, which can then outcompete nonwoody plants for sunlight and grow for hundreds or even thousands of years. Herbaceous plant stems lack wood, and die at the end of the growing season. Some trunks and trunk-like stems store water against drought in arid climates ( Figure 1.56). The trunks of baobabs in Africa, Madagascar, and Australia are an extreme example, storing up to 120,000 liters (32,000 gallons). The stems of cactus plants, native to the Americas, are thick and hard-walled but ﬂeshy, storing water when it rains.

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FIGURE 1.55 The stems of vascular plants are divided into nodes, where buds can produce leaves, ﬂowers, fruits, cones, or additional stems, and internodes, which separate the nodal structures and communicate/transport between them.

Because cactus’ leaves are reduced to protective spines to minimize water loss, their swollen stems also function as primary photosynthetic organs. A thick, waxy coating (the cuticle) covers the stems to prevent water loss, and stomata dot the epidermis to facilitate gas exchange for photosynthesis. Some species have adopted near-spherical shapes, maximizing volume for water storage while minimizing surface area for water loss and protecting the body from excessive sunlight.

FIGURE 1.56 Baobab trees (left) store up to 120,000 litres (32,000 gallons) of water in their trunks. This individual, a hollow baobab in Western Australia, is reputed to have been used to lock up indigenous pris- oners on their way to sentencing during the 1890s. Stems of cacti store water and have taken over photosynthetic duties from the leaves, which are reduced to spines. The two species shown here maintain near-spherical shapes to minimize surface area for water loss and maximize volume for water storage.

Avoiding the energy investment of trunk-building, some plants develop long, flexible climbing stems which cling to and wrap around other plants or structures, forming vines ( Figure 1.57). Tendrils may be modified, threadlike stems (although they may also derive from petioles or leaves) which which twine around and cling to surfaces in order to anchor and support vines as they grow. Corms ( Figure 1.58) are short, upright but underground stems which store food; they may appear similar to bulbs, but bulbs store food in modified leaves, and will be discussed in the next lesson. Corms are indeed protected by thin, papery modified leaves, but when cut through, the flesh is solid rather than layered. Examples of plants which form corms include the crocus, taro, and gladiolus. Although the plant stores food in the corm as a hedge against drought, famine, or winter, humans cultivate some plants to harvest the corm as a starchy vegetable; taro is thought to be one of the earliest cultivated plants. Rhizomes ( Figure 1.59) are horizontal, usually underground stems. The advantage of this growth form is the potential for asexual reproduction. At nodes spaced relatively close together, rhizomes send out new roots downward and new shoots upward –a form of reproduction by cloning. Ferns, asparagus, ginger, nettles, and Lily of the Valley

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FIGURE 1.57 Vines avoid the energy costs of building trunks, opting for thin, ﬂexible climbing stems which cling to and wind around other structures (including, of course, tree trunks). (left) Vines climb a palm. (right) Vines have ﬂexible, climbing stems.

FIGURE 1.58 Corms are basal or underground stems which store food in stems for plants to aid survival during drought, heat, or cold. The familiar crocus ( left) grows from corms. We cultivate taro ( right) in order to harvest the starchy corm as a vegetable.

have rhizomes. Stolons ( Figure 1.60) are very similar to rhizomes, but these stems are often above ground or just below the surface, and their nodes are typically spaced further apart, resulting in greater “dispersal” of “offspring”. Strawberries and spider plants readily reproduce using stolons.

FIGURE 1.59 Horizontal underground stems known as rhizomes allow asexual reproduction of plants through the growth of new roots and shoots at nodes. Note the nodes which will give rise to new plants in the rhizomes of nettle ( left). Rhizomes of ginger ( right) provide one of our most popular spices.

Some plants modify stolons or parts of stolons for food storage, forming tubers ( Figure 1.61). Potatoes are the most familiar tubers, recognized in their scientiﬁc name, Solanum tuberosum. Potato “eyes”, spiralling around the

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FIGURE 1.60 Strawberry plants ( left) and spider plants ( right) reproduce asexually at the nodes of stolons, stems which “run” above ground –or in the case of spider plants, reach into the air.

tuber, are the nodes. You may have seen the eyes “sprout” shoots; roots are produced by the shoots after they reach the surface. We humans intercept the potato plant’s store of food, although its purpose for the potato plant is to fuel the following year’s growth and reproduction.

FIGURE 1.61 Potatoes are a kind of stem tuber, a section of a stolon which stores food for the following year’s growth. The “eyes” are the stem’s nodes, which spiral around the potato and sprout shoots, as shown here. When the shoots reach the surface, they begin to form roots.

Many plants, especially desert plants, reduce some of their stems to produce rigid, rounded, sharply-pointed woody thorns ( Figure 1.62) which protect water, food, or woody tissue supplies. Certain plants more famous for their “thorns”, such as roses and cacti, do not have true thorns. Roses have prickles, which are highly modiﬁed epidermal cells. Cactus spines are modiﬁed parts of leaves, to be discussed in the next lesson.

Vocabulary

• bud: Embryonic tissue in the axil or at the tip of a plant, which can develop into new leaves, ﬂowers, or stems.

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FIGURE 1.62 Many plants, such as the honey locust trees, modify stems to produce thorns, which protect the plant from herbivores. Although roses ( below) are famous for their “thorns,” botanically they have “prickles”, instead. Prickles are primarily outgrowths of the epidermis, rather than modiﬁed stems.

• corm: Short, upright but underground stems which store food.

• cuticle: A thick organic layer surrounding the outer surface of nematodes and arthropods; a waxy waterproof covering over the aerial surfaces of a plant.

• herbaceous plants: Plants whose stems lack wood, and therefore die back at the end of the growing season.

• internode: The section of a plant stem between nodes.

• node: The place on a plant stem where a leaf is attached.

• prickle: A small, sharp outgrowth of epidermis or bark; for defense.

• rhizome: A modiﬁed subterranean stem of a plant that is usually found underground, often sending out roots and shoots from its nodes.

• spine: A rigid, slender, sharp-pointed modiﬁed leaf, arising from below the epidermis; for defense.

• tendril: Modiﬁed, threadlike, ﬂexible stems, petioles, or leaves which anchor and support vines.

• thorn: A rounded, rigid, woody modiﬁed stem with a sharp point; functions for defense.

• trunk: The main structural support for a tree; a stem which has added extensive woody tissue and bark.

• tuber: A stolon or section of a stolon modiﬁed for food storage and reproduction, such as a potato.

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Summary

Practice

Use this resource to answer the questions that follow.

Review

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1.34 Stem Structure and Function - Advanced

Stem Structures and Functions

Like roots, the stems of vascular plants are organs made of dermal, ground, and vascular tissues ( Figure 1.63). The cells and tissues should begin to sound familiar to you. As for roots and all other plant parts, the structures are closely related to their functions, so they will be treated together in this section. As you read and look at the diagrams, work to connect the form and location of each part with the job it carries out. Shape is usually related to purpose; function depends on form. There is no point to learning the name of a structure without learning its use, and you will understand much more quickly if you practice seeing connections between the two! In young and herbaceous plants, epidermis, a thin, single-celled layer of closely packed dermal tissue, protects and waterproofs the stem and controls gas exchange. Ground tissue is mostly parenchyma, and ﬁlls in around the

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FIGURE 1.63 A cross-section of a the herbaceous stem of a ﬂax plant shows these tissues:

vascular tissue; a central core of light-weight, parenchyma is the pith, and parenchyma outside the vascular tissue is the cortex. In most plants, pith and cortex cells function to support the plant. You may remember that these parenchymal cells closely resemble generalized plant cells, with large vacuoles. When these large vacuoles are filled with water, they push outward toward the cell walls, making them taut in the same way blowing up a balloon causes it to assume a firm shape. You can see the importance of the cortex-and-pith water-support system in flowers or plants you’ve forgotten to water or celery stalks which haven’t been kept moist; they “wilt” or go limp. In some plants, cells of the cortex contain cloroplasts for photosynthesis; in others, these cells may function to store starch. As in roots, vascular tissue functions for transport and support. Thick-walled, semi-porous, straw-like columns of dead xylem cells transport water and mineral ions upward under the force of transpirational pull. The cohesion- tension theory explains how water moves in xylem. The whole process is similar to sucking liquid up through a straw. A continuous flow of liquid is required for the process to be effective. As water is removed from the leaves, it pulls other water molecules up with it:

1. Water is removed from the leaves –through chemical processes such as photosynthesis, or, more often, by evaporation (transpiration). Transpiration provides the major force or tension. 2. Water moves from high (water) concentration in the xylem into the low-water leaf, creating a negative pressure in the xylem. Although it is a less important force, osmosis may carry water from high (water) concentrations in the soil to low water concentrations in root tissue and force a watery solution into the xylem, creating a positive root pressure. Both types of pressure work in the same upward direction. 3. Polar water molecules attract one another, forming hydrogen bonds (cohesion). The combination of transpirational pull, root pressure, and narrow xylem tubes allow water molecules to maintain their hydrogen bonds, each molecule pulling the next up from the xylem in an unbroken chain. 4. Eventually, the chain of rising water molecules leaves the roots low in water, causing more water to move by osmosis from the soil into the root tissue and xylem.

Living phloem cells (sieve tube elements), connected by sieve plates and plasmodesmata and bordered by companion cells, carry food from the leaves (sugar sources) to growing parts of the plant or to roots, for storage (sugar sinks). The difference in sugar concentrations between sources and sinks creates a concentration gradient which leads to bulk flow of sugar solution, according to the pressure flow hypothesis. Pressure flow involves the following sequence of events:

1. Photosynthesis produces sugars, creating a high concentration of sugar in the leaves (or stem): a sugar source. 2. Active transport carries sugars into companion cells in the leaf phloem, where the highly-concentrated sugar diffuses into the low-sugar sieve tube cells.

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3. Osmosis draws water into the sugar-loaded sieve tube cells, creating pressure. 4. Elsewhere in the plant, perhaps in a developing fruit, sugar is actively transported out of the sieve tubes, leaving a lower concentration, or sink. 5. Water follows the sugar by osmosis, so that sieve tube pressure drops near the sink. 6. Driven by the pressure gradient between source and sink, water moves through sieve tubes and their connecting plasmodesmata, from source to sink.

Arrangement and composition of the vascular tissue differs among plants; two major groups are recognized within the ﬂowering plants. Monocots (the name refers to the single cotyledon, or seed leaf - see Figure below) have bundles of vascular tissue scattered throughout the cortex. Monocots rarely develop woody xylem, although they may grow in diameter by expanding parenchyma or thickening primary meristem (recall that meristem tissue is capable of dividing and differentiating, like stem cells). Monocots include orchids, bananas, palms, bamboo, grasses, and grains such as rice, wheat, and corn.

Monocots (left) have a single seed leaf, and while eudicots have two (right). Although the term “cotyledon” refers to seed leaves, these are still visible as the seeds sprout.

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Dicots (two seed leaves –compare in Figure above), which include roses, mints, daisies, maples, oaks, and many more ﬂowering plants and trees, have vascular bundles arranged in a ring. These familiar dicots are now known as eudicots. In woody eudicots and some nonwoody eudicots, secondary growth (growth in diameter) is mediated by two secondary meristems, vascular cambium and cork cambium. Vascular cambium between the xylem (internal) and the phloem (external) forms a continuous cylinder to connect the vascular bundles, and divides to produce secondary xylem to the inside and secondary phloem to the outside. Secondary xylem, which contains the cross- bridging molecule lignin, is wood. Cells of the cambium divide rapidly early in the year to form less dense “early wood” which contrasts visibly with the slower-growing, more dense “late wood”; the result is an annual or growth ring ( Figure below). Counting growth rings allows aging of trees, as for the Bristlecone Pines mentioned at the beginning of this lesson.

A cross section of a Coastal Redwood tree cut in 1934 shows annual rings labeled with major historical events from 1215 to 1620 AD: Rings form annually because rapid growth early in the year produces xylem cells which are less dense and lighter in color, in visible contrast to darker, more dense cells later in the year.

1620: Pilgrims arrive at Plymouth Rock 1579: Frances Drake lands in California 1513: Balboa “discovers” the Paciﬁc Ocean 1300: Aztec Civilization, Mexico 1215: Magna Carta signed 1066: William the Conqueror

As trees age, inner xylem dies; usually this wood darkens visibly as well, becoming heartwood ( Figure below). Sapwood, the newer, outermost layers of xylem, remains active in transport of water and nutrients.

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A cross section of the trunk of a yew shows living, light-colored sapwood and darker, nonliving heartwood, which may remain active in fighting disease. The central dark center is the pith. How old was this tree when it was cut? Easily confused with but distinct from these are the terms hardwood, which refers to flowering plant trees (such as maples, oaks, and birches), and softwood, which refers to cone-bearing trees (such as pines, spruces, and firs). The latter terms are general, and do not accurately describe all species. As was discussed in the lesson on plant cells and tissues, the primary difference between flowering plant xylem and conifer xylem is the presence of wider, more efficient vessel elements in the former, and the predominance of narrower tracheids in the latter. As the stem increases in size due to new woody tissue formation, the epidermis and cortex erode and cork cambium, a secondary meristem, develops just outside the phloem. Cork cambium divides to form waterproofing, disease- resistant cork cells externally and sometimes an additional layer internally. Cork is made mostly of waxy suberin, which protects against dehydration. Within the cork layer, loosely packed cells called lenticels may allow gas exchange through the new “skin” formed by the cork cambium. In woody plants such as shrubs and trees, the thickened cork (which is often dead in older branches and the trunk), the cork cambium, and phloem form the familiar tough external bark, which may contain useful chemicals such as aspirin, antiseptic tannins, the spice cinnamon, or the antimalarial quinine.

Vocabulary

• bark: A protective, multi-layered outer covering of the stems and roots of many older woody plants, which replaces the epidermis.

• cohesion-tension theory: Theory that describes the movement of water and minerals up through the xylem (due to evaporation of water from leaves and the cohesion and adhesion of water molecules below).

• companion cells: Cells in the phloem (of a plant) which support and load the conducting sieve tube elements cells.

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• cortex: The outer portion of the stem or root of a plant.

• dicot: A ﬂowering plant with two embryonic seed leaves or cotyledons that usually appear at germination; dicotyledon.

• eudicots: Vascular plants which produce seeds that develop from ﬂowers and are enclosed in fruits.

• monocot: A ﬂowering plant with one embryonic seed leaf or cotyledon that usually appears at germination; monocotyledon.

• organ: A structure composed of two or more tissues that work together for a common purpose.

• phloem: Vascular tissue which transports food from leaves to storage or growth areas in other parts of the plant; includes sieve tube elements and companion cells.

• pith: Ground (parenchymal) tissue internal to the vascular tissue, in the center of a stem or root.

• plasmodesmata (singular, plasmodesma): Microscopic channels which traverse the cell walls of plant cells; enables transport and communication between them.

• pressure ﬂow hypothesis: Explains transport of sap by phloem according to differences in sugar concentration between sugar sources and sugar sinks.

• primary meristem: Embryonic plant tissues which allow growth in length or height.

• secondary growth: Growth that results from cell division in the cambia or lateral meristems (of a plant); causes the stems and roots to thicken.

• secondary meristem: Embryonic plant tissues which allow for growth in diameter; also known as lateral meristem.

• sieve plates: Pores in plant cell walls that facilitate transport of materials between them; found at the interface between two sieve tube members.

Summary

Practice

Use this resource to answer the questions that follow.

Review

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1.35 Stem Growth - Advanced

Stem Growth

Stems, like roots, may grow in two different dimension. All vascular plants grow in length or height through primary growth. Woody and many herbaceous dicots grow in diameter through secondary growth. Monocots lack true secondary growth, so that very few are woody, but some grow in diameter through division of other tissues. Primary growth can occur at the terminal bud (tip) and at axillary buds (buds at the nodes just above the leaf stems). Primary (apical) meristems, like our stem cells, retain cells which can continue to divide and differentiate, allowing primary growth to continue indeﬁnitely. Cells in the apical meristems in buds can differentiate to form leaves, sepals, petals, stamens, ovaries or new stems. Terminal buds of stems grow faster than buds on lateral

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branches, a phenomenon known as apical dominance. Pruning trees and shrubs removes these terminal buds and encourages growth of side branches. Secondary growth ( Figure 1.64) occurs in the interior of dicot stems in secondary (lateral) meristem tissues called cambium. Vascular cambium forms between primary xylem and phloem and divides to form lignin-strengthened secondary xylem (wood) internally and secondary phloem externally. As the stem grows in diameter, the epidermis and often the cortex are eroded and sloughed off, and a cork cambium forms external to the phloem. The cork cambium secretes protective layers of cork externally, forming bark.

FIGURE 1.64 Secondary growth involves two types of secondary or lateral meristems. Vascular cambium divides to produce secondary xylem internally and secondary phloem externally (top right). Eventually, epidermis and cortex slough off, and cork cambium develops, producing cork (middle right), which may contribute to a thickened, tough layer of bark (bottom right). The secondary xylem forms the wood of trees and woody shrubs.

Because the meristem tissue continually renews itself as well as dividing and differentiating, plants can continue to grow indefinitely, unlike humans which reach a definite height at the end of adolescence. However, the growth of many plants is limited by seasonal patterns of growth. As you learned in the lesson on plant cells and tissues, annuals die at the end of a single growing season; new individuals grow from seed the following year. Biennials grow from seed one season, store food over their first winter, and then use that food to grow flowers and reproduce during their second growing season. Perennials may lose their leaves or go dormant during the winter, but they continue to grow from season to season. Amd some of these perennials, supported by their sturdy stems, may live as long as 5000 years, grow as tall as 115 meters, or accumulate a mass of as many as 3600 tons!

Structure-Function Relationship: Stems

TABLE 1.7: Stem Structure

Name of Structure Form Function Cuticle Thick, waxy coating Prevents water loss 208 www.ck12.org Chapter 1. Plant Biology - Advanced

TABLE 1.7: (continued)

Name of Structure Form Function Tendril modiﬁed, threadlike stems (some- Anchor and support times petioles or leaves) which which twine around and cling to surfaces Corm short, upright but underground Store food stems Rhizome horizontal, usually underground Anchoring, asexual reproduction stems Stolons Stems which run above ground or Asexual reproduction, dispersal of just below the surface, with nodes “offspring” are typically spaced further apart than rhizomes Tubers Modiﬁed stolon or part of stolons Food storage for following year’s for food storage growth Thorns reduced stems having rigid, Protect water, food, or woody tissue rounded, sharply-pointed woody tip supplies Epidermis a thin, single-celled layer of closely Protects and waterproofs the stem packed dermal tissue and controls gas exchange. Pith central core of light-weight, Support, especially for herbaceous parenchyma plants Cortex parenchyma outside the vascular tissue Support, especially for herbaceous plants Xylem Elongated cells with thickened, pit- Absorbing and transporting water ted walls, connected end to end and ions Phloem Sieve tube elements connected by Carry food from the leaves to grow- sieve plates and plasmodesmata and ing parts of the plant or to roots, for bordered by companion cells storage Primary meristem Stem cell-like tissues found in buds Dividing and differentiating to pro- and tips of branches duce growth in length/height (primary growth) Vascular cambium Undifferentiated, rapidly dividing Growth in diameter cells between the xylem and phloem Cork cambium Undifferentiated, rapidly dividing Producing cork/bark cells which replace sloughed epidermis and cortex Lignin Highly cross-branched polymer Strengthens cell walls, xylem, and found in cell walls of woody plants wood Suberin Waxy molecule which coats leaves Protects against dehydration and stems Bark Thickened cork (dead in older Protection against dehydration, branches and the trunk), the cork abrasion, and infection cambium, and phloem

Vocabulary

• apical meristem: Embryonic plant tissues which allow growth in length or height.

• cork cambium: Undifferentiated, rapidly dividing cells which replace sloughed epidermis and cortex.

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• primary meristems: Embryonic plant tissues which allow growth in length or height.

• secondary meristem: Embryonic plant tissues which allow for growth in diameter; also known as lateral meristem.

Summary

Practice

Use this resource to answer the questions that follow.

Review

210 www.ck12.org Chapter 1. Plant Biology - Advanced

1.36 Leaves - Advanced

Leaves

Meinrad Craighead stated that the roots held the power of life, but a strong argument can be made that it is the leaves of plants which are the essential parts of our “green inheritance.” Support for leaves as key to not only plant life but also the entire living world, includes:

• The primary role of leaves is photosynthesis, which stores carbon and energy in a form which is food for plants –and food for all Life. • Photosynthesis is virtually the only ultimate source of food (stored, usable chemical energy and carbon) for Life on Earth. • A primary role of roots is to absorb water - to provide hydrogen and oxygen atom for photosynthesis. • A primary role of stems is to transport that water (as well as essential ions) to leaves. • Both roots and stems have the responsibility of supporting leaves so that they can collect sunlight and CO2 –for photosynthesis. • Photosynthesis produces and releases O2 gas, restoring that 20% of the atmosphere. • Oxygen from photosynthesis helps to maintain the ozone layer, which absorbs UV radiation which could otherwise damage DNA. • Photosynthesis absorbs CO2 from the atmosphere, limiting the effects of that greenhouse gas. • Leaves release massive amounts of water vapor by transpiration, affecting humidity and rainfall. • Leaves color and design our natural living world, providing habitat for Life.

"With critical roles in food production, habitat design, and regulation of Earth’s atmosphere, temperature, and water and carbon cycles, leaves are indeed our “green inheritance. . . without [which] we will surely perish.” How does the structure of a leaf help it to carry out photosynthesis –a process no animal or fungus has ever learned? In the Leaves concepts, we will analyze the tissues, specialized cells, and some organelles of the leaf organ to discover how their form and function contribute to photosynthesis. We will look at basic leaf types and variations on

211 1.36. Leaves - Advanced www.ck12.org the basic themes. You may want to refer to the concepts on photosynthesis to synthesize the chemical and cellular processes with tissue- and organ-level activities. At this point in your study of biology, you should be able to relate them in a continuum of structure and function which characterizes the Plant Kingdom.

Vocabulary

• leaf: An organ of a vascular plant; typically, a thin, ﬂattened above ground organ specialized for photosynthesis.

• photosynthesis: The process by which carbon dioxide and water are converted to glucose and oxygen, using sunlight for energy.

• transpiration: A process by which plants lose water; occurs when stomata in leaves open to take in carbon dioxide for photosynthesis and lose water to the atmosphere in the process.

Summary

Practice

Use this resource to answer the questions that follow.

• http://www.hippocampus.org/HippoCampus/Biology?loadLeftClass=Course&loadLeftId=37&loadTopicId=38 74

1. Which organs comprise the plant’s vegetative system? 2. Describe the main role of the leaf. 3. What anther roles do some leaves perform?

Practice Answers

1. A plant’s vegetative system consists of roots, stems, and leaves. 2. Leaves are primarily used for producing food via photosynthesis and removing metabolic wastes. 3. Leaves can also be specialized for protection and reproduction.

Review

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1.37 Leaf Types - Advanced

Types of Leaves: Diversity and Adaptations

You have probably noticed two truths about leaf forms:

1. An amazing diversity of leaf forms exists ( Figure 1.65). 2. Within individuals and species, leaf forms and shapes are surprisingly uniform.

As vital photosynthetic organs, leaves of different species vary in shape, texture, size, and arrangement. Clearly, there is no single way to solve the problems of collecting solar energy to make food. As you might expect, most of these differences relate closely to differences in purpose and environment, reinforcing yet again the relationship between form and function. Ferns are among the oldest of vascular plants, among the first to adapt to terrestrial environments with veins of xylem and phloem to carry water, minerals, and food between roots and aerial leaves. Fronds are their finely divided leaves, which bear spore-producing structures beneath their photosynthetic surfaces. Because the stolon or rhizome stems of ferns run horizontally across or underneath the ground, fronds often stand upright, supported by their rigid, often woody, veined central stems. Although the structure of a frond is quite similar to that of a flowering plant leaf, it develops differently, unfurling as a “fiddlehead” early in the spring. Representative of the earliest true leaves, the microphylls of club mosses have just a single vein. The leaves of ferns are mostly upright fronds, whose veins of vascular tissue allowed these plants to be among the earliest to colonize dry land. The noble fir, like most conifers, bears thin, needle-like leaves which conserve water and shed snow Figure 1.66; this particular species has flattened needles limited to two rows, maximizing their exposure to sunlight. A few cone-bearing species, such as cedars, have tiny, scale-like leaves. Among the flowering plants, grasses have some of the most specialized leaves. A sheath attaches the long, narrow blade to the hollow stem. Gritty textures and growth centered at the base of the blade rather than the tip adapt grasses to heavy grazing by herbivores. Evolutionarily older that the ferns are the club mosses, whose microphylls (“tiny leaves”) have a single small vein each, perhaps representing the very first true leaves.

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FIGURE 1.65 An amazing diversity of leaf shapes, mar- gins (edges) and patterns of venation exists. This chart shows only some of the variations among ﬂowering plant leaves. Yet within individuals and species, leaves are remarkably uniform, suggesting that each feature is an adaptation for function and/or environment.

Most conifers have long, thin, needle-like leaves covered with a waxy cuticle. Such leaves conserve water in arid or wintry environments, and together with tree shape, often shed snow. These adaptations allow many conifers to retain their leaves throughout the year. The dark green color of many conifer needles may help them to absorb weak sunlight in northern climates or shady understories. Leaves of species that live in higher-sunlight or warmer climates often have yellow-green needles, and other species have needles covered with a whitish wax which reflects damaging ultraviolet rays. Some conifers such as cedar have scale-like leaves rather than needles. In flowering plants, most leaves have two basic parts: a broad, flat blade and a petiole, or leaf stem ( Figure 1.67). The petiole attaches to the stem at a node to form an axil, somewhat like the armpit for which it was named. Buds for new stems or leaves often form at the base of the petiole in the axil; these are appropriately called axillary buds, as opposed to terminal buds at the tip of the stem. The blade may be divided into leaflets (compound) or a single piece (simple). One way to tell whether you are looking at a simple leaf or a leaflet of a compound leaf is to look for this “armpit bud”; it usually lies at the base of the petiole, whether the leaf is simple or compound. If multiple leaf parts (leaflets) are found along the petiole extending below the axillary bud, the leaf is compound. If a single blade is found at the end of the petiole extending below the axillary bud, the leaf is simple. You learned in the last lesson that the stems of monocots differ from those of dicots; the same is true of leaves. Dicots have branching or netted veins, which allows many to have broader leaves, capturing more sunlight. Monocots too have “broad leaves”, but their leaves have parallel veins; many have long, narrow leaves. Figure 1.65 illustrates the parallel venation of monocots and several variations of branching or netted veins found in dicots. Sheath leaves of grasses, which are monocots, are specialized to withstand grazing. Long and narrow with parallel veins, they grow on hollow stems at nodes. The lower, sheath part of the leaf hugs the stem, and the upper blade often contains gritty, sandy particles which discourage herbivores and in some species can cut through human skin. A fringe of hairs or membrane keeps water and insects out of the sheath. Despite their grittiness and sharp edges,

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FIGURE 1.66 The noble ﬁr, like most conifers, bears thin, needle-like leaves which conserve water and shed snow; this particular species has ﬂattened needles limited to two rows, maximizing their exposure to sunlight.

grazing is a fact of life for grasses; their unique pattern of growing from the base of the blade, near the ground, rather than the tip, adapts them to withstand grazing –and lawn-mowing. By far the most successful plants today are the ﬂowering plants, and their diversity in leaf form within the general pattern described above merits further discussion. Leaves of ﬂowering plants have the basic structure shown in Figure 1.67, but vary in vary widely from one species to another:

Arrangement of Leaves on the Stem

In the alternate pattern, shown in Figure 1.68, single leaves emerge from each node, “taking turns” in two different directions. In the opposite pattern, two leaves emerge from every node, one on one side of the stem and the other directly “opposite” the ﬁrst. Whorled leaves emerge in groups of three or more from a single node, and are usually spaced evenly about the stem. Some plants have only a rosette of leaves on the ground at the base of the ﬂower stem. Most of these arrangements spread leaves out to maximize exposure to the sun under different conditions. Which leaf arrangement pattern do you think would best adapt a plant to a single, upright stem? Which two would spread leaves out on many horizontal branches? Which pattern would best take advantage of warmer air near the ground?

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FIGURE 1.67 The most diverse and abundant group of plants, Flowering Plants, have leaves with two basic parts: a broad, ﬂat blade and a petiole (leaf stem). The petioles attach to the plant stem at nodes, forming axils. New buds form in the axils.

FIGURE 1.68 The arrangement of leaves on a stem can be alternate (A), opposite (B), whorled (C), or a basal rosette (D). Leaf distribution can affect the amount of sunlight or heat the plant receives, and thus the rate of photosynthesis.

TABLE 1.8:

Leaf Arrangement Form and Function Basal rosette A circle or whorl of leaves at the base of the stem can take advantage of the warmer temperatures near the soil surface. Whorls Leaves encircling upright stems at intervals, as in horsetails, collect sunlight from all directions. Alternate or opposite branching Arranges ﬂattened layers of leaves along a horizontal stem to maximize sunlight. On smaller, upright stems, these patterns can also arrange individual or pairs of leaves at intervals to distribute “solar collection surfaces” evenly.

Divisions of the Blade

Simple216 vs. compound leaves were mentioned above, although their adaptive value was not emphasized. Compound leaves, shown in Figure 1.69 have the advantage of spreading out the leaf tissue while minimizing the surface area, effectively reducing wind resistance and water loss by evaporation. Desert plants such as mesquite often have compound leaves for this reason. Simple leaves have more surface area for absorbing sunlight, and are more often found in habitats where water loss is not a serious problem. Leaves may be pinnately compound, with each leaﬂet directly opposite another along a central vein, or palmately compound, having leaﬂets which radiate out from a single point. A few species, such as honey locust, have doubly compound leaves.

FIGURE 1.69 Many plants have leaves divided into leaﬂets, spreading out leaf surface with less water loss than for a similar-sized simple leaf (A). Types of compound leaves include pinnately compound (B), palmately compound (C), and doubly compound (D).

Additional Diversity in Leaf Adaptations

Botanists have developed an extremely long list of technical terms which describe structural details of plant leaves; these are helpful in identifying and classifying species, but too numerous for our purposes here. Websites at http://w ww.biologie.uni-hamburg.de/b-online/e02/02c.htm and http://www.cas.psu.edu/docs/CASDEPT/Hort/LeafID/ can provide you with further information if you are interested. In any case, be aware that leaf margin patterns, leaf tip and base shapes, leaf surface texture, and hairiness can influence many important factors for plants: insect and large herbivore resistance, water retention and shedding, solar and UV absorption and reflection, temperature, water loss, and probably a great deal more. A few of the many leaf-modification adaptations are highlighted below: www.ck12.org Chapter 1. Plant Biology - Advanced

Many plants have modified leaves for storage and protection of the stored food or water. Pebble Plants ( A) are succulents, with greatly thickened leaves for storing water in their hot, dry habitats. The pebble- or stone-like shape is also a camouflaging adaptation. Like all cacti, Teddy Bear Cholla ( B) has leaves modified into spines to protect the water supply stored in the stem, which has taken over the leaves’ former job of photosynthesis. Bulbs, such as these tulip bulbs ( C), modify leaves into thickened layers to store food underground during periods of dormancy. Although not for storage, deeply cut Philodendron leaves ( D) serve a protective function, lessening the effect of wind on the leaves’ large solar collectors.

Adaptations that limit transpiration and other forms of water loss • Hairs, by trapping moisture and reducing the evaporative effect of wind. • A waxy leaf surface. • Reduced numbers of stomata. • Compound leaves. • Reduction in leaf size. • Complete loss of leaves, assigning photosynthesis to the stems (the Cactus family). • Deciduous leaves: lost at the beginning of the dry season, and later re-grown. • Metabolic adaptations (CAM metabolism), discussed in the chapter on photosynthesis.

Adaptations for storing water and/or food: • Thickened, ﬂeshy leaves of succulents, such as the pebble plant. • Bulbs: short, upright but underground stems with layers of specialized, food- and water- storing leaves, such as onions and tulips.

Adaptations to reduce wind resistance: • Compound leaves.

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• Cut leaves, as the cut-leaf philodendron. • Basal rosettes: leaves limited to a circular cluster at the (warmer and less windy) ground.

Adaptations that regulate temperature: • Shiny leaves help to reﬂect the sun’s rays where light is intense and temperatures high. • Basal rosettes: leaves limited to a circular cluster at the (warmer and less windy) ground.

Adaptations that discourage herbivores from eating leaves: • Secretion or synthesis of oils, as in Eucalyptus. • Secretion or synthesis of poisons, as in Poison Ivy and its relative, the Cashew. • Inclusion of crystalline minerals (similar to the grittiness in stems). • Spines, as in cactus. • Hairs.

Adaptations that attract pollinators: • Colorful bracts: leaves modiﬁed into colorful petal-like patterns to attract insects, as in the familiar “ﬂowers” Poinsettia and Dogwood ( Figure 1.70)

FIGURE 1.70 In some species, leaves have been modified to attract pollinators –or even prey! The familiar “flowers” of Poinsettias ( A) and Dogwood ( B) are actually modified leaves: note the mass of tiny reduced flowers in the center of their colorful bracts, which serve to attract insects for pollination. Glands on the leaves of sundew ( C) secrete sticky substances which attract and then trap insect prey. The African genus Nepenthes (D) has leaves modified into elaborate, colorful traps, which in larger species may even capture small lizards or rats.

Adaptations that attract prey (Carnivorous Plants): • Glands which secrete sticky “dewdrops” to attract and trap insects, as in Sundew. • Colorful traps with triggering hairs which close over and digest insects, as for Venus’ Flytrap http://www.b otany.org/Carnivorous_Plants/ . • Pouches or pitchers which drown and digest prey, as in Nepenthes.

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The genus Nepenthes, which includes 120 species ranging from China to India and Australia, demonstrates arguably the most amazing leaf modiﬁcations. Tendrils at the ends of long petioles gracefully suspend colorful pouches, complete with lids to keep the rain out. Within, nectar glands secrete substances which attract prey and a syrupy ﬂuid which drowns victims. Waxes and/or hairs line the cavities, preventing victims’ escape. Additional glands digest and absorb nutrients. Larger species can catch small vertebrates, such as lizards and rats! A common name, Monkey Cup, derives from sightings of monkeys drinking from the pouches. Many carnivorous plants photosynthesize using traditional leaves, using predation to out-compete other species for nutrients (often nitrogen) which are scarce in certain environments (such as bogs). Let’s return from this world of elaborate variations on the theme of leaves to look at the intricate structure of a basic leaf, from which most of the above were derived.

Vocabulary

• axil: The angular space formed where a leaf’s petiole joins the stem.

• blade: The broad, ﬂat part of a typical leaf from a ﬂowering plant.

• bulb: Short, upright but underground stem attached to specialized, food- and water- storing leaves, as in onions.

• compound leaf: Describes a leaf whose blade divided into leaﬂets.

• deciduous leaves: Refers to leaves which are lost seasonally, either in winter or in dry seasons.

• fronds: The leaves of ferns; a large divided leaf.

• microphylls: Single-veined "tiny leaf" of club mosses, perhaps similar to the ﬁrst true leaves.

• petiole: The stem of a leaf.

• sheath leaves: Leaves of grasses, adapted to withstand grazing.

• simple leaf: Describes a leaf whose blade is all one piece.

• succulent: Plant which stores water in ﬂeshy leaves, stems, and/or roots as an adaptation to dry climate.

• tendril: Modiﬁed, threadlike, ﬂexible stems, petioles, or leaves which anchor and support vines.

• veins: Vessels that carry blood toward the heart; the vascular bundle in a leaf, containing xylem to transport water and minerals and phloem to transport food.

Summary

Practice

Use this resource to answer the questions that follow.

Review

219 1.38. Leaf Structure and Function - Advanced www.ck12.org

1.38 Leaf Structure and Function - Advanced

Leaf Structure and Function

Factories for Photosynthesis

A leaf is a highly organized factory –an organ constructed of several kinds of specialized tissues, each of which has its own duties. The product of the factory is no less than the food which supports nearly all life on Earth (although we must not forget that roughly half of Earth’s photosynthetic productivity is the province of algae and bluegreen bacteria, both evolutionary ancestors of plants.) In this section, we will explore how each tissue’s structure contributes to the production of food.

FIGURE 1.71 A cross section of a leaf shows that it is a complex organ built of several different kinds of specialized tissues. The tissues, in turn, are built of specialized cells, and the cells, of organelles.

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Epidermis covers the upper and lower surfaces of the leaf. Usually a single layer of tightly-packed cells, the epidermis mediates exchanges between the plant and its environment, limiting water loss, controlling gas exchange, transmitting sunlight for photosynthesis, and discouraging herbivores. The epidermis secretes a waxy cuticle of suberin, which restricts evaporation of water from the leaf tissue. This layer may be thicker in the upper epidermis compared to the lower, and in dry climates compared to wet ones. As noted in the section on adaptation, epidermal hairs can discourage herbivores, limit the effects of wind, and trap a layer of moisture to reduce water loss. The presence of the cuticle limits water loss, but also inhibits absorption of carbon dioxide and excretion of oxygen. These functions are served by stomata (singular, stoma), “little mouths” which regulate water loss, O2 release, and CO2 intake. In most leaves, stomata are more abundant in the lower epidermis, limiting water loss due to direct sunlight. More movement without muscles! How do they work?

An microphotograph of a stoma shows the two guard cells which regulate its opening and closure to limit water loss, excrete oxygen, and absorb carbon dioxide. The openings or pores in stomata are formed by two specialized sclerenchymal cells, the guard cells ( Figure above). As you will recall from our discussion of tissues, sclerenchymal cells have greatly thickened cell walls. In guard cells, the thickenings spiral around the cell, preventing them from increasing in diameter. Epidermal cells stitch the guard cells firmly together so that they can expand only by bowing apart, forming the opening, or pore. But what makes them expand? Have you guessed? Although the mechanism is not completely understood, it is known to harness the power of osmosis. In daylight or high humidity, when CO2 is needed for photosynthesis and water loss can be minimized, the guard cells use active transport to alter the concentration of hydrogen and potassium ions so that water flows into the cells. Pressure builds up, and expands the cells to open the pores. Closing the stomata is controlled by signals from roots when they sense water shortage. Under these conditions, the roots release a hormone, abscisic acid (ABA), which binds to receptors in the guard cell membranes and alters ion uptake and concentrations. Consequent reduction in cytoplasm ion concentrations leads to the loss of water by osmosis, the guard cells “relax”, and the pores close. Mesophyll (“middle leaf” –refer again to Figure above) includes the tissues which build most of the interior of the leaf. These tissues conduct most of the photosynthesis for most plants, so most are made of thinner-walled parenchymal cells or collenchymal cells with chloroplasts. In flowering plants and ferns, two different layers make up the mesophyll:

• The upper, palisade layer captures most of the sunlight and carries out most of the photosynthesis. The columnar cells of the palisade layer contain many chloroplasts. Slight but precise separations between the

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cells maximize availability of the raw materials for photosynthesis by allowing diffusion of CO2 and capillary movement of H2O. Leaves exposed to high levels of sunlight contain as many as ﬁve layers of palisade cells, while shade leaves may contain only one. • The lower spongy layer contains more rounded cells with fewer chloroplasts. The cells are loosely packed, separated by larger, airy spaces. This lower layer of cells is closely associated with the stomata, and the airy spaces allow diffusion of oxygen, water vapor, and carbon dioxide through the stomata when they are open.

The veins of leaves are made primarily of vascular tissue, surrounded by parenchymal pith and collenchyma. An upper layer of xylem transports water and minerals from the roots and stem into and throughout the leaves. Recall that xylem is made of dead cells, with heavily thickened but pitted cell walls. The cells are arranged end-to-end, straw-like, allowing hydrogen bonds between water molecules ( cohesion) to pull each other (and hitch-hiking mineral ions) through the xylem columns when stomatal evaporation begins the transpirational pull. Living cells of the lower layer of phloem, also arranged in bundles of straw-like columns but connected by sieve plates and plasmodesmata, transport sugars made in the leaves (“sugar sources”) to parts of the plants which need these fuels (“sugar sinks”). Recall that companion cells in the phloem actively transport and concentrate sugars produced in leaves, causing water to follow and increase the pressure, which leads to ﬂow of sap from source to sink. Like guard cells, vascular tissue harnesses the power of osmosis to accomplish movement without muscles. This feat is especially impressive because osmosis itself is a passive, entirely physical process.

A cross-section of the needle-like leaf of a pine shows tissues similar to those of a ﬂowering plant: protective epidermis with stomata, photosynthetic mesophyll, and vascular xylem and phloem. Differences include not only the water-conserving shape, but also a thicker, two-layered epidermis, a thicker cuticle, a single central “vein” of vascular tissue, and resin ducts which secrete resin in response to injury. The needle-like leaves of conifers ( Figure above) have similar tissues and several additional adaptations. A single, central core of vascular tissue consists of xylem surrounded by phloem. Photosynthetic mesophyll surrounds the vascular tissue; some conifers (pines) lack a palisade layer, while others have both palisade and spongy layers (balsam ﬁr, for example). Stomata open to allow entry of carbon dioxide, but the double-layered epidermis and waxy cuticle are often thicker and tougher than in dicot leaves. Many conifer leaves secrete resin through resin ducts in response to injury in order to discourage herbivores.

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Additional Leaf Functions

As indicated above, many plants lose their leaves seasonally; though this occurs most often in temperate zones before the cold, dry season of winter, some plants lose their leaves in advance of hot, dry seasons. Deciduous leaves have a region of cells at the base of the petiole specialized for this purpose. Weak-walled cells make up its top layer, and expansion of a bottom layer of cells in the fall (or other appropriate season) breaks the upper cells’ walls to release the leaf. Evergreen plants also lose leaves by this mechanism, but their loss occurs randomly throughout the year. Hormones including ethylene and auxins help to control the timing of leaf loss; although abscisic acid (ABA) was named after the process ( abscission), it was later discovered to be of minor importance to leaf loss. Plant hormonoes will be discussed in the following chapter. Closely related to loss of leaves is their change in color –not a function in itself, but a beautiful side effect of seasonal loss of function. Plants respond to seasonal decreases in levels of light and lower temperatures by reducing their production of the green pigment, chlorophyll. The loss of chlorophyll reveals accessory pigments such as yellow xanthophylls and orange carotenoids, which function to absorb other colors of light for photosynthesis. The red pigments (anthocyanins) are now thought to be added during the transition, protecting leaves’ nitrogen supplies from potentially harmful exposure to sunlight as they are salvaged from leaf tissue to be stored elsewhere for later use. Figure below shows the presence of these leaf pigments and reviews ﬂowering plant leaf tissues structures and functions.

Fall color change results from a decrease in the production of chlorophyll, which reveals accessory pigments such as xanthophylls and carotenoids. The leaves synthesize anthocyanins during the time of transition to protect supplies of nutrients such as nitrogen from sunlight damage as they are relocated to other parts of the plant before the leaf is lost. This diagram also reviews the structures and functions of leaf tissues; use it to check your understanding!

Structure-Function Relationship: Leaves

The Table below summarizes the structures which build leaves. Note once again how the structure’s form suits its function.

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TABLE 1.9: Leaves-Form Fits Function

Name of Structure Structure Function ABA Hormone molecule which binds to Regulates closure of guard cells guard cell membrane receptors Chloroplasts Green organelles with stacked Photosynthesis membranes Cuticle Layer of suberin Limiting water loss Epidermis Single layer of thin, closely packed Transmit light, limit water loss and cells control gas exchange Guard cells Spiral walls, bound at ends Open and close to control gas exchange Palisade cells Tall, many chloroplasts, precisely Photosynthesis spaced Phloem Elongated cells, living but without Transport of sugars in sap nucleus or ER, connected end to end by sieve plates and plasmodesmata Pith Parenchyma with vacuoles and Storage, support plastids Plasmodesmata Openings between sieve tubes con- Transport of sap necting cytoplasm Spongy cells Rounded, widely spaced, near Allow gas exchange stomata Suberin Waxy molecule Waterprooﬁng Vascular cambium Undifferentiated, rapidly dividing Growth in diameter cells between the xylem and phloem Xylem Elongated cells with thickened, pit- Absorbing and transporting water ted walls, connected end to end and ions

Food production, habitat design, and regulation of Earth’s atmosphere, temperature and water and carbon cycles make leaves our “green inheritance. . . without [which] we will surely perish.

Vocabulary

• abscisic acid (ABA): A plant hormone; among many other functions, acts on guard cells to close stomata.

• abscission: The shedding of various parts of an organism; the process by which a plant drops one or more of its parts, such as a leaf, fruit, ﬂower or seed.

• cohesion: The tendency for water molecules to stick together.

• collenchymal cells: One of the three major plant cell types; irregularly thickened walls and elongated cells arranged in strands help to support growing parts of the plant.

• companion cells: Cells in the phloem (of a plant) which support and load the conducting sieve tube elements cells.

• cuticle: A thick organic layer surrounding the outer surface of nematodes and arthropods; a waxy waterproof covering over the aerial surfaces of a plant.

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• deciduous leaves: Refers to leaves which are lost seasonally, either in winter or in dry seasons.

• epidermis: The outermost layer of skin, composed of epithelial cells; the outermost layer of a plant.

• evergreen: Refers to plants which lose and grow leaves throughout the year, rather than seasonally.

• guard cells: Pairs of specialized epidermal cells which regulate water and gas exchange in leaves and stems by opening and closing stomata.

• mesophyll: The ground tissue which makes up most of the photosynthetic interior of leaves; "middle leaf."

• palisade layer: One or two rows of photosynthetic columnar cells in the mesophyll.

• phloem: Vascular tissue which transports food from leaves to storage or growth areas in other parts of the plant; includes sieve tube elements and companion cells.

• pith: Ground (parenchymal) tissue internal to the vascular tissue, in the center of a stem or root.

• plasmodesmata (singular, plasmodesma): Microscopic channels which traverse the cell walls of plant cells; enables transport and communication between them.

• resin: A mixture of chemicals secreted by conifers in response to injury, to discourage herbivores.

• sclerenchymal cells: One of the three major plant cell types; greatly thickened, rigid secondary walls containing lignin and elongated, often-dead (at maturity) cells provide strong support.

• spongy layer: A layer of loosely packed and irregularly shaped chlorophyll-bearing cells; ﬁlls the part of a leaf between the palisade layer and the lower epidermis.

• stomata (singular, stoma): Openings on the underside of a leaf which allow gas exchange and transpiration.

• transpirational pull: The movement of water and minerals up through the xylem due to evaporation of water from leaves and the cohesion and adhesion of water molecules below.

• vascular tissue: A type of tissue in plants that transports ﬂuids through the plant; includes xylem and phloem.

• veins: Vessels that carry blood toward the heart; the vascular bundle in a leaf, containing xylem to transport water and minerals and phloem to transport food.

• xylem: Vascular tissue which transports water and minerals from the roots to stems and leaves; includes tracheids and vessel elements.

Summary

Practice

Use this resource to answer the questions that follow.

Review

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1.39 Life Cycles of Non-ﬂowering Plants - Ad- vanced

Reproduction in Plants

Keiki is the Hawaiian word for “baby.” If certain hormones accumulate at a node along the ﬂowering stalk of some orchids, keikis grow from the stalk ( Figure 1). Eventually, these leaf-and-root sprouts drop off and form new plants –a simple form of reproduction, creating a straightforward, uncomplicated life cycle.

Some orchids reproduce asexually with “keikis” (left), and sexually with flowers elaborately designed to trick certain species of bees into transferring pollen from the blossom to other flowers. Asexual reproduction results in identical offspring, while sexual reproduction provides variety in offspring, whose increased chance of surviving change apparently offsets the energy expense of building flowers.

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Life Cycles of Plants: Alternation of Generations

Orchids also grow flowers. Ophrys speculum ( Figure 1), for example, fashions a “lower lip” petal into an elaborate lobe bordered by two “wings” and fringed with a thick layer of brown hairs. Three maroon-striped sepals frame the lip, one curving overhead to display a dangling packet of thousands of pollen grains (the pollinium) and at the same time shield a carefully hidden stigma. Intricate details give the entire structure a remarkable resemblance to a specific bee, and the plant enhances this by manufacturing a long-range scent molecule identical to that of a receptive female. The entire structure is completed exactly when male bees become active, but before females appear. The stage is set for the following drama: a male bee, attracted by the “receptive female” scent, arrives in the vicinity and is drawn to the flower - a visual replica of “his” female. In a passionate attempt to mate, he sticks to a carefully placed patch of “fly paper”, which pulls off the pollinium as he leaves. The structure of the pollinium shifts in such a way that, if the frustrated male “carrier” attempts to mate with another flower, the pollinium brushes off against the stigma and fertilizes as many as 12,000 eggs in the ovary. After months of development, the ovary/capsule matures and wind blows the thousands of microscopic seeds away. With no endosperm, the seeds require a precise habitat and specific mycorrhizal fungi to absorb nutrients to launch them into new individual lives; their chance of survival is minute. Germination, if it happens at all, may require up to 15 years. Why do plants invest so much time and energy in flowers, when keikis would allow much faster, more reliable and less “expensive” reproduction? As you have probably noted, keikis are a means of asexual reproduction, so offspring are clones –exact copies of the parent. Flowers, however, facilitate sexual reproduction, and offspring of two parents vary genetically. Such variation allows at least some offspring to survive in new or changing environments, offsetting the increased energy costs and risk of engineering two-parent reproduction in immobile plants. All plants have “sex lives”; sexual reproduction is certainly not limited to flowering plants. Moreover, reproduction in plants is not limited to sexual reproduction, as is so often true for animals. Most animals, including humans, spend their entire lives as diploid, multicellular individuals; only their gametes are haploid. At the opposite end of the spectrum are many fungi, which live their multicellular lives as haploid individuals, joining nuclei to become truly diploid only as zygotes. Plants –especially early plants - have the best of both worlds. Every generation includes a multicellular diploid stage and a multicellular haploid stage; plants have an alternation of generations life cycle ( Figure 2).

227 1.39. Life Cycles of Non-ﬂowering Plants - Advanced www.ck12.org

The life cycle of plants (A) shows alternation of generations, with multicellular individuals in both haploid and diploid stages. Most animals produce only diploid multicellular individuals (B), and many fungi are multicellular only when haploid (C). In plants, meiosis (1) produces haploid spores, which divide by mitosis (2) to produce a multicellular haploid stage. Eventually, haploid individuals produce haploid gametes for sexual reproduction (3), and mitosis produces a multicellular diploid stage. In typical animals, meiosis (1) produces haploid gametes directly; these fuse (3) and divide by mitosis (2) to produce a single, multicellular diploid individual. In many fungi, haploid spores divide by mitosis (2) to produce multicellular haploid individuals and eventually gametes, which fuse (3) to form a diploid zygote which undergoes meiosis (1) to produce more spores. Because the diploid stage of the plant life cycle produces spores (resistant reproductive cells), it is known as a sporophyte (“spore plant”). Similarly, because the haploid stage produces gametes, or haploid sex cells, it is known as the gametophyte. An entire life cycle includes:

1. meiosis in the sporophyte to produce haploid spores 2. mitosis of spores to produce the multicellular haploid gametophyte 3. mitosis within the gametophyte to produce gametes 4. fertilization: the fusion of two haploid gametes to form a diploid zygote 5. mitosis of the zygote to form the multicellular diploid sporophyte.

Alternation of generations involves multicellular diploid (sporophyte –2n) and multicellular haploid (gametophyte - n) stages. Although relative size and independence of the two stages vary, all members of the Plant Kingdom include both. This life cycle may seem complex, but it is worth noting that all plants include each stage in some form. Differences are merely changes in the relative size and importance of each stage throughout the evolution of plants. The details of plant reproduction, which at ﬁrst appear strange and unnecessarily complicated, “make sense” in terms of this basic pattern - the evolutionary inheritance handed down to every member of the Plant Kingdom. The advantages of alternation of generations are both sexual and genetic:

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• In the haploid (gametophyte) stage, every allele is expressed; none can be masked by a dominant alternative. Lethal mutations are quickly eliminated from the gene pool. • In the diploid (sporophyte) stage, meiosis involves crossing over and independent assortment of genes, resulting in diversity among offspring. • Fertilization restores a diploid state, in which harmful recessive mutations may be masked.

In these concepts, we will explore the ways in which three major groups of plants have modiﬁed alternation of generations to sustain life, including sexual reproduction –even without ﬂowers.

Vocabulary

• alternation of generations: A life cycle that alternates between diploid and haploid phases.

• gamete: A sexually reproducing organism’s reproductive cells, such as sperm and egg cells.

• gametophyte: The gamete-producing phase in the alternation of generations life cycle; a haploid structure which produces gametes by mitosis.

• sexual reproduction: Reproduction involving the joining of haploid gametes, producing genetically diverse individuals.

• spore: A haploid reproductive cell; found in plants, algae and some protists; can fully develop without fusing with another cell.

• sporophyte: The spore-producing phase in the alternation of generations life cycle; a diploid structure which produces spores by meiosis.

• zygote: A fertilized egg; the ﬁrst cell of a new organism.

Summary

Practice

Use this resource to answer the questions that follow.

Review

229 1.40. Life Cycles of Bryophytes - Advanced www.ck12.org

1.40 Life Cycles of Bryophytes - Advanced

Life Cycles of Bryophytes

In bryophytes (liverworts, hornworts, and mosses), the gametophyte stage is dominant http://www.biology.duke.edu/ bryology/LiToL/Lifecycle.html . The leafy green structures we recognize ( Figure below) are haploid, and perform most of the photosynthesis.

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The familiar, green photosynthetic stages of mosses (left), liverworts (upper right), and hornworts (lower right) are haploid gametophytes. These gametophytes produce organs for sexual reproduction: male antheridia and female archegonia. In dioecious (“two house”) species, individual plants are either male or female, producing just one type of sex organ. In monoecious (“one house”) species, both antheridia and archegonia form on the same plant. Occasionally, the environment determines whether a species is monoecious or dioecious.

In bryophytes such as the liverwort Marchantia, gametophytes produce organs for sexual reproduction: male antheridia (above) and female archegonia (below). By mitosis, antheridia produce many biﬂagellate sperm, and each archegonium produces a single egg. The sperm must swim through a layer of moisture into the archegonium (red arrow) to fertilize the egg. The zygote undergoes mitosis to produce a new diploid sporophyte, which emerges from the archegonium.

231 1.40. Life Cycles of Bryophytes - Advanced www.ck12.org

Each archegonium produces a single egg cell by mitosis. Within each antheridium, mitosis produces many biﬂag- ellate (two ﬂagella) sperm. Consult the drawings in Figure 5 to visualize these microscopic structures and the Lilliputian “umbrellas” that raise them above the leafy surface in the liverwort, Marchantia. Sperm must swim through a layer of moisture from antheridia to archegonia to fertilize the eggs. The resulting zygote divides by mitosis –still within the archegonium - to produce a diploid sporophyte. In many bryophytes, the sporophyte is almost parasitic on its haploid gametophyte parent, although it may be green for part of its life. In mosses, the sporangium takes the form of a capsule, raised high above the green gametophyte by a stalk ( Figure 6). In liverworts, the sporangium is a tiny sphere which hangs from the female “umbrella”. And in hornworts, the sporangium is the tall spike or “horn” which gives the group their common name. In all three groups, sporophytes eventually form sporangia, which produce haploid spores by meiosis. These spores are dispersed with the help of moisture-sensitive teeth or “springs” ( elaters), to grow new gametophyte plants.

In bryophytes, the sporophyte stage usually grows from the female gametophyte and is often dependent on it. Moss sporophytes form stalked capsules (A). Liverwort sporophytes (B) are small spheres hanging from female “umbrellas”. Hornworts are named for their prominent horn-like sporophytes (C). In all three groups, sporophytes develop sporangia (D) which produce spores by meiosis. Some species of bryophytes reproduce asexually during the gametophyte stage by means of gemmae, small masses of cells that are splashed out of tiny cups ( Figure 7) by raindrops. The gemmae then form new gametophytes.

Some bryophytes reproduce asexually during the gametophyte stage. Raindrops may splash gemmae (right) out of tiny cups (left) or crescents (right). The gemmae then form new gametophytes. Review bryophyte reproduction by tracing the life cycles for mosses, liverworts, and hornworts in Figure 8.

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233 1.40. Life Cycles of Bryophytes - Advanced www.ck12.org

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Mosses (upper ﬁgure), liverworts (lower left), and hornworts (lower right) have dominant gametophytes, which produce eggs and biﬂagellate sperm by mitosis. Sperm must swim to eggs for fertilization, restricting these plants to moist habitats. Zygotes develop into dependent sporophytes, which produce haploid spores by meiosis, beginning the cycle again.

Vocabulary

• antheridium (plural, antheridia): A haploid male reproductive organ, which produces male gametes by mitosis.

• archegonia (singular, archegonium): A haploid female reproductive organ, which produces female gametes by mitosis.

• dioecious: Having individuals of separate sexes for gamete production.

• elater: Cell or cell structure which absorbs moisture from its environment, causing it to change shape and help to disperse associated spores.

• gemmae (singular, gemma): A mass of cells or a modiﬁed bud of tissue, that detaches from the parent and develops into a new individual.

235 1.40. Life Cycles of Bryophytes - Advanced www.ck12.org

• monoecious: Also called hermaphroditic; individuals capable of producing both eggs and sperm.

• sporangia (singular, sporangium): Asexual reproductive organ which produces spores.

Summary

Practice

Use this resource to answer the questions that follow.

• http://www.hippocampus.org/Biology Biology for AP* Search: Colonizing the Land ! !

1. What is the dominant generation of a bryophyte? 2. In a moss, how does the sperm reach an egg prior to fertilization? 3. Where does the sporophyte form? 4. What is a sporangium? 5. How are spores spread? 6. Spores develop into what structures?

Review

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1.41 Life Cycles of Seedless Vascular Plants - Advanced

Life Cycles of Seedless Vascular Plants

Like bryophytes, early vascular plants alternate generations. However, club mosses, horsetails, and ferns have a dominant sporophyte stage and a greatly reduced gametophyte stage. In each group, the familiar plant is diploid and produces sporangia ( Figure 9).

Early vascular plants, including ferns (A), clubmosses (B), horsetails, (C,D, and E) have a dominant diploid sporophyte stage, in which sporangia (A-D) produce haploid spores (E) by meiosis. Within the sporangia, meiosis produces haploid spores, which disperse to form tiny (2-5 mm), heart-shaped, haploid gametophyte plants ( Figure 10). These inconspicuous gametophytes develop sperm-producing antheridia and egg- bearing archegonia –sometimes on the same plant, and in other species on two different plants. Dioecious species are heterosporous –smaller spores ( microspores) produce male plants and antheridia, and larger spores ( megaspores) produce female plants with archegonia. In monoecious species, spores are often the same size ( homosporous).

237 1.41. Life Cycles of Seedless Vascular Plants - Advanced www.ck12.org

The haploid gametophyte stages of ferns, clubmosses, and horsetails are seldom seen. Usually between 2 and 5 mm in width, they develop egg-producing archegonia (if female, as in the drawing above) or sperm-producing antheridia (if male). In some species, the gametophyte produces both types of reproductive organs. The biﬂagellate sperm must swim into the archegonium to fertilize the egg cell. The familiar diploid plant grows out of the tiny heart-shaped gametophyte. Pr = prothallus; ar = developing archegonium; emb = embryos –developing sporophytes. Eggs and sperm are produced within antheridia and archegonia by mitosis. The possibility that the biﬂagellate sperm will be able to swim to and fertilize eggs within archegonia is greater for species which produce both organs on the same plant. However, guaranteed cross-fertilization provides more genetic variety in offspring for plants whose gametophytes are essentially two different sexes. In either case, clubmosses, horsetails, and ferns are limited to moist habitats by this phase of their life cycle. The zygote formed when the sperm fertilizes an egg is, of course, diploid –the beginning of the next sporophyte generation. The familiar fern, or horsetail, or clubmoss grows out of the small gametophyte, which is seldom noticed as the “launching pad” of the new plant. Underground rhizomes and above-ground stolons facilitate asexual reproduction in some early vascular plants ( Figure 11). This type of reproduction results in colonies of genetically uniform plants (clones).

Some early vascular plants reproduce asexually by sprouting new plants from underground rhizomes.

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Vocabulary

• heterosporous: Producing spores of two different sizes –male microspores and female megaspores.

• homosporous: Producing spores of the same size, which develop into gametophytes with both antheridia and archegonia.

• megaspore: Larger spore resulting from meiosis, which produces a female gametophyte and archegonia.

• microspore: Smaller spore resulting from meiosis, which produces a male gametophyte and antheridia.

• rhizome: A modiﬁed subterranean stem of a plant that is usually found underground, often sending out roots and shoots from its nodes.

• stolon: Stem similar to a rhizome, but often above or just below the surface; nodes typically spaced further apart.

Summary

FIGURE 1.72

Practice

Use this resource to answer the questions that follow.

239 1.41. Life Cycles of Seedless Vascular Plants - Advanced www.ck12.org

• http://www.hippocampus.org/Biology Biology for AP* Search: The Rise of Vascular Plants ! !

1. Why is liquid water required for sexual reproduction of a fern? 2. What is a gametangium? 3. Is the sporophyte haploid or diploid? 4. Where do the spores form? 5. Is a spore haploid or diploid? 6. Spores germinate into what structures?

Review

240 www.ck12.org Chapter 1. Plant Biology - Advanced

1.42 Life Cycles of Gymnosperms - Advanced

Life Cycles of Gymnosperms

Although gymnosperms are named for their “naked seeds”, their success on dry land depends just as much or more on another, equally revolutionary reproductive innovation: pollen. To clarify the relationship between gymnosperm seeds and pollen and reproductive structures in earlier plants, let’s begin with the sporophyte, which is even more dominant in gymnosperms than their predecessors, the ferns, clubmosses, and horsetails. Most gymnosperm sporophytes are large, single-trunk trees: conifers such as pines, cedars, spruces, redwoods, sequoias, and firs; and the ancient cycads and ginkgos. As in ferns and other early vascular plants, modified leaves bear sporangia, but in most gymnosperms, many highly modified leaves combine to form a distinct, cone-shaped structure which gives the conifers (“cone-bearers”) their name. Moreover, gymnosperms are heterosporous, with two distinct types of sporangia produced on two distinct types of cones.

241 1.42. Life Cycles of Gymnosperms - Advanced www.ck12.org

In gymnosperms, modiﬁed leaves form male pollen cones which bear microsporangia (red arrows). Meiosis within the microsporangia produces haploid microspores which develop into pollen (C and D). Photos show pollen cones from pines (A), cycads (B), a gnetophyte (Ephedra –E), and ginkgo (F, preserved). In smaller, herbaceous male cones, microsporangia produce haploid microspores by meiosis, and the microspores develop within the sporangia into male gametophytes, which we know as pollen ( Figure 12). Gymnosperm pollen consists of just a few cells, surrounded by layers of cellulose and extremely durable organic molecules; in many species, the pollen is winged to facilitate wind dispersal. Because the male gametophytes mature as pollen within male cones, the microsporangia are often called pollen sacs. In larger, often woody cones, megasporangia produce haploid megaspores by meiosis, but instead of dispersing, the megaspores, together with diploid maternal tissue, develop into ovules ( Figure 13), which are borne “naked” on the scales of the cone. The female gametophyte, which develops within a megaspore, is entirely dependent on surrounding sporophyte tissues for nutrition and protection –the opposite extreme from bryophytes, in which the sporophyte is dependent on the gametophyte. The much-reduced, haploid gametophyte produces one or more egg cells, which remain within the ovule.

242 www.ck12.org Chapter 1. Plant Biology - Advanced

In gymnosperms, female gametophytes are entirely dependent on the parent sporophyte for nutrition and protection. Megasporangia, “naked” on scales or modified leaves of often-woody cones, produce haploid megaspores by meiosis. The much-reduced gametophyte develops from a megaspore, producing one or more egg cells. Together with the megasporangium and layers of parental tissue (the integument), the gametophyte forms an ovule (C), which eventually develops into a seed. Cones are woody in most conifers (A), opening to release seeds for wind dispersal. In species such as Jack Pine (B), cones may remain closed for up to 80 years until fire opens them to release seeds for “pioneer” conditions. Female cones in cycads (D) and the gnetophyte Ephedra (E) show cleary the evolution of scales as modified leaves. Female cones in juniper (F) and yew (G) are fleshy and colorful rather than woody, encouraging dispersal by birds. Most conifers are monoecious, although many avoid self-fertilization by differential timing of male and female cone maturity or by locating female cones higher on the tree. Most cycads, ginkgos, and gnetae are dioecious, encouraging cross-fertilization and greater variety among offspring. In nearly all gymnosperms, wind carries pollen grains –containing and protecting fragile male gametophytes –to the female cones. Pollen enters the ovule through a single opening in the integument (maternal covering tissues), and the male gametophyte develops a pollen tube, which releases sperm to fertilize egg nuclei. Cycad and ginkgo sperm retain their flagella, and must swim a very short way to reach the egg, but conifer sperm are non-motile, fully transported by the pollen tube. All, however, are protected from drying by the durable pollen coatings. After fertilization, the diploid zygote –still within the ovule –develops into a sporophyte embryo with primordial root and leaves. Gametophyte tissue provides a stored food supply for the embryo, and the parental sporophyte integuments develop into a protective seedcoat. Together, embryo, food supply, and seedcoat form a new, highly resistant means of dispersal –a seed ( Figure 14).

243 1.42. Life Cycles of Gymnosperms - Advanced www.ck12.org

In gymnosperms, the dispersal stage is the seed, which develops from the ovule and contains maternal sporophyte tissue (the seedcoat), new sporophyte tissue (the embryo), and female gametophyte tissue which serves as a stored food supply. Seeds are borne “naked” on the scales of female cones, and adapted for dispersal by wind (many pines, A), birds and/or mammals (cycads, B, and some pines, C). D shows ginkgo seeds. The seeds may have wings, which promote wind dispersal or bright colors and/or extensive food supplies to encourage birds and animals to help with dispersal. Seeds may also be dormant, requiring chemical or physical signals to stimulate germination only when conditions for growth are optimal. Requirements such as periods of cold, changes in light, or minimum warmth or moisture help to control germination. Closed cones assist in Jack Pine seed dormancy ( Figure 12B); only the intense heat of a forest ﬁre will open the scales and release the seeds into soils rich in nutrients and sunlight, which inevitably follow a ﬁre. In summary, gymnosperms have three major adaptations which help them to reproduce successfully in dry habitats:

1. Pollen carries sperm to the female gametophyte without the need for moisture. 2. Ovules allow sporophytes to nourish and protect female gametophytes and next-generation sporophytes. 3. Seeds give next-generation sporophytes a “jump start” by providing food and ensuring dormancy until optimal conditions return.

For a remarkably detailed 6-part animation of the conifer life cycle, see http://video.google.com/videoplay?docid= 4704824509001888937 .

Vocabulary

• megaspore: Larger spore resulting from meiosis, which produces a female gametophyte and archegonia.

• ovule: Structure in seed plants which produces the egg cell and develops into a seed.

• pollen: Plant reproductive structure which protects male sex cells during pollination.

• seed: An embryonic plant and food supply stored within a protective seed coat

244 www.ck12.org Chapter 1. Plant Biology - Advanced

FIGURE 1.73 The gymnosperm life cycle follows the general plant life cycle, but with some new adaptations. Can you identify them?

Summary

Practice

Use this resource to answer the questions that follow.

• http://www.hippocampus.org/Biology Biology for AP* Search: Cones, Flowers, and Seeds ! !

1. Are there male and female cones? 2. Is the sporangia haploid or diploid? 3. Are the spores haploid or diploid? How do spores form? 4. What is pollen? How does it travel to a female gametophyte? 5. Where is the egg located? 6. Distinguish between pollination and fertilization. 7. Where does the embryo form? 8. Where does the seed come from? 9. What happens to the seed?

Review

245 1.43. Life Cycles of Angiosperms - Advanced www.ck12.org

1.43 Life Cycles of Angiosperms - Advanced

Reproduction in Flowering Plants

The largest (Rafflesia) is 1 meter wide, weighs up to 11 kg (24 pounds), and produces an overpowering smell of rotten flesh. The smallest (Wolffia) is dwarfed by its single 1 mm leaf. If we consider inflorescence (a cluster of flowers on a single stem), two more compete for honors. The Talipot Palm takes up to 80 years to prepare for its single, 8-meter (24 feet!) branching spike of several million flowers, and then dies after the fruits of this labor ripen. The Titan Arum produces a 3-meter (10 foot) “loaf of French bread” –an unbranched cluster of thousands of male and female flowers, and competes with Rafflesia not only in size, but also in odor, for the title of “corpse flower”. Textures waxy to fuzzy, colors from vibrant to camouflaged, shapes symmetrical or exotic, scents intoxicating or repulsive, and nectars described as “food for the gods”: as many as 400,000 species of flowers illustrate all of these and more.

246 www.ck12.org Chapter 1. Plant Biology - Advanced

Flowering plants are the most abundant and diverse plants on Earth today. The largest flower is rotten-flesh-scented Rafflesia (A), up to 1 meter in diameter and 11 kg (24 pounds) in mass. The largest unbranched cluster of flowers is the Titan Arum (D) –equally famous for its corpse-like odor. The Talipot Palm has the largest branched cluster, with millions of flowers in an 8-meter-high “bouquet”. The smallest flower is a single pistil paired with a single stamen, tucked into a 1-mm floating leaf: Wolffia (F). Other flowers have unique textures (pussy willow, B), colors and nectars (Euphorbia, C), or simple, unadorned reproductive organs (foxtail grass, E). Angiosperms (“covered seed” or flowering plants) are fast living, flamboyant, and diverse –today’s success story in the Plant Kingdom. While gymnosperms (“naked seed” plants) are arguably the most successful plants in evolutionary history (the tallest, most massive, and oldest living things today are conifers –see the introduction to the lesson on stems), angiosperms dominate our contemporary terrestrial landscape. This lesson will explore the ways in which several reproductive adaptations in angiosperms have helped them to out-compete gymnosperms to become the most diverse and abundant plants on Earth.

Life Cycles of Angiosperms

As for all plants, the key to understanding the life cycle of angiosperms is their evolutionary history of alternation of generations. Like gymnosperms and early vascular plants, angiosperms have a dominant, diploid sporophyte stage. Like gymnosperms, angiosperms are heterosporous, producing two types of reproductive spores. Like gymnosperms, angiosperm gametophytes and the next-generation sporophytes develop within the sporophyte par-

247 1.43. Life Cycles of Angiosperms - Advanced www.ck12.org ent, rather than living independently as in early vascular and bryophyte plants. Like gymnosperms, angiosperms accomplish fertilization with pollen, and package next-generation embryos in seeds for dispersal. How, then, do angiosperms differ from other groups of plants?

First, angiosperms continue several evolutionary trends:

• 1. Male and female gametophytes are further reduced –to just 3 cells for pollen, and 7 cells/8 nuclei per ovule ( Figure 2). These reductions expedite pollination, fertilization, and seed development. Whereas gymnosperms may require up to three years for pollination, fertilization, and seed development, angiosperms can complete an entire life cycle within a single season, allowing more rapid colonization and adaptation.

In angiosperms, both male and female gametophytes are minimized, allowing streamlined, rapid fertilization and development. Male gametophytes (pollen, left) are reduced to three cells –not visible beneath the resistant, protective coating. Female gametophytes are reduced to seven cells and eight nuclei (ovule, right).

• 2. Female gametophytes and young sporophytes are highly protected by the parent sporophyte, forming within a carpel. This highly modiﬁed leaf wraps around the ovule (megasporangium), forming an ovary. In many species, several carpels fuse to form a pistil, consisting of stigma, style, and ovaries. Each part of the pistil may be adapted for specialized pollination. Eventually, the carpel/ovary ripens into a fruit, which usually has adaptations for specialized dispersal.

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In angiosperms, a modiﬁed leaf, the carpel, forms an ovary around the ovule. One or more carpels together form the pistil, consisting of stigma, style, and ovary –the female reproductive organ of the ﬂower. Eventually, the ovary and/or associated structures may mature into a fruit for dispersal.

• 3) Food supplies in seeds are enriched. Double fertilization of the female gametophyte nuclei by male sperm nuclei produces not only a zygote, but also highly nutritious, often triploid endosperm ( Figure 4). Endosperm within the seed provides starch, oil, and/or protein for the developing embryo, cotyledons, and/or seedling. We use the endosperm of many plant seeds –wheat, corn, barley, coconut, and banana –for food.

Angiosperms such as common wheat have seeds enriched with triploid endosperm, formed by the fusion of one sperm nucleus with two polar nuclei in the female gametophyte.

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The efficiency gained by maximizing these previous evolutionary changes permits additional angiosperm innovations: • 1) Male reproductive organs (recall that these were housed in herbaceous cones in gymnosperms) are streamlined to become lightweight, easily modified stamens ( Figure 5). Stalks ( filaments) of variable heights support anthers, which contain four microsporangia (pollen sacs). Within the pollen sacs, meiosis produces microspores, which become male gametophytes enclosed within durable pollen grains. The height, shape, number, and position of the stamens can be modified to suit specialized pollination schemes from wind to insects, birds, and bats. Timing of maturation and position can help to prevent self-pollination.

Angiosperms have modiﬁed male reproductive organs (formerly within pollen cones) into lightweight, stalked stamens. Anthers atop the ﬁlament stalks contain pollen sacs (microsporangia) which produce microspores by meiosis. Spores develop by mitosis into 3-celled pollen (male gametophytes).

• 2) Flowers ( Figure 6) unite stamens and pistil into structures whose dramatic colors, shapes, scents, and even food supplies attract animals and enlist them to carry pollen from one individual to another, ensuring efﬁcient cross-pollination and maximum variety in offspring.

The ﬂower combines all of the angiosperm reproductive innovations into a single sexual reproductive organ, designed to encourage animals to carry out cross-pollination.

Summary

Let’s look at how these angiosperm innovations streamline the life cycles of ﬂowering plants, beginning with the familiar diploid sporophyte. The steps below will summarize a generalized life cycle. Following sections will detail

250 www.ck12.org Chapter 1. Plant Biology - Advanced variations on this basic pattern. Trace these steps through the diagram in Figure 7.

FIGURE 1.74 A generalized angiosperm life cycle shows alternation of generations, extreme reduction and dependency of gametophyte stages, and greater efﬁciency in pollination and seed development. Use this summary diagram to follow the life cycle steps described in the text.

1. Diploid sporophytes produce flowers, including colorful petals and sepals, “designer” shapes, scents, and nectars, a female pistil, and multiple male stamens. 2. Within the ovule (megasporangium), in the ovary of the pistil, meiosis produces a single haploid megaspore. 3. The megaspore divides by mitosis to form an embryo sac –the female gametophyte - containing an egg cell, two polar nuclei, and two synergids (See Figure 2). The 3 antipodal cells degenerate. 4. Within the anthers, microsporangia (pollen sacs) produce haploid microspores by meiosis. Each develops into a male gametophyte (pollen grain) containing one vegetative and two germinative (reproductive) cells. 5. An insect or other animal (or wind) carries pollen from the anther of one flower to the stigma of another (cross-pollination) –or in some cases, the stigma of the same flower (self-pollination). 6. Pollen sticks to the stigma and the tube nucleus grows a pollen tube down into the pistil, through the style, into the ovary and through the micropyle of the ovule. Synergid cells may help to guide the pollen tube. 7. Two sperm nuclei are released by the pollen tube. One fertilizes the egg cell to form the zygote, and the second joins the two polar nuclei to form the endosperm. 8. The zygote divides by mitosis to form the embryo –the next generation sporophyte. Endosperm develops starches, oils, and proteins to nourish the embryo, cotyledons, and/or seedling. The integuments develop into a protective seedcoat. Embryo, endosperm, and seedcoat form the seed. 9. The ovary matures into a fruit, with adaptations which promote the dispersal of its seed contents by animals. 10. When favorable conditions end dormancy, the seed germinates and grows into a new sporophyte.

251 1.43. Life Cycles of Angiosperms - Advanced www.ck12.org