Plant Reproduction 24 and Response

694 S ON CU O F TEKS

10B

The Green Lemons For years, a California warehouse had stored freshly picked green lemons before they were shipped to market. The warehouse managers knew that the lemons would always be ripe and yellow Pollen grains from the common and ready to ship to market about five ­ragweed (SEM 1000∙) days after they arrived. Or so they thought. One year, for safety reasons, they decided to replace the warehouse’s kerosene heaters with modern electric ones. Then, much to their surprise, when they began to pack their first shipment of five-day-old lemons, they had to stop. The fruit they expected to ship were still a bright, and very unripe, green. All the lemons had been grown under similar conditions. A variable had changed after the lemons were harvested, and the lemons had failed to ripen. What could it have been? Look for clues in this chapter to the mystery of the still- green lemons. Then, solve the mystery.

Never Stop Exploring Your World. Finding the solution to The Green Lemons mystery is only the beginning. Take a video field trip with the ecogeeks of Untamed Science to see where this mystery leads.

Texas Essential Knowledge and Skills

READINESS TEKS: 10B Describe the interactions that occur among systems that perform the functions of transport, reproduction, and response in plants. SUPPORTING TEKS: 5B Examine specialized cells, including roots, stems, and leaves of plants; and animal cells such as blood, muscle, and epithelium. 12B Compare variations and adaptations of organisms in different ecosystems. TEKS: 2G Analyze, evaluate, make inferences, and predict trends from data. Also covered: TEKS 2H.

Plant Reproduction and Response 695 Reproduction in 24.1 Flowering Plants

In this lesson you will learn about flowers and the angiosperm life cycle (TEKS 10B). In the Quick Lab, you will examine specialized leaves (TEKS 5B).

Key Questions Think about it What makes a flower beautiful? The symmetry of What are flowers? its petals, its rich colors, and, sometimes, its fragrance. But, at the heart of it, what’s behind all this beauty? The answer is, simply, angiosperm How does fertilization in sexual reproduction. To a plant, the whole point of a flower is to bring angiosperms differ from ­fertilization in other plants? gametes together for reproduction and to protect the resulting zygote and embryo. What is vegetative reproduction? The Structure of Flowers TEKS 10B Vocabulary What are flowers? stamen • anther • carpel • stigma • pistil • embryo sac • You may think of flowers as decorative objects that brighten our double fertilization • endosperm • world, and so they are. However, the beauty of flowers also reflects the vegetative reproduction • stunning evolutionary success of the angiosperms, or flowering plants. grafting The structure of a typical angiosperm flower is shown in Figure 24–1. Flowers are reproductive organs that are composed of four Taking Notes different kinds of specialized leaves: sepals, petals, stamens, and Two-Column Table Construct carpels. Flower development is triggered by interactions with the a two-column table with the plant’s genetic system when a gene called LEAFY is activated, causing headings Male Gametophyte and Female Gametophyte. As an ordinary meristem to develop into a flower. you read, take notes on the Sepals and Petals The outermost circle of floral parts contains characteristics of each type of gametophyte. the sepals (see pulz). In many plants, the sepals are green and closely Stamen Carpel resemble ordinary leaves. Sepals enclose the bud before it opens, and Anther Style Stigma protect the flower while it is develop- Filament Ovary ing. Petals, often brightly colored, are found just inside the sepals. The colors and shapes of such petals attract insects and other pollinators to the flower.

Figure 24–1 The Parts of a Flower This diagram shows the parts of a typical flower. The flowers of some angiosperm species, however, Petal do not have all the parts shown here. Ovary Sepal Ovule

696 Stamens Within the ring of petals are the structures that produce ELPS 4.G.2 male and female gametophytes. Thestamens are the male parts of Use the “Visual Summary” on the flower. Each stamen consists of a stalk called a filament with an page 701 to clarify what you are anther at its tip. Anthers are the structures in which pollen grains— reading and to help you recall main the male gametophytes—are produced. In most angiosperm species, ideas. Then, show the visual to a small group. Use it to help retell or the flowers have several stamens. If you rub your hand on the anthers summarize what happens during the of a flower, a yellow-orange dust may stick to your skin. This dust is life cycle of a flowering plant. made up of thousands of individual pollen grains. Carpels The innermost floral parts are the carpels.Carpels produce and shelter the female gametophytes and, later, seeds. Each carpel has a broad base forming an ovary, which contains one or more ovules where female gametophytes are produced. The diameter of the carpel narrows into a stalk called the style. At the top of the style is a sticky or feathery portion known as the stigma, which is specialized to capture pollen. Botanists sometimes call a single carpel or several fused carpels a pistil.

Online Journal Make a two-column table labeled Male and Figure 24–2 Variety Among Flowers Female. Then list and define the structures that make up a flower Flowers vary greatly in structure. in the appropriate columns. Form a Hypothesis How might it be an advantage for Variety in Flowers Flowers vary greatly in shape, color, and size, as a plant to have many flowers shown in Figure 24–2. A typical flowering plant produces both male clustered in a single structure? and female gametophytes. In some species, however, male and female gametophytes are produced on different plants. In some species, many flowers grow close together to form a composite structure that looks like a single flower, as seen in the Queen Anne’s lace at right. ▼ Iris The drooping petal-like Queen Anne’s Lace Some structures are in fact modified flowerlike structures are sepals. The fuzzy yellow stripe actually clusters of many

running down the center guides individual flowers. ▼ bees and other pollinators to the male and female parts at the interior of the flower. Wild Rose Passion Flower Some flowers This flower has have stamens and pistils many stamens you can easily count. In this surrounding a dramatic flower, five stamens tight cluster of lie beneath three pistils.▼ carpels at the

center. ▼

Plant Reproduction and Response 697 TEKS 5B What Is the Structure of a Flower? 1 Examine2 3 a4 flower 5 carefully.6 7 8 Make9 a Analyze and Conclude detailed drawing of the flower and label as 1. Observe Are many parts as you can. Note whether the the anthers in anthers are above or below the stigma. this flower 1 2 Remove3 4 an5 anther6 and7 place8 9 it on located above a slide. While holding the anther with or below the forceps, use the scalpel to cut one or more stigma? How thin slices across the anther. CAUTION: Be could this affect what happens to the pollen careful with sharp tools. Place the slide on a produced by the anthers? Explain your answer. flat surface before you start cutting. 2. Apply Concepts What structures did you identify 1 2 3 Lay4 the5 slices6 flat7 on8 the9 microscope in the anther? What is the function of these structures? slide and add a drop of water and a 3. Apply Concepts What structures did you identify coverslip. Observe the slices with the in the ovary? What is the function of these structures? microscope at low power. Make a labeled 4. Draw Conclusions Which parts of the flower will drawing of your observations. become the seeds? Which parts will become the fruit? 2 3 4 Repeat5 6 steps7 2 and8 93 with the ovary.

The Angiosperm Life Cycle TEKS 10B How does fertilization in angiosperms differ from fertilization in other plants? Like other plants, angiosperms have a life cycle that shows an alterna- tion of generations between a diploid sporophyte phase and a haploid gametophyte stage. Recall that in vascular plants, including ferns and gymnosperms, the sporophyte plant is much larger than the gameto- phyte. This trend continues in angiosperms, where male and female gametophytes live within the tissues of the sporophyte. Development of Male Gametophytes The male gametophytes— the pollen grains—develop inside anthers. This process is shown in the top half of Figure 24–3. First, meiosis produces four haploid spore cells. Each spore undergoes one mitotic division to produce the two haploid nuclei of a single pollen grain. The two nuclei are surrounded by a thick wall that protects the male gametophyte from dryness and damage when it is released. The pollen grains stop growing until they are released from the anther and land on a stigma.

Online Journal Make a flowchart that records the stages of development of an angiosperm’s male gametophyte.

698 Chapter 24 • Lesson 1 Development of Female Gametophytes While the male gameto- phytes are forming, female gametophytes develop inside each carpel of a flower. The ovules—the future seeds—are enveloped in a protective ovary—the future fruit. How do the female gametophytes form? As shown in the bottom half of Figure 24–3, a single diploid cell goes through meiosis to produce four haploid cells, three of which disintegrate. The remaining cell undergoes mitosis, producing eight nuclei. These eight nuclei and the surrounding membrane are called the embryo sac. The embryo sac, contained within the ovule, makes up the female gametophyte of a flowering plant. Next, cell walls form around six of the eight nuclei. One of the eight nuclei, near the base of the gametophyte, is the nucleus of the egg—the female gamete. If fertilization takes place, this egg cell will fuse with the male gamete to become the zygote that grows into a new sporophyte plant.

The Development of Gametophytes Figure 24–3 The diagrams show the development of the male gametophyte inside an anther and the development of the female gametophyte inside a single ovule. Interpret Visuals In each case—male and female—which cellular process produces the first haploid cell?

Male Pollen grain (Male gametophyte) 2N N MEIOSIS MITOSIS

Inside the anthers, The nucleus of each pollen spore meiosis produces four divides. The pair of nuclei and haploid spore cells their surrounding cell wall are called pollen spores. the male gametophyte.

Female Embryo sac (Female gametophyte) 2N N MEIOSIS MITOSIS

Inside the ovule, a single The remaining cell The eight nuclei and the diploid cell undergoes undergoes mitosis, surrounding membrane meiosis, producing four producing eight are called the embryo haploid cells. Three of nuclei. sac. This is the female the four cells disintegrate. gametophyte.

Plant Reproduction and Response 699 Figure 24–4 Pollination The appearance of a flower often indicates how it is pollinated. The flowers of an oak tree (left) are typical of wind-pollinated flowers in that they are small and not very showy but produce vast amounts of pollen. In contrast, many animal- pollinated flowers are large and brightly colored (right).

Pollination Pollination is the transfer of pollen to the female portions BUILD Vocabulary of the flower. Some angiosperms are wind pollinated, but most are Related Word Forms Several word forms are derived from the pollinated by animals. These animals, mainly insects, birds, and bats, word pollen. Pollination is the carry pollen from one flower to another. Because wind pollination is transfer of pollen from one flower less efficient than animal pollination, wind-pollinated plants, such as to another. A pollinator is an the oak tree in Figure 24–4, rely on favorable weather and sheer num- animal that moves pollen. bers of pollen grains to get pollen from one plant to another. Animal- pollinated plants have a variety of adaptations, such as bright colors and sweet nectar, to attract and reward animals. In turn, many animals have evolved bodies that enable them to reach nectar deep within certain flowers. For example, hummingbirds have long, thin beaks that can probe deep into flowers to reach their nectar. Insect pollination is beneficial to insects and other animals because it provides a dependable source of food—pollen and nectar. Plants also benefit because the insects take the pollen directly from flower to flower. This is more efficient than wind pollination, giving insect-pollinated plants a greater chance of reproductive success. The efficiency of insect pollination may be one of the main reasons why angiosperms displaced gymnosperms as the dominant land plants over the past 130 million years.

LAB See the Investigating Fertilization If a pollen grain lands on the stigma of a flower of the Pollen Tube Growth lab H same species, it begins to grow a pollen tube. This is the first in a series A K O on p 978 TEKS 10B N D B O of cellular interactions involved in angiosperm reproduction. Of the pollen grain’s two cells, one cell—the “generative” cell—divides and forms two sperm cells. As a result, the pollen tube now contains a tube Figure 24–5 Inside a Corn Kernel nucleus and the two sperm cells. The pollen tube grows into the style, The endosperm and embryo of a corn seed are the result of double where it eventually reaches the ovary and enters an ovule. fertilization. Inside the embryo sac, two distinct fertilizations take place— a process called double fertilization. First, one of the sperm Seed coat nuclei fuses with the egg nucleus to produce a diploid zygote that Endosperm becomes the new plant embryo. Second, the other sperm nucleus does something truly remarkable—it fuses with two polar nuclei Embryonic in the embryo sac to form a triploid (3N) cell. This cell will grow leaves into a food-rich tissue known as endosperm, which nourishes Embryo the seedling as it grows. The process of fertilization in Cotyledon angiosperms is distinct from that found in other plants. Two Primary root fertilization events take place—one produces the zygote and the other a tissue, called endosperm, within the seed. The rich supply of endosperm will nourish the embryo as it grows.

700 Chapter 24 • Lesson 1 Angiosperm Life Cycle Figure 24–6 In the life cycle of a typical angiosperm, the developing seeds of a flower are protected and nourished inside the ovary. Relate Cause and Effect Which two structures are the result of fertilization? Anther (2N) Pollen grains (N) (male gametophyte)

Haploid (N)

Diploid (2N) Stigma

Pollen tubes Style MEIOSIS

Ovule (2N) Haploid cell (N) Ovary Ovary (2N)

Embryo sac (N) (female gametophyte) Egg cell

Sperm

Pollen tube Polar nuclei

Endosperm nucleus (3N) Embryo Mature sporophyte (2N)

FERTILIZATION Seedling (2N) (new sporophyte) Endosperm Zygote Seed coat (2N) Seed

Double fertilization may be another reason why the angiosperms have been so successful. By using endosperm to store food, the flower- ing plant spends very little energy producing seeds from ovules until double fertilization has actually taken place. The energy saved can be used to make many more seeds. Figure 24–6 summarizes the life cycle of a typical angiosperm.

Online Journal Make an outline detailing the key features of angiosperm reproduction.

Plant Reproduction and Response 701 Vegetative Reproduction What is vegetative reproduction? Although angiosperms are best known by their patterns of sexual reproduction, many flowering plants can also reproduce asexually. This process, also known asvegetative reproduction, enables a single plant to produce offspring genetically identical to itself. This process takes place naturally in many plants, and horticulturists also use it as a technique to produce many copies of an individual plant. Vegetative reproduction is the formation of new individuals by mitosis. It does not require gametes, flowers, or fertilization. Types of Vegetative Reproduction Vegetative reproduction takes place in a number of ways. For example, new plants may grow from roots, leaves, stems, or plantlets. Figure 24–7 shows several ways plant species reproduce vegetatively. Because vegetative reproduction does not involve pollination or seed formation, a single plant can reproduce quickly. In addition, asexual reproduction allows a single plant to produce genetically identical offspring. This enables well-adapted individuals to rap- idly fill a favorable environment. One of the obvious drawbacks of asexual reproduction is that it does not produce new combinations of genetic traits, which may be valuable if conditions in the physical environment change.

Figure 24–7 Examples of Vegetative Reproduction Stem adaptations play a role in the vegetative repro­ duction of these three plants. Apply Concepts Describe how asexual reproduction might allow a plant to become rapidly established in a new area.

▼ A potato is an underground stem called a tuber that can grow whole new plants from buds called eyes.

Cholla and many other cactus species can reproduce by dropping sections of their stems. The small individuals growing at the base of the larger adults are, in fact, clones.

▼ Strawberry plants send out long, trailing stems called stolons. Nodes that rest on the ground produce roots and upright stems and leaves.

702 Chapter 24 • Lesson 1 Figure 24–8 Grafting When just starting to bud, a branch from a lemon tree is grafted onto the branch of an established orange tree. Months later, the mature branch bears lemon fruit. Grafting leads to a single plant bearing more than one species of fruit.

Plant Propagation Horticulturists often take advantage of vegeta­ tive reproduction. To propagate plants with desirable characteristics, horticulturists use cuttings or grafting to make many identical copies of a plant or to produce offspring from seedless plants. One of the simplest ways to reproduce plants vegetatively is by cuttings. A grower cuts from the plant a length of stem that includes a number of buds containing meristem tissue. That stem is then partially buried in soil or in a special mixture of nutrients that encourages root formation. Grafting is a method of propagation used to reproduce seedless plants and varieties of woody plants that cannot be propagated from cuttings. To graft, a piece of stem or a lateral bud is cut from the parent plant and attached to another plant, as shown in Figure 24–8. Grafting works only when the two plants are closely related, such as when a bud from a lemon tree is grafted onto an orange tree. Grafting usually works best when plants are dormant, which allows the wounds created by the cut to heal before new growth starts.

24.1 Review Key Concepts TEKS 10B 1. a. Review Name and describe four kinds of specialized leaves that make up a flower. b. Classify Which of the structures of a flower are the male 4. Review the life cycle of the sexual organs? Which are the female organs? green alga Chlamydomonas 2. a. Review Describe the features of fertilization that are char­ in Lesson 22.2. Make a acteristic of angiosperms. ­compare/contrast table com­ b. Explain How is fertilization in angiosperms different from par­ing alternation of gen­ fertilization in other types of plants? erations in flowering plants and in Chlamydomonas. Note c. Apply Concepts Relate the characteristics of angiosperm the dominant stage (haploid reproduction to angiosperm success. or diploid) of each organism’s 3. a. Review Define vegetative reproduction. life cycle and the point at b. Compare and Contrast Compare the advantages and which ­meiosis occurs. dis­advantages of sexual reproduction versus asexual reproduction in flowering plants.

Plant Reproduction and Response 703 24.2 Fruits and Seeds

In this lesson you will learn about fruits and seeds and their role in plant reproduction (TEKS 10B). In addition, you will learn about some of the adaptations in plants that allow them to survive in different ecosystems (TEKS 12B).

Key Questions THINK ABOUT IT What are fruits, and what purpose do they serve for the plants that produce them? Would it surprise you to learn that if How do fruits form? you ate a meal of corn on the cob and baked beans, from the point of How are seeds dispersed? view of a biologist, you were actually eating fruit? And have you ever What factors influence the wondered why plants go to the “trouble” of surrounding their seeds dormancy and germination of with tasty, fleshy fruits like those produced by apples, oranges, and seeds? grapes? Here’s a hint: You, and all the animals that enjoy those fruits, are being used. Plants may be smarter than you think. Vocabulary dormancy germination Seed and Fruit Development TEKS 10B How do fruits form? Taking Notes Flowchart Make a flowchart that The development of the seed, which protects and nourishes the plant shows the process of germina- embryo, contributed greatly to the success of plants on land. The tion and the factors that influ- angiosperm seed, protected by a fruit, was an even better adaptation. ence it. Indicate the differences By helping a seed get into the best possible location to start its new life, between monocots and dicots. fruits were immediately favored by natural selection. Interactions between the flower and the vascular system draw a steady flow of nutrients to support the growing embryo within the seed. As angiosperm seeds mature, ovary walls thicken to form a fruit that encloses the Strawberries  developing seeds. A fruit is simply a matured angio- sperm ovary, usually containing seeds. Examples of fruits are shown in Figure 24–9. Bigger fruits, such Rose Hips  as apples, tomatoes, and pears, require the transport of large amounts of sugars and other nutrients from the plant’s vascular system. The term fruit applies to the sweet things we usually think of as fruits, such as apples, grapes, and Peanut Shell  strawberries. However, foods such as peas, corn, beans, rice, cucumbers, and tomatoes, which we commonly call vegetables, are also fruits. The ovary wall surrounding a simple fruit may be fleshy, as it is in grapes and tomatoes, or tough and dry, like the shell that surrounds peanuts. (The Figure 24 –9 Variety Among peanuts themselves are the seeds.) Fruits Like the flowers from which they develop, fruits vary Online Journal Make a list of the first ten in structure. Observe Which “vegetables” that come to mind and place a example is a dry fruit? check mark next to ones you think are fruits. Explain why.

704 Seed Dispersal TEKS 12B How are seeds dispersed? What are fleshy fruits for, and why have they been favored by natural selection? They are not there to nourish the seedling—the endosperm does that. So why should these plants have seeds that are wrapped in an additional layer of nutrient-packed tissue? It seems pointless, but in evolutionary terms, it makes all the sense in the world. Think of the blackberries that grow wild in the forests of North America. Each seed is enclosed in a sweet, juicy fruit, making it a tasty treat for all kinds of animals. What good is such sweetness if all it does is get the seed eaten? Well, believe it or not, that’s exactly the point.

Figure 24 –10 Mechanisms of Seed Dispersal A Bohemian waxwing feasts on mountain ash berries (left) and will later disperse the seeds in its feces. Parachute-like dandelion fruits catch the wind, carrying the tiny attached seeds far and wide (middle). The buoyant coconut fruit can disperse its seed over great distances of water (right).

Dispersal by Animals The seeds of many plants, especially those encased in sweet, fleshy fruits, are often eaten by animals. The seeds are covered with tough coatings, allowing them to pass through an animal’s digestive system unharmed. The seeds then sprout in the feces elimi- nated from the animal. These fruits provide nutrition for the animal and also help the plant disperse its seeds—often to areas where there is less competition with the parent plants. Seeds contained in fleshy, nutritious fruits are usually dispersed by animals. Three mechanisms of seed dispersal are shown in Figure 24 –10. Animals also disperse many dry fruits, but not necessarily by eating them. Dry fruits sometimes have burs or hooks that catch in an animal’s fur, enabling them to be carried many miles from the parent plant. Dispersal by Wind and Water Animals are not the only means by which plants can scatter their seeds. Seeds are also adapted for dispersal by wind and water. Seeds dispersed by wind or water are typically contained in lightweight fruits that allow them to be carried in the air or in buoyant fruits that allow them to float on the surface of the water. A dandelion seed, for example, is attached to a dry fruit that has a parachute- like structure. This adaptation allows the seed to glide considerable distances away from the parent plant. Some seeds, like the coconut, are dispersed by water. Coconut fruits are buoyant enough to float in seawater for many weeks, enabling the seeds to reach and colonize even remote islands.

Plant Reproduction and Response 705 TEKS 12B

Temperature and Seed Germination Effect of Temperature on Seed Germination

Arisaema dracontium—“green dragon”—is Stored at 24°C Stored at 3°C a plant that grows from the southern United 100 States to Canada. The graph shows germina- tion rates of Arisaema seeds gathered from 80 two locations and stored at two different temperatures. 60 1. Interpret Graphs What effect does chilling 40 have on germination of seeds from Ontario? How does it affect the seeds from Louisiana? 20 2. Form a Hypothesis Describe how the Germination (%) 0 different rates of seed germination might be Seeds from Seeds from explained in terms of adaptation to the local Clinton, Ontario Baton Rouge, LA climate.

BUILD Vocabulary Seed Dormancy and Germination TEKS 12B Word origins The What factors influence the dormancy and germination of seeds? word dormancy comes from the Latin word Some seeds sprout so rapidly that they are practically instant plants. dormire, meaning “to Bean seeds are a good example. With proper amounts of water and sleep.” warmth, a mature bean seed rapidly sprouts and develops into a green plant. But many seeds will not grow when they first mature. Instead, these seeds enter a period of dormancy, during which the embryo is alive but not growing. The length of dormancy varies in different spe- cies. Germination is the resumption of growth of the plant embryo. Environmental factors such as temperature and moisture can cause a seed to end dormancy and germinate. How Seeds Germinate Before germinating, seeds absorb water. The absorbed water causes food-storing tissues to swell, cracking open the seed coat. Through the cracked seed coat, the young root emerges and begins to grow. The shoot—the part of the plant that will grow above ground—emerges next. The Role of Cotyledons Cotyledons are a flowering plant’s first leaves. Their job is to store nutrients and then transfer them to the growing embryo as the seed germinates. Figure 24 –11 compares germination in a monocot and a dicot. Monocots have a single coty- ledon, which usually remains underground while it passes nutrients to the young plant. The growing monocot shoot emerges from the soil protected by a sheath. In dicots, which have two cotyledons, there is no sheath to protect the tip of the young plant. Instead, the upper end of the shoot bends to form a hook that forces its way through the soil. This protects the delicate tip of the plant, which straightens as it emerges into the sunlight. In some species, the cotyledons appear above ground as the plant emerges, while in others, such as the garden pea, the cotyledons remain underground. 706 Chapter 24 • Lesson 2 Corn (monocot) Bean (dicot) Foliage Foliage leaves leaves Young shoot Cotyledons Young shoot

Seed coat Germinating Primary Germinating Primary seed root seed root

Advantages of Dormancy Seed dormancy can be adaptive to differ- Figure 24 –11 Germination: A Comparison The monocot corn ent ecosystems in several ways. For one, it can allow for long-distance seedling (left) grows directly upward, dispersal. And it also allows seeds to germinate under ideal growth protected by a sheath of tissue that conditions. The seeds of most temperate plants, for example, germi- surrounds the developing leaves. nate in the spring, when conditions are best for growth. For some In contrast, the garden bean (right) species, a period of cold temperatures during which the seeds are forms a hook in its stem that gently pulls the new plant tissues through the dormant is required before growth can begin. Seeds can easily survive soil. Predict What might happen to a winter cold, but many young green plants cannot. The period of cold that is germinating seedling that lacked such required is long enough that seeds will not germinate until the dangerous adaptations? winter season has passed. Sometimes, only extreme environmental conditions can end seed dormancy. Some pine trees, for example, produce seeds in cones that remain sealed until the high temperatures generated by forest fires cause the cones to open. The high temperature both activates and releases the seeds, allowing the plants to reclaim the forest quickly after a fire.

24.2 Review Key Concepts TEKS 10B, 12B 1. a. Review Describe how fruits form. c. Apply Concepts The seeds of a bishop pine b. Infer Is a pumpkin a fruit? Describe any germinate only after exposure to the extreme evidence you use in making your inference. heat of a forest fire. What might be the signifi- 2. a. Review Describe two methods of seed cance of this adaptation? dispersal. b. Pose Questions A new angiosperm species is discovered. What questions would you ask Creative Writing before predicting how its seeds are dispersed? 4. Imagine that you are writing a children’s book Explain the rationale for asking each question. about seeds and that you are working on the 3. a. Review Summarize the environmental chapter on dispersal. Write a paragraph on seed ­factors that affect seed germination. dispersal by wind. (Hint: Try to include details b. Explain Why is it adaptive for some seeds that you would have found appealing when you to remain dormant before they germinate? were about eight years old.)

Plant Reproduction and Response 707 24.3 Plant Hormones

In this lesson you will learn about plant hormones and how plants respond to different factors (TEKS 10B). In addition, you will learn about plant adaptations (TEKS 12B). You will also interpret data and predict trends from data (TEKS 2G).

Key Questions THINK ABOUT IT Plants, like all organisms, are collections of What roles do plant cells. Plants grow in response to environmental factors such as light, ­hormones play? moisture, temperature, and gravity. But how do roots “know” to grow down, and how do stems “know” to grow up? How do the tissues of What are some examples of environmental stimuli to a plant determine the right time of year to produce flowers? In short, which plants respond? how do the collections of cells in a plant manage to act together as a single organism? Is something carrying messages from cell to cell? How do plants respond to seasonal changes? Hormones TEKS 10B Vocabulary What roles do plant hormones play? hormone • target cell • receptor • auxin • Hormones are chemical signals produced by living organisms that apical dominance • cytokinin • affect the growth, activity, and development of cells and tissues. In gibberellin • abscisic acid • plants, hormones may act on the same cells in which they are made, ethylene • tropism • or they may travel to different cells and tissues. This is in contrast phototropism • gravitropism • to animal hormones, which typically act at a location some distance thigmotropism • photoperiodism away from the cells that produce them. Taking Notes Plant hormones serve as ­signals that control development Concept Map As you read, build a concept map sum- of cells, tissues, and organs. They marizing the effects of also coordinate responses to the different hormones on Hormone- environment, including the process plant growth. producing of reproduction. The two functions fit cells together well, because plants respond to the environment mainly by chang- ing their development. One role of hormones in plant Movement reproduction is shown in Figure 24–12. Target of hormone Hormones released from a mature cells flower inhibit the development of other flowers nearby.

Figure 24–12 Hormones and Flower Development In some species, hormone-producing cells in a mature flower release hormones that travel into flower buds and inhibit development. Once the mature flower is done blooming, production of the inhibiting hormone will decline, and the flower bud can then begin its bloom.

708 How Hormones Act Cells in an organism affected by a particular hormone are called target cells. To respond to a hormone, a cell must contain hormone receptors—usually proteins—to which hormone molecules bind. The response will depend on what kinds of receptors are present in the target cell. One kind of receptor might alter metabo- lism; a second might speed growth; a third might inhibit cell division. Thus, depending on the receptors present, a given hormone may cause a different response in roots than it does in stems or flowers—and the effects may change as cells add or remove receptors. Cells that do not contain receptors are generally unaffected by hormones. Figure 24–13 How Plants Detect Light The Darwins conducted controlled experiments to determine which region of the plant senses light. When they removed the seedling tip or placed an opaque cap over the tip, they observed no bending toward light. But when they placed a clear cap on the tip or an opaque shield around the base, they observed bending similar to that seen in the control. Control Variables What variable did the Darwins control for by comparing the results of seedlings treated with a clear cap Control Tip Opaque Clear Opaque shield versus no cap? removed cap cap over base

Auxins The first step in the discovery of plant hormones came over a century ago, and was made by a scientist already familiar to you. In 1880, Charles Darwin and his son Francis published the results of a series of experiments exploring the mechanism behind a grass seed- ling’s tendency to bend toward light as it grows. Figure 24–14 Auxins and The results of their experiments, shown in Figure 24–13, suggested Cell Elongation Cells elongate more on the shaded side of the that the tip of the seedling somehow senses light. The Darwins hypothe- shoot, where there is a higher sized that the tip produces a substance that regulates cell growth. More concentration of auxins. than forty years later, the regulatory substances produced by the tips of growing plants were identified and named auxins. Auxins stimulate cell elongation and the growth of new roots, among other roles that they play. They are produced in the shoot apical meri- More stem and transported to the rest of the plant. auxin  Auxins and Cell Elongation One of the effects of auxins is to stimulate cell elongation, as shown in ­Figure 24–14. In the Darwins’ experiment, when light hits one side of the shoot, auxins collect in the Less shaded part of the shoot. This change in concentra- auxin tion ­stimulates cells on the dark side to lengthen. As a result, the shoot bends away from the shaded side and toward the light.

Online Journal Review Figure 24–13. Describe how the results led the Darwins to conclude that the tip of the seedling senses light.

Plant Reproduction and Response 709 TEKS 2G

Auxins and Plant Growth Effects of Hormone Concentration This graph shows the results of experiments in on Plant Growth which carrot cells were grown in the presence of varying concentrations of auxins. The blue Stems line shows the effects on root growth. The red line shows the effects on stem growth. promotes 1. Analyze Data At what auxin concentration are the stems stimulated to grow the most? 0 Growth 2. Analyze Data How is the growth of the Roots roots affected by the auxin concentration at which stems grow the most? inhibits 3. Infer If you were a carrot farmer, what −11 −9 −7 −5 −3 −1 concentration of auxin should you apply to 10 10 10 10 10 10 your fields to produce the largest carrot roots? Increasing Auxin Concentration (particles/L)

 Auxins and Branching Auxins also regulate cell division in meri- stems. As a stem grows in length, it produces lateral buds. As you may have observed, the buds near the apex grow more slowly than those near the base of a plant. The reason for this delay is that growth at the lateral buds is inhibited by auxins. Because auxins move out from the apical meristem, the closer a bud is to the stem’s tip, the more it is inhibited. This phenomenon is called apical dominance. If you snip off the tip of a plant, these lateral buds begin to grow more quickly. The plant becomes bushier. This is because the apical meristem—the source of the growth-inhibiting auxins—has been eliminated. Cytokinins Cytokinins are plant hormones that are produced in growing roots and in developing fruits and seeds. Cytokinins stimu- late cell division, they interact with auxins to help to balance root and shoot growth, and stimulate regeneration of tissues damaged by injury. Figure 24–15 Apical Dominance Cytokinins also delay the aging of leaves and play important roles in The basil plant on the right has had the early stages of plant growth. its apical meristem pinched off, Cytokinins often produce effects opposite to those of auxins. For in contrast to the plant on the left, example, root tips make cytokinins and send them to shoots; shoot which hasn’t. Observe How are the two plants different? tips make auxins and send them to roots. This exchange of signals can restore lost organs and keep root and shoot growth in balance. Auxins stimulate the initiation of new roots, and they inhibit the initiation and growth of new shoot tips. Cytokinins do just the opposite. So if a tree is cut down, the stump will often make new shoots because auxins have been removed and cytokinins accumulate near the cut.

Online Journal Make a 2×2 table labeled Shoot and Root across the top and Auxins and Cytokinins down the side. Then, fill in the effects of these hormones.

710 Chapter 24 • Lesson 3 Gibberellins For years, farmers in Japan knew of a disease that weak- See the Plant Hormones LAB and Leaves lab on ened rice plants by causing them to grow unusually tall. The plants would H A K p 979. TEKS 2H, 10B O flop over and fail to produce a high yield of rice grain. Farmers called the N D B O disease the “foolish seedling’’ disease. In 1926, Japanese biologist Eiichi Kurosawa discovered that a fungus, Gibberella fujikuroi, caused this extraordinary growth. His experiments showed that the fungus pro­duced a growth-promoting substance. In fact, the chemical produced by the fungus mimicked hormones produced naturally by plants. These hormones, calledgibberellins,­ stim- ulate growth and may cause dramatic increases in size, particularly in stems and fruits. Abscisic Acid Gibberellins also interact with another hormone, abscisic acid, to control seed dormancy. Abscisic acid inhibits cell division, thereby halting growth. Recall that seed dormancy allows the embryo to rest until con- ditions are good for growth. When seed development is complete, abscisic acid stops the seed’s growth and shifts the embryo into a dor- mant state. The embryo rests until environmental events shift the bal- ance of hormones. Such events may include a strong spring rain that washes abscisic acid away. (Gibberellins do not wash away as ­easily.) Without the opposing effect of abscisic acid, the gibberellins can signal germination. Abscisic acid and gibberellins have opposite effects, much like the auxins and cytokinins. The opposing effects of plant hormones contribute to the balance necessary for homeostasis. Ethylene One of the most interesting plant hormones, ethylene, is actually a gas. Fruit tissues release small amounts of the hormone ethylene, causing fruits to ripen, an essential part of the reproduc- tive process. Ethylene also plays a role in causing plants to seal off and drop certain structures. Petals drop after flowers have been pollinated, leaves drop in autumn, and fruits drop after they ripen. In each case, ethylene signals cells at the base of the structure to seal off from the rest of the plant by depositing waterproof materials in their walls.

Figure 24–16 A Summary of Plant Hormones A Summary of Plant Hormones This table lists some of the effects Hormone Some of the Effects Where Found of the major plant hormones and where the hormones can Auxins Promote cell elongation and apical Produced in shoot

dominance; stimulate growth of apical meristem and be found in the plant body. new roots transported elsewhere Interpret Tables Name two pairs of hormones that work in Cytokinins Stimulate cell division; affect root Growing roots opposition to each other. growth and differentiation; may work in opposition to auxins Gibberellins Stimulate growth; influence various Meristems of shoot, developmental processes; promote root, and seed germination embryo Abscisic acid Inhibits cell division; promotes seed Terminal buds; seeds dormancy Ethylene Stimulates fruits to ripen; causes plants Fruit tissues; aging to seal off and drop unnecessary leaves and flowers organs, such as leaves in autumn

Plant Reproduction and Response 711 Tropisms and Rapid Movements TEKS 10B What are some examples of environmental stimuli to which plants respond? Like all living things, plants need the power of movement to respond to the environment. Many plant movements are slow, but some are so fast that even animals cannot keep up with them. Tropisms Plant sensors that detect environmental stimuli signal BUILD Vocabulary ­elongating organs to reorient their growth. These growth responses word origins The word tropism are called tropisms. Plants respond to environmental stimuli comes from a Greek word that such as light, gravity, and touch. means “turning.”

 Light The tendency of a plant to grow toward a light source is called phototropism. This Figure 24–17 Three Tropisms response can be so quick that young seedlings reorient themselves in a matter of hours. Recall that changes in auxin concentration are respon- sible for phototropism. Experiments have shown that auxins migrate toward shaded tissue, pos- sibly due to changes in membrane permeability in response to light.  Gravity Auxins also affectgravitropism, the response of a plant to gravity. For reasons still not understood, auxins migrate to the lower sides of horizontal roots and stems. In horizontal stems, the migration causes the stem to bend upright. In horizontal roots, however, the migra- tion causes roots to bend downward.  Touch Some plants even respond to touch, a process called ­thigmotropism. Vines and ­climbing plants exhibit thigmotropism when Figure 24–18 Rapid Movement they encounter an object and wrap around it. The mimosa plant responds to touch Other plants, such as grape vines, have extra by folding in its leaves quickly. This growths called tendrils that emerge near the response is produced by decreased osmotic pressure in cells near the base base of the leaf and wrap tightly around any of each leaflet. Infer What adaptive object they encounter. value might this response have?

Rapid Movements Some plant responses are so rapid that it would be a mistake to call them tropisms. Figure 24–18 shows what happens if you touch a leaf of Mimosa pudica, appropriately called the “sensitive plant.” Within only two or three seconds, its two leaflets fold together completely. The carnivorous Venus’ fly- trap also demonstrates a rapid response. When an insect lands on a flytrap’s leaf, it triggers sensory cells on the inside of the leaf, send- ing electrical signals from cell to cell. A combination of changes in osmotic pressure and cell wall expansion interact in a way that causes the leaf to snap shut, trapping the insect inside.

712 Chapter 24 • Lesson 3 Response to Seasons TEKS 10B, 12B How do plants respond to seasonal changes? “To every thing there is a season.” Nowhere is this more evident than in the regular cycles of plant growth. Year after year, some plants flower in the spring, others in summer, and still others in the fall. Plants such as chrysanthemums and poinsettias flower when days are short and are therefore called short-day plants. Plants such as spinach and irises flower when days are long and are therefore known as long-day plants. Photoperiod and Flowering How do all these plants manage to time their flowering so precisely in response to the environment? In the early 1920s, scientists discovered that tobacco plants flower according to their photoperiod, the number of hours of light and darkness they receive. Additional research showed that many other plants also respond to changing photoperiods, a response called photoperiodism. This type of response is summarized in Figure 24–19. Photoperiodism is a major factor in the timing of seasonal activities such as flowering Figure 24–19 Effects of Photoperiod Changes in the and growth. photoperiod can affect the seasonal It was later discovered that a plant pigment called phytochrome timing of flowering. Form an (fyt oh krohm) is responsible for plant responses to photoperiod. Opinion Are “short-day plant” and Phytochrome absorbs red light and activates a number of signaling “long-day plant” the best names for categorizing these plants, or would pathways within plant cells. By mechanisms that are still not under- it be better to name plants after their stood completely, plants respond to regular changes in these pathways. responses to night length? Explain These changes determine the patterns of a variety of plant responses. your reasoning.

Effect of Photoperiod on Flowering

Long Day Short Day Interrupted Night Midnight Midnight Midnight

Noon Noon Noon

Short-day plants Short-Day flower only when Plant exposed to an extended period of darkness.

Long-day plants Long-day plants also flower if a Long-Day flower when brief period of Plant exposed to a light interrupts short period the darkness–this of darkness. essentially divides one long night into two short nights.

Plant Reproduction and Response 713 Winter Dormancy Phytochrome also regulates the changes in activity that prepare many plants for dormancy as win- ter approaches. Recall that dormancy is the period during which an organism’s growth and activity decrease or stop. As cold weather approaches, deciduous plants turn off photosynthetic pathways, transport materials from leaves to roots, and seal off leaves from the rest of the plant.  Leaf Loss In temperate regions, many flowering plants lose their leaves during the colder months. At summer’s end, the phytochrome in leaves absorbs less light as days shorten and nights become longer. Auxin production drops, but the production of ethylene increases. The change in the relative amounts of these two hormones starts a series of events that gradually shut down the leaf. As chlorophyll breaks down, other pigments that have been present all along—including yellow and orange carotenoids—become visible for the first time. The brilliant reds come from freshly made anthocy- anin pigments.  Changes to Meristems Hormones also produce important changes in apical meristems. Instead of continuing to produce leaves, meri- Figure 24–20 Adaptations stems produce thick, waxy scales that form a protective layer around for Winter In autumn, leaves new leaf buds. Enclosed in its coat of scales, a terminal bud can sur- shut down photosynthesis and fall vive the coldest winter days. At the onset of winter, xylem and phloem from deciduous trees. Meanwhile, tissues pump themselves full of ions and organic compounds. The meristems at the tips of the branches produce thick, waxy scales that cover resulting solution acts like antifreeze in a car, preventing the tree’s sap and protect new stem and leaf buds from freezing. This is one of several mechanisms plants use to survive through the harsh winter. the bitter cold.

24.3 Review Key Concepts TEKS 10B, 11A, 12B

1. a. Review Describe how plant hormones con- c. Design an Experiment How could a garden- tribute to homeostasis. store owner determine what light conditions b. Infer Why should a person who trims trees are needed for a particular flowering plant to for a living know about the effect of apical bloom? Design a controlled experiment to dominance on the shapes of trees? Explain. find out. 2. a. Review Give three examples of plant 4. Describe Review what you learned about evo- responses to external stimuli. lution by natural selection in Chapter 16. Then, b. Apply Concepts Describe how you might using what you know about natural selection, use a houseplant, a marker, and a sunny win- describe how plant adapta­tions for dormancy dowsill to describe a plant’s response to light. may have devel­oped over time. 3. a. Review Summarize plant responses to seasonal changes. b. Explain Which type of plant—short-day or long-day—is likely to bloom in the summer? Explain your answer.

714 Chapter 24 • Lesson 3 24.4 Plants and Humans

In this lesson you will learn about the history of agriculture and the uses of plants.

THINK ABOUT IT A stroll through the produce section of a grocery Key Questions store will convince you that plants are important. Even a medium-sized Which crops are the major food store will contain products made from hundreds of different plant food supply for humans? species. But which ones are the most important? Are there certain plants that we simply couldn’t live without? What are some examples of benefits besides food that humans derive from plants?

Agriculture Vocabulary Which crops are the major food supply for humans? green revolution The importance of agriculture—the systematic cultivation of plants— should be obvious, even to those of us who live in urban areas and sel- Taking Notes dom visit a farm. Modern farming is the foundation on which human Preview Visuals Preview society is built. North America has some of the richest, most produc- Figure 24–24. Identify what tive cropland in the world. As a result, farmers in the United States and plants provided the raw materi- Canada produce so much food that they are able to feed millions of als for the products shown in the photos. Then list any other people around the world as well as their own citizens. products you can think of that Worldwide Patterns Many scholars now trace the beginnings of come from plants. human civilization to the cultivation of crop plants. Evidence suggests that agriculture developed separately in many parts of the world about 10,000 to 12,000 years ago. Once people discovered how to grow plants for food, the planting and harvesting of crops tended to keep them in one place for much of the year, leading directly to the establishment of Figure 24–21 Plants and social institutions. Even today, agriculture is the principal occupation Agriculture Rice is a staple of more human beings than any other activity. crop in China and many nations of Southeast Asia. Thousands of different plants—nearly all of which are angiosperms­— are raised for food in various parts of the world. Yet, despite this diversity, much of human society depends upon just a few of these plants. Worldwide, most people depend on a few crop plants, such as rice, wheat, soybeans, and corn, for the bulk of their food supply. The same crops are also used to feed livestock.

715 Teosinte

Modern Corn

Modern corn has greatly exaggerated kernels compared to its ancestor, teosinte. Note how much larger corn is than teosinte when compared to the size of a quarter.

Figure 24–22 From Wild Grass to Staple Crop The selective You may not have thought of it this way, but the food we eat from breeding of a wild grass called most crop plants is taken from their seeds. For nutrition, most of teosinte about 8000 years ago led to the development of maize and humanity worldwide depends on the endosperm of only a few care- modern corn. fully cultivated species of grass. The pattern in the United States follows this trend. Roughly 80 percent of all U.S. cropland is used to grow just four crops: wheat, corn, soybeans, and hay. Of these crops, three—wheat, corn, and hay—are derived from grasses. New Plants The discovery and introduction of new crop plants has frequently changed human history. Before they were discovered in the Americas, many important crops—including corn, peanuts, and potatoes—were unknown in Europe. The introduction of these plants changed European agriculture rapidly. We think of boiled potatoes, for example, as a traditional staple of German and Irish cooking, but 400 years ago, potatoes were new items in the diets of Europeans. The efficiency of agriculture has been improved through the selec- tive breeding of crop plants and improvements in farming techniques. Recall from Chapter 15 that selective breeding is a method for improv- ing a species by allowing only organisms with certain traits to produce the next generation. The corn grown by Native Americans, for exam- ple, was developed more than 8000 years ago from teosinte, a wild grass found in Mexico. Further selective breeding has produced modern-day corn. The changes caused by selective breeding can be very dramatic, as shown in Figure 24–22. In more recent times, other familiar crops have been the product of selective breeding. Sugar beets, the source of most refined sugar from the United States, were produced from the ordinary garden beet using selective breeding. Plants as different as cabbage, broccoli, and Brussels sprouts have been developed from a single species of wild mustard.

716 Chapter 24 • Lesson 4 Since 1980, the amount of corn grown in the United States has increased dramatically.

Annual Corn Yield in the United States

200

175

150

125

100

Bushels per Acre 75 Changes in Agriculture Between 1950 and 1970, a world- wide effort to combat hunger and malnutrition led to dra- 50 matic improvements in farming techniques and crop yields. 1980 1990 2000 2010 This effort came to be called the green revolution because it Year greatly increased the world’s food supply. Green revolution technologies enabled many countries to end chronic food shortages and, in some cases, become exporters of surplus food. At the heart of the green revolution was the use of high-yield varieties of seed and fertilizer. For thousands of years, farmers have added essential nutrients in the form of natural fertilizers such as animal manure. While some farmers today still use these traditional methods, many farmers use artificial fertilizers. Fertilizers are labeled with three numbers that reflect the percentage by weight of three elements: nitrogen (N), phos- phorus (P), and potassium (K). A bag of garden fertilizer labeled “20-10-5’’ is 20 percent nitrogen, 10 percent phosphorus, and 5 percent potassium by weight. Fertilizers and pesticides must be used with great care. Overfertilizing can kill crop plants by putting too high a concentration of salts into the soil. The intensive use of fertil- izers can also affect the groundwater. When large amounts of nitrogen- and phosphate-containing fertilizer are used near wetlands and streams, runoff from the fields may contaminate the water. Pesticides can also pose a health risk. Chemical pes- ticides are poisons, and they have the potential to harm wildlife and leave dangerous chemical residues in food. Figure 24–23 Reading a Fertilizer Label Three numbers typically appear on a fertilizer label. Online Journal Write a paragraph summarizing Apply Concepts Describe what the the risks and benefits of modern agricultural practices. numbers on this label mean.

Plant Reproduction and Response 717 Fiber, Wood, and Medicine Figure 24–24 Products From Plants What are some examples of other benefits besides Plants provide the raw food that humans derive from plants? materials for many useful products. Some of the most important uses of plants have nothing to do with food. Plants produce the raw materials for ▼ The succulent plant our homes and clothes, and some of our most powerful Aloe vera contains and effective medicines. Some examples of plant products many chemicals that soothe and moisturize are shown in Figure 24–24. If you’re reading this page out the skin. Extracts of of the printed book, you are turning paper pages made this plant are used in from the conifer forests of North America, possibly sitting many skin lotions as on a chair made from oak tree xylem, and probably wear- well as in burn and ing at least one piece of clothing made from the fibers of the wound ointments. ­cotton plant.

Cotton is used in countless products The acoustical including thread, fabrics, bandages, properties of Sitka carpeting, and insulation. Cotton spruce wood make fibers are outgrowths of the

it ideal for use in seed coat epidermis. ▼ pianos, guitars, violins, and other musical instruments. ▼

24.4 Review Key Concepts 1. a. Review Name four crops that make up the base of the world’s food supply. b. Relate Cause and Effect Describe how selec- Use the line graph from Figure 24–22 to answer tive breeding was used to develop corn from an the questions below. ancestral grass that looked very different. 3. Calculate By about how much did the amount 2. a. Review Besides food, what other important of corn produced per acre of farmland increase products are developed from plants? between 1985 and 2005? b. Infer What effect could plant species extinc- 4. Interpret Graphs How would you describe the tion have on therapeutic drug development? overall trend in the data? 5. Predict What factors do you think might ­influence corn production in the next decade?

718 Chapter 24 • Lesson 4 TEKS 2H, 3F

The Evolution of Agriculture More than 10,000 years ago, humans began a gradual transition from hunter-gatherer societies to civilizations that were reliant on crops—many of which are still grown today.

7500 7000 6500 6000 5500 5000 4500 4000

b c 4500 b.c. 8000 . . Rice farming becomes well Inhabitants of the Middle established in southern China, East begin to farm wheat. southeast Asia, and northern The change from gather- India. Rice farming spreads widely ing a crop in the wild from these regions, and rice later to farming it eventually becomes a major Chinese export. contributes to the rise of one of the earliest Middle Eastern civilizations. b c 3500 b.c. 5500 . . The potato is farmed in the  Barley is grown in the Nile Andes Mountains of South Valley of Egypt. About 2000 years America. Early Andean farmers later, farming settlements are united eventually produce hundreds throughout the Nile Valley, and of different varieties of ­potatoes Egyptian culture flourishes. by growing them on irrigated terraces built on mountain 5000 b.c.  People in central Mexico grow a slopes. form of corn called maize. Early corncobs are only about an inch 7000 b.c. long and have a few dozen kernels.  Chilies and avocados become The ancestor of corn was a wild important additions to the diets grass called teosinte. of Mesoamerican people. Chilies are used for flavoring foods, and avocados provide vitamins and oils.

The domestication of all major crops had a huge impact on the growth of civiliza- tions. Choose one of the crops discussed above and research how that crop contributed to the rise of civilization and culture in the region discussed. Using technology, produce, publish, and update a poster display that communicates your findings in words, pictures, and other graphics.

Biology and History 719 Y TH PL E P

A TEKS

10B

the green lemons In trying to solve the mystery, the shippers remem- bered a story from the nineteenth century. In those days, gas streetlights were common in large cities. A few years after the gaslights were installed, city dwellers noticed that trees growing near them had developed short, thick stems and dropped their leaves much earlier than other trees of the same type. It was as if hormone levels in the trees had been affected. In fact, the levels had changed. One of the components of the gas used in the lights was ethylene. Recall that one effect of ethylene is to stimulate the ripen- ing of fruit. Whether the ethylene is manufactured by the plant or by an external source, such as a kerosene heater, doesn’t matter. Because ethylene is a gas, it can diffuse through the air, cell walls, and membranes of a plant and its fruit. When that source of ethylene was removed from the environment of the picked lemons (when the electric heat- ers replaced the kerosene ones)—the ripening stimulus was removed, and the lemons stayed green.

1. Relate Cause and Effect Tomatoes put in a 3. Infer Recent studies have shown that gaseous paper bag with ripe apples ripen much more hormones are involved in a plant’s system-wide quickly than those placed in the open air. What response to an attack by herbivores such as cater- might cause this effect? pillars. What might be the benefits of a gaseous hormone in such a situation? 2. Describe Would the green lemons have ripened faster had they been put in a larger warehouse full 4. Form a Hypothesis How could farmers, shippers, of ripening lemons? Explain your answer. and produce marketers use the effects of ethylene to their advantage?

720 Chapter 24 • Solve the Chapter Mystery 24 TEKS Practice TEKS 10B, 12B

Review Content 12. What is a tropism? Give one example of a tropism 1. In angiosperms, the structures that produce the that affects plant stems and another example of a male gametophyte are called the tropism that affects roots. a. anthers. c. pollen tubes. 13. Describe two different ways in which a plant may b. sepals. d. stigmas. respond to changes in photoperiod. 14. 2. Pollination occurs when pollen lands on Describe what happens to deciduous plants dur- ing winter dormancy. a. the style. c. the filament. b. the stigma. d. the anther. 15. Give an example of a plant you have eaten in the last 24 hours that you think is a product of selec- 3. The process in which a single plant produces tive breeding. Explain why you think so. many offspring genetically identical to itself is a. sexual reproduction. Think Critically b. agriculture. 16. Interpret Visuals The diagram below shows the c. dormancy. parts of a typical flower. d. vegetative reproduction. a. Inside which structure is pollen produced? 4. The thickened ovary wall of a plant may join with b. What structure is represented by A? What is its other parts of the flower to become the function? a. fruit. c. endosperm. c. In which structure do seeds develop? b. seed. d. cotyledon. d. What is the name of structure G? 5. The period during which the embryo is alive but B A C E not growing is called D a. fertilization. c. dormancy. b. vegetative growth. d. germination. 6. Chemical signals in plants affecting the growth, activity, and development of cells and tissues are called a. hormones. c. auxins. b. enzymes. d. phytochromes. 7. Substances that stimulate cell division and cause dormant seeds to sprout are a. gibberellins. c. cytokinins. F G H b. d. auxins. phytochromes. 17. Predict Some plants form flowers that produce 8. Photoperiod is a measurement of stamens but no carpels. Could fruit form on one a. water level. c. gravity. of these flowers? Cite textual evidence to support b. day length. d. nutrients. your answer. 18. Describe Describe the interactions that occur Understand Concepts among plant systems that perform the function of reproduction in plants. 9. Give examples of seed dispersal by animal, wind, and water. 19. Infer The seeds of lupines, an Arctic plant, can ­remain dormant for thousands of years. Why 10. What is the function of dormancy? might this trait be an important adaptation for a 11. Explain how auxins act in opposition to plant in an Arctic environment? c­ytokinins.

721 IEW TH V E E R Biology Chapter 24 TEKS Practice TEKS 20. Compare and Contrast Compare and contrast the benefits and the dangers of using pesticides Lesson 1 and fertilizers to grow food crops. In Lesson 24.1, you learned about reproduction 21. Describe Describe the interactions that occur in flowering plants. Flowers are reproductive among plant systems that perform the function of organs that are composed of specialized leaves: response in plants. sepals, petals, stamens, and carpels. The process of fertilization in angiosperms is different from that 22. Compare Compare variations of organisms living found in other plants. Two fertilization events take in different ecosystems. The range of the red place—one produces the zygote and the other, the maple species Acer rubrum, extends from south- endosperm. Some plants can reproduce asexually ern Canada in the north to Florida in the south, by means of vegetative reproduction. and westward to eastern Texas. Among members Readiness TEKS: 10B of this species, there is large variation in character- Supporting TEKS: 5B istics such as hardiness and the color of leaves in autumn. What might account for this variation? Lesson 2 In Lesson 24.2, you learned about flowers and Use Science Graphics seeds. As seeds mature, ovary walls thicken to Recall that growth responses of plants to external form a fruit that encloses the developing seeds. Seeds that are contained in fleshy fruits are usually stimuli are called tropisms. A tropism is positive if dispersed by animals. Seeds that are dispersed the affected plant part grows toward the stimulus. The by wind or water are typically contained in response is negative if the plant part grows away from lightweight fruits that allow them to be carried the stimulus. The experiment shown below was intend- in the air or in buoyant fruits that float in water. ed to test the effect of gravitropism on plant growth. Environmental conditions such as temperature and The conclusion drawn from the experiment was that moisture can cause a seed to end dormancy and the plant stems grow upward due to negative gravitro- germinate. pism. Use the diagram to answer questions 23–26. Readiness TEKS: 10B Supporting TEKS: 12B

Lesson 3 Lesson 24.3 discusses plant hormones. Plant hormones serve as signals that control the development of cells, tissues, and organs. They also coordinate responses to the environment. The opposing effects of plant hormones contribute to the balance of homeostasis. Plants respond to environmental stimuli such as light, gravity, and touch. Photoperiodism is a major factor in the timing of seasonal activities such as flowering and growth. As cold weather approaches, deciduous plants turn off photosynthetic pathways, transport 23. Interpret Visuals Describe the three experimental materials from leaves to roots, and seal off leaves. setups and the result of each. Readiness TEKS: 10B 24. Form a Hypothesis What was the probable hy- Supporting TEKS: 2G, 12B pothesis for this experiment? Lesson 4 25. Interpret Visuals From the experimental setups In Lesson 24.4, you learned how plants are parts shown, was the hypothesis successfully tested? of our everyday lives. Plants produce the raw Explain. materials for our homes and clothes, as well as 26. Evaluate and Revise Indicate what kinds of food and medicines. changes you would make to improve this experi- mental design.

722 Chapter 24 • TEKS Practice ★ TEKS Practice: Chapter Review

1 Which evidence best supports the conclusion that a plant’s transport systems are vital to the responses regulated by auxins?

A Auxins are produced in the shoot apical meristem, but affect many parts of the plant.

B Auxins regulate the plant’s response to light.

C Auxins stimulate cell elongation in shaded areas, causing the plant to grow toward a light source.

D Auxins can inhibit or promote growth within a plant.

2 The diagram shows the transport of substances that regulate plant responses.

Shoots

A B

Roots

What substances are represented by the labels “A” and “B”?

F Abscisic acid

G Auxin

H Cytokinin

J None of the above

Plant Reproduction and Response 723 3 The table shows experimental data about germination of two sets of seeds of the same plant species.

Effect of Temperature on Seed Germination

Stored at 24°C Stored at 3°C 100

80

60

40

Germination (%) 20

0 Seeds from Seeds from Clinton, Ontario Baton Rouge, LA

What conclusion can be drawn from the data?

A There are a wide variety of conditions in different ecosystems.

B Seeds of different plant species germinate under different conditions.

C Seeds of plants in different ecosystems germinate under different conditions.

D Seeds from warmer climates germinate at a greater rate than seeds from colder climates.

4 Under certain conditions, some flowering plants can undergo vegetative reproduction. Which best predicts the result of a plant undergoing vegetative reproduction?

F Great increase in the number of animal pollinators required

G Rapid production of individuals well-adapted to the ecosystem

H Small increase in the number of flowers produced in the ecosystem

J Large increase in genetic variation among individuals of this plant species

724 Chapter 24 • TEKS Practice ★ TEKS Practice: Cumulative Review

5 A scientist records the number of pollinators in an area of apple orchards each year and produces this graph. Number of Pollinators

Time

From the data in the graph, what inference could you make about apple production in the area?

A Apple production has gradually increased over the time shown in the graph.

B Apple production has gradually decreased over the time shown in the graph.

C Apple production has not likely changed over the time shown in the graph.

D Apple production has likely changed, but it cannot be predicted by these data.

6 Seeds are important to the reproduction of many kinds of plants. Some seeds, especially those dispersed by animals, are enclosed in a sweet, fleshy fruit. How would damage to the vascular tissue leading to a developing fruit affect the fruit?

F Water loss from the fruit would happen at an increased rate.

G The number of seeds enclosed in the fruit would be decreased.

H The amount of water and sugar stored in the fruit would be decreased.

J The raw materials needed for photosynthesis would not reach the fruit.

If You Have Trouble With . . . Question 1 2 3 4 5 6 See Lesson 24.3 24.3 24.2 24.1 22.4 23.5 TEKS 10B, 3A 10B 12B, 2H 12B 10B, 2G 10B

Plant Reproduction and Response 725