Botany Basics ❂1

lants are essential to life on earth. Either directly or indirectly, they are the primary food source for humans and other animals. Additionally, they P ❂ OPICS IN THIS CHAPTER provide fuel, replenish the earth’s oxygen supply, prevent T soil erosion, slow down wind movement, cool the atmosphere, provide wildlife habitat, supply medicinal com- ■ Plant life cycles pounds, and beautify our surroundings. ■ Internal plant parts Many plants are familiar to us, and we can identify and ■ External plant parts appreciate them based on their external structures. However, their internal structures and functions often are ■ Plant growth and overlooked. Understanding how plants grow and develop development helps us capitalize on their usefulness and make them ■ Environmental factors part of our everyday lives. affecting growth This chapter focuses on vascular plants—those that contain water-, nutrient-, and food-conducting tissues ■ Plants in communities called xylem and phloem. Ferns and seed-producing ■ Plant hormones and plants fall into this category. growth regulators In several cases, we will distinguish between monocoty- ledonous and dicotyledonous plants. Sometimes called monocots and dicots for short, these plants have several important distinguishing characteristics. For example, monocots (e.g., grasses) produce only one seed leaf, while dicots (broadleaf plants) have two. The vascular systems, flowers, and leaves of the two types of plants By Ann Marie VanDerZanden, Extension also differ (Table 1). These differences will be important master gardener state coordinator, Oregon in our discussion of plant growth and development. State University.

Table 1.—Comparison between monocots and dicots. Structure Monocots Dicots Seed leaves one two Vascular system Xylem and phloem are paired in bundles, Xylem and phloem form rings inside the stem. which are dispersed throughout the stem. The phloem forms an outer ring, the xylem an inner ring. Floral parts Usually in threes or multiples of three. Usually in multiples of four or five. Leaves Often parallel-veined. Generally net-veined. 4 • Botany Basics—Chapter 1

Botany terminology Monocot—Having one seed leaf. Anther—The pollen sac on a male flower. Node—An area on a stem where a leaf, Apex—The tip of a shoot or root. stem, or flower bud is located. Apical dominance—The tendency of an Ovary—The part of a female flower apical bud to produce hormones that where eggs are located. suppress growth of buds below it on Petiole—The stalk that attaches a leaf to the stem. a stem. Axil—The location where a leaf joins a Phloem—Photosynthate-conducting stem. tissue. Cambium—A layer of growing tissue that Photosynthate—A food product (sugar separates the xylem and phloem and or starch) created through photo- continuously produces new xylem and synthesis. phloem cells. Photosynthesis—The process in green Chlorophyll—The green pigment in plants of converting carbon dioxide leaves that is responsible for trapping and water into food (sugars and light energy from the sun. starches) using energy from sunlight. Chloroplast—A specialized component of Pistil—The female flower part; consists of certain cells; contains chlorophyll and a stigma, style, and ovary. is responsible for photosynthesis. Respiration—The process of converting Cortex—Cells that make up the primary sugars and starches into energy. tissue of the root and stem. Stamen—The male flower part; consists Cotyledon—The first leaf that appears on of an anther and a supporting filament. a seedling. Also called a seed leaf. Stigma—The top of a female flower part; Cuticle—A relatively impermeable collects pollen. surface layer on the epidermis of leaves Stoma (pl. stomates, stomata)—Tiny and fruits. openings in the epidermis that allow Dicot—Having two seed leaves. water, oxygen, and carbon dioxide to Epidermis—The outermost layer of plant pass into and out of a plant. cells. Style—The part of the female flower that Guard cell—Epidermal cells that open connects the stigma to the ovary. and close to let water, oxygen, and Pollen travels down the style to reach carbon dioxide pass through the the ovary, where fertilization occurs. stomata. Transpiration—The process of losing Internode—The space between nodes on water (in the form of vapor) through a stem. stomata. Meristem—Specialized groups of cells Turgor—Cellular water pressure; that are a plant’s growing points. responsible for keeping cells firm. Mesophyll—A leaf’s inner tissue, located Vascular tissue—Water-, nutrient-, and between the upper and lower photosynthate-conducting tissue epidermis; contains the chloroplasts (xylem and phloem). and other specialized cellular parts Xylem—Water- and nutrient-conducting (organelles). tissue. Chapter 1—Botany Basics • 5

Plant life cycles grow from the plant’s crown each spring. Trees and shrubs, on the Based on its life cycle, a other hand, have woody stems plant is classified as either that withstand cold winter an annual, biennial, or temperatures. They are perennial. referred to as woody perenni- An annual, such as a als. zinnia, completes its life cycle in 1 year. Annuals are said to go Internal plant parts from seed to seed in 1 year Cells are the basic structural or growing season. During and physiological units of plants. this period, they germinate, Most plant reactions (cell division, grow, mature, bloom, produce photosynthesis, respiration, etc.) occur at seeds, and die. Summer annuals the cellular level. Plant tissues (meristems, complete their life cycle during spring and xylem, phloem, etc.) are large, organized summer; most winter annuals complete groups of similar cells that work together their growing season during fall and to perform a specific function. winter. There are both winter and summer A unique feature of plant cells is that annual weeds, and understanding a they are readily totipotent. In other words, weed’s life cycle is important in control- almost all plant cells retain all of the ling it. See Chapter 3, genetic information (encoded in DNA) ✿ A biennial requires all or part of 2 years Propagation. necessary to develop into a complete to complete its life cycle. During the first plant. This characteristic is the main season, it produces vegetative structures reason that vegetative (asexual) reproduc- (leaves) and food storage organs. The tion works. For example, the cells of a plant overwinters and then produces small leaf cutting from an African violet flowers, fruit, and seeds during its second have all of the genetic information neces- season. Swiss chard, carrots, beets, sweet sary to generate a root system, stems, William, and parsley are examples of more leaves, and ultimately flowers. biennials. Specialized groups of cells called Sometimes biennials go from seed meristems are a plant’s growing points. germination to seed production in only Meristems are the site of rapid, almost one growing season. This situation occurs continuous cell division. These cells when extreme environmental conditions, either continue to divide or begin to such as drought or temperature variation, differentiate into other tissues and organs. cause the plant to pass rapidly through How they divide, and whether they ulti- the equivalent of two growing seasons. mately become a tissue or an organ, is This phenomenon is referred to as bolting. controlled by a complex array of internal Sometimes bolting occurs when biennial plant hormones but also can be influ- plant starts are exposed to a cold spell enced by environmental conditions. In before being planted in the garden. many cases, you can manipulate mer- Perennial plants live more than 2 years istems to make a plant do something you and are grouped into two categories: want, such as change its growth pattern, herbaceous perennials and woody peren- flower, alter its branching habit, or pro- nials. Herbaceous perennials have soft, duce vegetative growth. nonwoody stems that generally die back to the ground each winter. New stems 6 • Botany Basics—Chapter 1

External plant parts vegetative. Sexual reproductive parts produce seed; they include flower buds, External plant structures such as flowers, fruit, and seeds. Vegetative parts leaves, stems, roots, flowers, fruits, and (Figure 1) include roots, stems, shoot seeds are known as plant organs. Each buds, and leaves; they are not directly organ is an organized group of tissues that involved in sexual reproduction. Vegeta- work together to perform a specific tive parts often are used in asexual forms function. These structures can be divided of reproduction such as cuttings, budding, into two groups: sexual reproductive and or grafting. Leaf primordia Roots Shoot apex Often roots are overlooked, probably because they are less visible than the rest of the plant. However, it’s important to understand plant root systems because Leaf they have a pronounced effect on a plant’s size and vigor, method of propagation, adaptation to soil types, and response to cultural practices and irrigation. Bud Roots typically originate from the lower portion of a plant or cutting. They have a root cap, but lack nodes and never bear leaves or flowers directly. Their principal functions are to absorb nutrients and moisture, anchor the plant in the soil, Stem support the stem, and store food. In some plants, they can be used for propagation. Structure Internally, there are three major parts of Soil Vascular a root (Figure 2): line tissue • The meristematic zone is at the tip and manufactures new cells; it is an area of cell division and growth. • Behind the meristem is the zone of elongation. In this area, cells increase in size through food and water absorp- Lateral root tion. As they grow, they push the root Primary through the soil. root • The zone of maturation is directly beneath the stem. Here, cells become specific tissues such as epidermis, cortex, or vascular tissue. A root’s epidermis is its outermost layer of cells (Figure 3). These cells are responsible for absorbing water and minerals Figure 1.—Principal parts of a vascular plant. (Adapted by dissolved in water. Cortex cells are permission from Plant Physiology, The Benjamin/Cummings involved in moving water from the Publishing Company, Inc., 1991.) Chapter 1—Botany Basics • 7

epidermis to the vascular tissue (xylem undercutting it can encourage the plant to and phloem) and in storing food. Vascular produce a fibrous root system. Nurseries tissue is located in the center of the root use this technique with trees that natu- and conducts food and water. rally produce a taproot because trees with Externally, there are two areas of impor- a compact, fibrous root system are trans- tance: the root cap and the root hairs planted more successfully. (Figure 2). The root cap is the root’s outermost tip. It consists of cells that are How roots grow sloughed off as the root grows through During early development, a seedling the soil. Its function is to protect the root absorbs nutrients and moisture from the meristem. Root hairs are delicate, elongated epidermal cells that occur in a small zone Lateral just behind the root’s growing tip. They root generally appear as fine down to the Primary Zone of naked eye. Their function is to increase root maturation the root’s surface area and absorptive Root capacity. Root hairs usually live 1 or hairs 2 days. When a plant is transplanted, they are easily torn off or may dry out. Root tip Many roots have a naturally occurring Zone of elongation Root cap symbiotic (mutually beneficial) relationship with certain fungi, which improves Meristematic zone the plant’s ability to absorb water and Figure 2.—Root structure. (Adapted by permission from Plant nutrients. This beneficial association is Physiology, The Benjamin/Cummings Publishing Company, called mycorrhizae (fungus + root). Inc., 1991.) Types of roots There are two major types of roots: Epidermis primary and lateral roots. Cortex A primary root originates at the lower Ground end of a seedling’s embryo (Figure 2). If Pericycle tissues the primary root continues to elongate Endodermis downward, becomes the central feature of the root system, and has limited second- Xylem Vascular tissue ary branching, it is called a taproot Phloem (Figure 4a). Hickory and pecan trees, as well as carrots, have taproots. Figure 3.—Cross section of a root. (Adapted by permission from Plant Physiology, The Benjamin/Cummings Publishing A lateral, or secondary, root is a side or Company, Inc., 1991.) branch root that arises from another root. If the primary root ceases to elongate, and numerous lateral roots develop, a fibrous root system is formed (Figure 4b). These lateral roots branch repeatedly to form the network of feeding roots found on most plants. Some plants, such as grasses, naturally produce a fibrous root system. In other cases, severing a plant’s taproot by (a) Taproot (b) Fibrous root Figure 4.—Taproot of a carrot (a) and fibrous root of grass (b). 8 • Botany Basics—Chapter 1

soil around the sprouting seed. A band of Roots as food fertilizer several inches to each side and An enlarged root is the edible portion of slightly below newly planted seeds helps several vegetable crops. Sweet potatoes early growth of most row crops. are a swollen tuberous root; and carrots, As a plant becomes well established, parsnips, salsify, and radishes are elon- the quantity and distribution of its roots gated taproots. strongly influence its ability to absorb moisture and nutrients. For most plants, Stems the majority of the absorbing (feeder) Stems support buds and leaves and roots are located in the top 12 inches of serve as conduits for carrying water, soil. The soil environment in this region minerals, and food (photosynthates). The generally is best for root growth, with a vascular system inside the stem forms a good balance of fertility, moisture, and air continuous pathway from the root, spaces. through the stem, and finally to the The following factors are important in leaves. It is through this system that water root growth: and food products move. ✿See Chapter 2, • Roots in water-saturated soil do not Soils and grow well and ultimately may die due Fertilizers. Structure to lack of oxygen. Vascular system • Roots penetrate much deeper in loose, This system consists of xylem, phloem, well-drained soil than in heavy, poorly and vascular cambium. It can be thought drained soil. of as a plant’s plumbing. Xylem tubes • A dense, compacted soil layer can conduct water and dissolved minerals; restrict or terminate root growth. phloem tubes carry food such as sugars. • Container plants not only have a The cambium is a layer of meristematic restricted area for root growth, but tissue that separates the xylem and also are susceptible to cold damage phloem and continuously produces new because the limited amount of soil xylem and phloem cells. This new tissue is surrounding their roots may not responsible for a stem’s increase in girth. provide adequate insulation. The vascular cambium is important to • In addition to growing downward, gardeners. For example, the tissues on a roots grow laterally and often extend grafted scion and rootstock need to line well beyond a plant’s dripline. Keep up. In addition, careless weed trimming this extensive root system in mind can strip the bark off a tree, thus injuring when disturbing the soil around the cambium and causing the tree to die. existing trees and shrubs.

Phloem Phloem Stem terminology Cambium Shoot—A young stem (1 year old or Xylem less) with leaves. Twig—A young stem (1 year old or Xylem less) that is in the dormant winter stage (has no leaves). Pith Branch—A stem that is more than (a) Monocot (b) Dicot 1 year old, typically with lateral Figure 5.—Cross sections of stems: (a) Discontinuous vascular stems radiating from it. system of a monocot stem; (b) continuous vascular system of Trunk—A woody plant’s main stem. a woody dicot stem. Chapter 1—Botany Basics • 9

locate a plant’s nodes. Generally, you want to make a pruning cut just above, but not Bud too close to, a node. Pruning in this ✿See Chapter 4, manner encourages the buds at that node Pruning. to begin development and ultimately form new stems or leaves. Node The area between two nodes is called an internode. Its length depends on many Internode factors, including genetics. Several other factors also can influence internode length: • Reduced soil fertility decreases internode length, while an application of high-nitrogen fertilizer can greatly Figure 6.—Stem structure. (Adapted by permis- increase it. sion from Plant Physiology, The Benjamin/ • Lack of light increases internode Cummings Publishing Company, Inc., 1991.) length and causes spindly stems. This situation is known as stretch, or The vascular systems of monocots and etiolation, and often occurs in seed- dicots differ (Figure 5). Although both lings started indoors and in house- contain xylem and phloem, these struc- plants that do not get enough sunlight. tures are arranged differently in each. In a • Internode length also varies with the monocot, the xylem and phloem are season. Early-season growth has long paired in bundles, which are dispersed internodes, while late-season growth throughout the stem. In a dicot, the generally has much shorter inter- vascular system is said to be continuous nodes. because it forms rings inside the stem. • If a stem’s energy is divided among The phloem forms the outer ring and three or four side stems, or is diverted eventually becomes part of the bark in into fruit growth and development, mature woody stems. The xylem forms internode length is shortened. the inner ring. In woody plants, it is called • Plant growth regulator substances and the sapwood and heartwood. herbicides also can influence inter- The difference in the vascular systems node length. of monocots and dicots is of practical interest to gardeners because some Types of stems herbicides affect only one group. For Stems may be long, with great dis- example, 2,4-D kills only plants with a tances between the leaves and buds continuous vascular system (dicots). (e.g., branches of trees, runners on Nonselective herbicides, on the other strawberries) or compressed, with short hand (e.g., glyphosate), kill plants regard- distances between buds or leaves less of their type of vascular system. (e.g., crowns of strawberry plants, fruit spurs, and African violets). Although Nodes stems commonly grow above ground, they A node is an area on a stem where buds sometimes grow below ground in the form are located (Figure 6). It is a site of great of rhizomes, tubers, corms, or bulbs. All cellular activity and growth, where small stems must have buds or leaves to be buds develop into leaves, stems, or flow- classified as stem tissue. ers. When pruning, it is important to 10 • Botany Basics—Chapter 1

Spur

Stolon Crown

Figure 7.—Diversified aboveground stem development.

Specialized aboveground stems is difficult to distinguish between roots Some plants have specialized above- and stems, but one sure way is to look for ground stems known as crowns, spurs, or nodes. Stems have nodes; roots do not. stolons (Figure 7). Crowns (on strawber- In potato tubers, for example, the “eyes” ries, dandelions, and African violets) are are actually the stem’s nodes, and each compressed stems with leaves and flow- eye contains a cluster of buds. When ers on short internodes. Spurs are short, growing potatoes from seed pieces, it is stubby side stems that arise from a main important that each piece contain at least stem. They are the fruit-bearing stems on one eye and be about the size of a golf ball pear, apple, and cherry trees. If severe so there will be enough energy for early pruning is done too close to fruit-bearing growth of shoots and roots. spurs, they can revert to nonfruiting Rhizomes resemble stolons because stems, thus eliminating the year’s poten- they grow horizontally from plant to tial fruit crop. plant. Some rhizomes are compressed and Stolons are fleshy or semiwoody, elon- fleshy (e.g., iris), while others are slender gated, horizontal stems that often lie and have elongated internodes along the soil surface. Strawberry runners (e.g., bentgrass). Johnsongrass is an are stolons that have small leaves at the insidious weed principally because of the nodes. Roots develop from these nodes, spreading capability of its rhizomes. and a daughter plant is formed. This type Tulips, lilies, daffodils, and onions of vegetative reproduction is an easy way produce bulbs, which are shortened, to increase the size of a strawberry patch. compressed underground stems sur- Spider plants also produce stolons, which rounded by fleshy scales (leaves) that ultimately can become entirely new envelop a central bud at the tip of the plants. stem. In November, you can cut a tulip or daffodil bulb in half and see all of the Specialized below-ground stems flower parts in miniature. Potato tubers, iris rhizomes, and tulip After a bulb-producing plant flowers, its bulbs are underground stems that store phloem transports food reserves from its food for the plant (Figure 8). It sometimes leaves to the bulb’s scales. When the bulb Chapter 1—Botany Basics • 11

Leaves

Stem Tuberous stem

Tunicate bulb Nontunicate bulb Lateral bud

Internode

Node

Old corm

Tubers Corm Rhizome Figure 8.—Diversified below-ground stem development. begins growing in the spring, it utilizes the solid, swollen stem with dry, scale-like stored food. For this reason, it is impor- leaves. Gladiolus and crocuses produce tant not to remove the leaves from daffo- corms. dils, tulips, and other bulb-producing Some plants (e.g., tuberous begonias plants until after they have turned yellow and cyclamen) produce a modified under- and withered. At that time, they have ground stem called a tuberous stem. These finished producing the food that will be stems are short, flat, and enlarged. Buds used for next year’s flowering. and shoots arise from the top (crown), There are two types of bulbs: tunicate and fibrous roots grow from the bottom. and nontunicate (Figure 8). Tunicate bulbs Other plants (e.g., dahlias and sweet (e.g., daffodils, tulips, and onions) have a potatoes) produce underground storage thin, papery covering, which actually is a organs called tuberous roots, which often modified leaf. It helps protect the bulb are confused with bulbs and tubers. from damage during digging and from However, these are root tissue, not stem drying out once it is out of the soil. tissue, and have neither nodes nor inter- Nontunicate bulbs (e.g., lilies) do not have nodes. this papery covering. They are very susceptible to damage and drying out, so Stems and propagation handle them very carefully. Stems often are used for vegetative Corms are another kind of below-ground plant propagation. Using sections of stem. Although both bulbs and corms are aboveground stems that contain nodes and internodes is an effective way to ✿See Chapter 3, composed of stem tissue, they are not the Propagation. same. Corms are shaped like bulbs, but do propagate many ornamental plants. These not contain fleshy scales. A corm is a stem cuttings produce roots and, eventually, new plants. 12 • Botany Basics—Chapter 1

Below-ground stems also are good Stems as food propagative tissues. You can divide rhi- The edible portion of several cultivated zomes into pieces; remove small bulblets plants, such as asparagus and kohlrabi, is or cormels from their parent; and cut an enlarged, succulent stem. The edible tubers into pieces containing eyes and parts of broccoli are composed of stem nodes. All of these tissues will produce tissue, flower buds, and a few small new plants. leaves. The edible tuber of a potato is a fleshy underground stem. And, although Types of plants and their stems the name suggests otherwise, the edible Trees generally have one, but occasion- part of cauliflower actually is proliferated ally several, main trunks, which usually stem tissue. are more than 12 feet tall when mature. In contrast, shrubs generally have several Buds main stems, which usually are less than 12 feet tall when mature. A bud is an undeveloped shoot from Most fruit trees, ornamental trees, and which leaves or flower parts grow. The shrubs have woody stems. These stems buds of temperate-zone trees and shrubs contain relatively large amounts of hard- typically develop a protective outer layer ened xylem tissue in the central core of small, leathery scales. Annual plants (heartwood or sapwood). and herbaceous perennials have naked Herbaceous or succulent stems contain buds with green, somewhat succulent, only a little xylem tissue and usually live outer leaves. for only one growing season. In perennial Buds of many plants require exposure plants, new herbaceous stems develop to a certain number of days below a from the crown (root–stem interface) each critical temperature before resuming year. growth in the spring. This period, often Canes are stems with relatively large referred to as rest, varies for different pith (the central strength-giving tissue). plants. Forsythia, for example, requires a They usually live only 1 or 2 years. relatively short rest period and grows at ✿See Chapter 11, Examples of plants with canes include the first sign of warm weather. Many Berry Crops. roses, grapes, blackberries, and raspber- peach varieties, on the other hand, ries. For fruit production, it is important require 700 to 1,000 hours of temperatures to know which canes to prune, how to below 45°F. During rest, dormant buds can prune them, and when to prune them. withstand very low temperatures, but A vine is a plant with long, trailing after the rest period is satisfied, they are stems. Some vines grow along the ground, more susceptible to damage by cold while others must be supported by temperatures or frost. another plant or structure. Twining vines A leaf bud is composed of a short stem circle a structure for support. Some circle with embryonic leaves. Leaf buds often clockwise (e.g., hops and honeysuckle), are less plump and more pointed than while others circle counterclockwise flower buds (Figure 9). (e.g., pole beans and Dutchman’s pipe A flower bud is composed of a short vine). Climbing vines are supported either stem with embryonic flower parts. In the by aerial roots (e.g., English ivy and case of fruit crops, flower buds sometimes poison ivy), by slender tendrils that are called fruit buds. This terminology is encircle a supporting object (e.g., cucum- inaccurate, however; although flowers bers, gourds, grapes, and passionflowers), have the potential to develop into fruits, or by tendrils with adhesive tips they may not do so because of adverse (e.g., Virginia and Japanese creeper). weather conditions, lack of pollination, or other unfavorable circumstances. Chapter 1—Botany Basics • 13

Leaf bud edible part of brussels sprouts. In the case of globe artichoke, the fleshy basal Flower bud portion of the flower bud’s bracts is eaten, along with its solid stem. Broccoli is the most important horticultural plant with edible flower buds. In this case, portions of the stem, as well as small leaves associ- ated with the flower buds, are eaten. Leaves Function and structure The principal function of leaves is to absorb sunlight to manufacture plant sugars through a process called photosyn- Figure 9.—Elm leaf and flower buds. thesis. Leaf surfaces are flattened to present a large area for efficient light Location absorption. The blade is the expanded Buds are named for their location on thin structure on either side of the midrib the stem (Figure 10). Terminal buds are and usually is the largest, most conspicu- located at the apex (tip) of a stem. Lateral ous part of a leaf (Figure 11). (axillary) buds are located on the sides of A leaf is held away from its stem by a a stem and usually arise where a leaf stem-like appendage called a petiole, and meets a stem (an axil). In some instances, the base of the petiole is attached to the an axil contains more than one bud. stem at a node. Petioles vary in length or Adventitious buds arise at sites other may be lacking entirely, in which case the than the terminal or axillary position. leaf blade is described as sessile or stalk- They may develop from roots, a stem less. internode, the edge of The node where a petiole meets a stem a leaf blade, or callus is called a leaf axil. The axil contains Terminal bud tissue at the cut end single buds or bud clusters, referred to as of a stem or root. axillary buds. They may be either active Adventitious buds or dormant; under the right conditions, allow stem, leaf, and they will develop into stems or leaves. root cuttings to develop into entirely Leaf axil with axillary bud new plants. Twig Buds as food Lateral Enlarged buds or (axillary) parts of buds form bud Blade the edible portion of some horticultural crops. Cabbage and Midrib head lettuce are examples of unusu- Petiole Node ally large terminal Stem Figure 10.—Bud buds. Succulent Figure 11.—Leaf parts. (Adapted by permission location. axillary buds are the from Plant Physiology, The Benjamin/ Cummings Publishing Company, Inc., 1991.) 14 • Botany Basics—Chapter 1

Cuticle Upper spray additive to help the product adhere epidermis to, or penetrate, the cutin layer. Special epidermal cells called guard Palisade layer cells open and close in response to environmental stimuli such as changes in weather and light. They regulate the passage of water, oxygen, and carbon dioxide into and out of the leaf through Vascular tiny openings called stomata. In most bundle species, the majority of the stomata are located on the undersides of leaves. Spongy Conditions that would cause plants to mesophyll lose a lot of water (high temperature, low humidity) stimulate guard cells to close. Intercellular In mild weather, they remain open. Guard chamber cells also close in the absence of light. Located between the upper and lower Lower epidermis epidermis is the mesophyll. It is divided Guard cells Stoma into a dense upper layer (palisade meso- Figure 12.—Leaf cross section. (Reprinted by permission phyll) and a lower layer that contains lots from Plant Science: Growth, Development, and Utilization of air space (spongy mesophyll). Located of Cultivated Plants, Prentice Hall, 1988.) within the mesophyll cells are chloroplasts, where photosynthesis takes place. A leaf blade is composed of several layers (Figure 12). On the top and bottom Types of leaves is a layer of thick, tough cells called the There are many kinds of plant leaves. epidermis. Its primary function is to The most common and conspicuous protect the other layers of leaf tissue. The leaves are referred to as foliage and are arrangement of epidermal cells deter- the primary location of photosynthesis. mines the leaf’s surface texture. Some However, there are many other types of leaves, such as those of African violets, modified leaves: have hairs (pubescence), which are exten- • Scale leaves (cataphylls) are found on sions of epidermal cells that make the rhizomes and buds, which they leaves feel like velvet. enclose and protect. The cuticle is part of the epidermis. It • Seed leaves (cotyledons) are found on produces a waxy layer called cutin, which embryonic plants. They store food for protects the leaf from dehydration and the developing seedling. disease. The amount of cutin on a leaf • Spines and tendrils, such as those on increases with increasing light intensity. barberry and pea plants, protect a For this reason, when moving plants from plant or help support its stems. shade into full sunlight, do so gradually • Storage leaves, such as those on over a period of a few weeks. This gradual bulbous plants and succulents, store exposure to sunlight allows the cutin layer food. to build up and protect the leaves from • Bracts often are brightly colored. For rapid water loss or sunscald. example, the showy structures on The waxy cutin also repels water. For dogwoods and poinsettias are bracts, this reason, many pesticides contain a not petals. Chapter 1—Botany Basics • 15

Pinnate

Palmate

(a) Parallel-veined (b) Net-veined Figure 13.—Types of venation: (a) Parallel-veined; (b) net-veined.

Venation In palmate venation, the principal veins The vascular bundles of xylem and extend outward, like the ribs of a fan, from phloem extend from the stem, through the the base of the leaf blade (e.g., grapes and petiole, and into the leaf blade as veins. maples). The term venation refers to how veins are distributed in the blade. There are two Leaves as plant identifiers principal types of venation: parallel- Leaves are useful for plant identifica- veined and net-veined (Figure 13). tion. A leaf’s shape, base, apex, and In parallel-veined leaves, numerous margin can be important identifying veins run essentially parallel to each characteristics (Figures 14–16). other and are connected laterally by Leaf type (Figure 17) also is important minute, straight veinlets. Parallel-veined for identification. There are two types of leaves occur most often on monocotyle- leaves: simple and compound. In simple donous plants. The most common type of leaves, the leaf blade is a single, continu- parallel veining is found in plants of the ous unit. Compound leaves are composed grass family, whose veins run from the of several separate leaflets arising from leaf’s base to its apex. Another type of the same petiole. Some leaves are doubly parallel venation is found in plants such compound. Leaf type can be confusing as banana, calla, and pickerelweed, whose because a deeply lobed simple leaf may veins run laterally from the midrib. look like a compound leaf. In net-veined leaves (also called reticu- Leaf arrangement along a stem also is late-veined), veins branch from the main used in plant identification (Figure 18). rib or ribs and subdivide into finer vein- There are four types of leaf arrangement: lets. These veinlets then unite in a compli- • Opposite leaves are positioned across cated network. This system of enmeshed the stem from each other, with two veins makes the leaf more resistant to leaves at each node. tearing than does a parallel vein structure. • Alternate (spiral) leaves are arranged Net-veined leaves occur on dicotyledon- in alternate steps along the stem, with ous plants. only one leaf at each node. Net venation may be either pinnate or • Whorled leaves are arranged in circles palmate. In pinnate venation, the veins along the stem. extend laterally from the midrib to the • Rosulate leaves are arranged in a edge (e.g., apples, cherries, and peaches). rosette around a stem with extremely short nodes. continues on page 18 16 • Botany Basics—Chapter 1

Leaves and plant identification

Common blade shapes (Figure 14) Lanceolate—Longer than wide and tapering toward the apex and base. Linear—Narrow, several times longer than wide and of approximately the same width throughout. Cordate (heart-shaped)—Broadly ovate, tapering to an acute apex, Lanceolate Linear Cordate with the base turning in and forming a notch where the petiole is attached. Elliptical—About two or three times as long as wide, tapering to an acute or rounded apex and base. Ovate—Egg-shaped, basal portion wide, tapering toward the apex. Elliptical Ovate Figure 14.—Common leaf blade shapes.

Common margin forms (Figure 15) Entire—Having a smooth edge with no teeth or notches. Crenate—Having rounded teeth. Dentate—Having teeth ending in an acute angle pointing outward. Serrate—Having small, sharp teeth pointing toward the apex. Incised—Having a margin cut into sharp, deep, irregular teeth or Entire Crenate Dentate incisions. Lobed—Having incisions that extend less than halfway to the midrib.

Serrate Incised Lobed Figure 15.—Common leaf margin shapes.

continues on next page Chapter 1—Botany Basics • 17

Leaves and plant identification (continued)

Common apex and base shapes (Figure 16) Apex shapes Acute—Ending in an acute angle, with a sharp, but not acuminate, point. Acuminate—Tapering to a long, narrow point. Obtuse—Tapering to a rounded edge. Acute Acuminate Obtuse Cuneate—Wedge-shaped; triangular with the narrow end at the point of Base shapes attachment. Cordate (heart-shaped)—Turning in and forming a notch.

Cuneate Obtuse Cordate Figure 16.—Common leaf apex and base shapes.

Simple Palmate compound Pinnate compound Double pinnate compound Figure 17.—Leaf types.

Opposite Alternate Whorled Rosulate Figure 18.—Leaf arrangement. 18 • Botany Basics—Chapter 1

Leaves as food Structure The leaf blade is the principal edible As a plant’s reproductive part, a flower part of several horticultural crops, includ- contains a stamen (male flower part) and/ ing chives, collards, endive, kale, leaf or pistil (female flower part), plus acces- lettuce, mustard, parsley, spinach, Swiss sory parts such as sepals, petals, and chard, and other greens. The edible part nectar glands (Figure 19). of leeks, onions, and Florence fennel is a The stamen is the male reproductive cluster of fleshy leaf bases. The petiole is organ. It consists of a pollen sac (anther) the edible product in celery and rhubarb. and a long supporting filament. This filament holds the anther in position, Flowers making the pollen available for disperse- Flowers, which generally are the showi- ment by wind, insects, or birds. est part of a plant, have sexual reproduc- The pistil is a plant’s female part. It tion as their sole function. Their beauty generally is shaped like a bowling pin and and fragrance have evolved not to please is located in the flower’s center. It consists humans but to ensure continuance of the of a stigma, style, and ovary. The stigma is species. Fragrance and color attract located at the top and is connected by the pollinators (insects or birds) that play an style to the ovary. The ovary contains important role in the reproductive pro- eggs, which reside in ovules. If an egg is cess. fertilized, the ovule develops into a seed. Flowers are important for plant classifi- Sepals are small, green, leaf-like struc- cation. The system of plant nomenclature tures located at the base of a flower. They we use today was developed by Carl von protect the flower bud. Collectively, the ✿See Chapter 22, sepals are called a calyx. Linné (Linnaeus) and is based on flowers Plant Petals generally are the highly colored Identification. and/or reproductive parts of plants. One reason his system is successful is because portions of a flower. Like nectar glands, flowers are the plant part least influenced petals may contain perfume. Collectively, by environmental changes. Thus, a knowl- the petals are called a corolla. The num- edge of flowers and their parts is essential ber of petals on a flower often is used to for anyone interested in plant identifica- help identify plant families and genera. tion. Flowers of dicots typically have four or five sepals and/or petals, or multiples thereof. In monocots, these floral parts typically come in threes or multiples of Stigma three. Petal Style Types of flowers Anther If a flower has a stamen, pistil, petals, and sepals, it is called a complete flower Pistil Ovules Stamen (Figure 19). Roses are an example. If one Filament Ovary of these parts is missing, the flower is called incomplete. The stamen and pistil are the essential parts of a flower and are involved in seed Sepal production. If a flower contains both Pedicel functional stamens and pistils, it is called Figure 19.—Complete flower structure. a perfect flower, even if it does not contain petals and sepals. If either stamens or pistils are lacking, the flower is called Chapter 1—Botany Basics • 19

imperfect (Figure 20). Pistillate (female) flowers possess a functional pistil or pistils, but lack stamens. Staminate (male) Stigma flowers contain stamens, but no pistils. Petal Plants with imperfect flowers are fur- Style ther classified as monoecious or dioecious. Monoecious plants have separate Pistil Ovules male and female flowers on the same plant (e.g., corn and pecans). Some mono- Ovary ecious plants bear only male flowers at the beginning of the growing season, but later develop both sexes (e.g., cucumbers and squash). Sepal Dioecious species have separate male Pedicel and female plants. Examples include holly, Figure 20.—Imperfect (pistillate) flower structure. ginkgos, and pistachios. In order to set fruit, male and female plants must be planted close enough together for insects, animals, or birds often have pollination to occur. In some instances brightly colored or patterned flowers that (e.g., holly), the fruit is desirable. In the contain fragrance or nectar. While search- case of ginkgos, however, the fruit gener- ing for nectar, pollinators transfer pollen ally is not desirable due to its putrid smell from flower to flower, either on the same when ripe. Kiwis are complicated because plant or on different plants. Plants they may have one plant with bisexual evolved this ingenious mechanism in flowers and another plant with only male order to ensure their species’ survival. flowers. The plant world doesn’t always Wind-pollinated flowers often lack showy have absolutes! floral parts and nectar because they don’t need to attract pollinators. Types of inflorescences A chemical in the stigma stimulates Some plants bear only one flower per pollen to grow a long tube down the style stem, which is called a solitary flower. to the ovules inside the ovary. When Other plants produce an inflorescence— pollen reaches the ovules, it releases a cluster of flowers. Each flower in an sperm, and fertilization typically occurs. inflorescence is called a floret. Fertilization is the union of a male sperm Most inflorescences belong to one of nucleus from a pollen grain with a female two groups: racemes and cymes. In the egg. If fertilization is successful, the ovule racemose group, the florets start blooming develops into a seed. It is important to from the bottom of the stem and progress remember that pollination is no guarantee toward the top. In a cyme, the top floret that fertilization will occur. opens first and blooms progress down- Cross-fertilization combines genetic ward along the stem. Detailed discussions material from two parent plants. The of flower types are found in many botany resulting seed has a broader genetic base, textbooks. (See “For more information” at which may enable the population to the end of this chapter.) survive under a wider range of environmental conditions. Cross-pollinated plants How seeds form usually are more successful than self- Pollination is the transfer of pollen from pollinated plants. Consequently, more an anther to a stigma, either by wind or plants reproduce by cross-pollination by pollinators. Species pollinated by than by self-pollination. 20 • Botany Basics—Chapter 1

Fruit (drupe), pears and apples (pome), and Structure tomatoes (berries). Although generally referred to as a vegetable, tomatoes Fruit consists of fertilized, mature technically are a fruit because they ovules (seeds) plus the ovary wall, which develop from a flower. Squash, cucum- may be fleshy, as in an apple, or dry and bers, and eggplants also develop from a hard, as in an acorn. In some fruits, the seeds are enclosed within the ovary single ovary and are classified botanically as fruits. (e.g., apples, peaches, oranges, squash, Other types of simple fruit are dry. and cucumbers). In others, seeds are Their wall is either papery or leathery and situated on the outside of fruit tissue hard, as opposed to the fleshy examples (e.g., corn and strawberries). just mentioned. Examples are peanuts The only part of the fruit that contains genes from both the male and female (legume), poppies (capsule), maples (samara), and walnuts (nut). flowers is the seed. The rest of the fruit An aggregate fruit develops from a arises from the maternal plant and is single flower with many ovaries. Examples genetically identical to it. are strawberries, raspberries, and black- Types of fruit berries. The flower is a simple flower with Fruits are classified as simple, aggre- one corolla, one calyx, and one stem, but gate, or multiple (Figure 21). Simple fruits it has many pistils or ovaries. Each ovary develop from a single ovary. They include is fertilized separately. If some ovules are fleshy fruits such as cherries and peaches not pollinated successfully, the fruit will be misshapen.

(a) Simple fruits Hard stone enclosing seed

Fleshy ovary

Seeds Seed

Apple (pome) Peach (drupe) Maple samara (dry)

(b) Aggregate fruits (c) Multiple fruit

Berry Cone Pineapple Figure 21.—Types of fruit: (a) Simple fruits (apple, peach, and maple); (b) aggregate fruits (berry and cone); (c) multiple fruit (pineapple). Chapter 1—Botany Basics • 21

Plumule Embryo Hypocotyl Radicle Seed coat Endosperm Plumule Micropyle Seed coat Plumule (sheathed) Hypocotyl Cotyledon Endosperm Perisperm Hypocotyl Seed coat Radicle Cotyledon Radicle Cotyledons

(a) Beet (b) Bean (c) Onion Figure 22.—Parts of a seed: (a) Beet; (b) bean; (c) onion.

Cotyledon Cotyledons

Hypocotyl Hypocotyl Radicle Radicle

(a) Germination of a bean (dicot) (b) Germination of an onion (monocot) Figure 23.—Germination of a dicot (a) and a monocot (b).

Multiple fruits are derived from a tight • The seed coat, a hard outer covering, cluster of separate, independent flowers protects the seed from disease and borne on a single structure. Each flower insects. It also prevents water from has its own calyx and corolla. Pineapples entering the seed and initiating germi- and figs are examples. nation before the proper time. Seeds Germination Germination is a complex process A seed contains all of the genetic infor- whereby a seed embryo goes from a mation needed to develop into an entire dormant state to an active, growing state plant. It is made up of three parts (Figure 23). Before any visible signs of (Figure 22): germination appear, the seed must absorb • The embryo is a miniature plant in an water through its seed coat. It also must arrested state of development. It will have enough oxygen and a favorable begin to grow when conditions are temperature. Some species, such as favorable. celery, also require light. Others require • The endosperm (and in some species darkness. the cotyledons) is a built-in food If these requirements are met, the supply (although orchids are an radicle is the first part of the seedling to exception), which can be made up of emerge from the seed. It develops into the proteins, carbohydrates, or fats. 22 • Botany Basics—Chapter 1

primary root and grows downward in dormancy is broken, other factors also response to gravity. From this primary affect germination: root, root hairs and lateral roots develop. • The seed’s age greatly affects its Between the radicle and the first leaf-like viability (ability to germinate). Older structure is the hypocotyl, which grows seed generally is less viable than upward in response to light. young seed, and if it does germinate, The seed leaves, or cotyledons, encase the seedlings are less vigorous and the embryo. They usually are shaped grow more slowly. differently than the leaves the mature • The seedbed must be properly pre- plant will produce. Monocots produce one pared and made up of loose, fine- cotyledon, while dicots produce two. textured soil. Because seeds are reproductive struc- • Seeds must be planted at the proper tures and thus important to a species’ depth. If they are too shallow, they survival, plants have evolved many may wash away with rain or watering; mechanisms to ensure their survival. One if too deep, they won’t be able to push such mechanism is seed dormancy. through the soil. Dormancy comes in two forms: seed coat • Seeds must have a continual supply of dormancy and embryo dormancy. moisture; however, if over-watered, In seed coat dormancy, a hard seed coat they will rot. does not allow water to penetrate. Red- Many weed seeds are able to germinate bud, locust, and many other ornamental quickly and under less than optimal trees and shrubs exhibit this type of conditions. This is one reason they make dormancy. such formidable opponents in the garden. A process called scarification is used to break or soften the seed coat. In nature, scarification is accomplished by means Plant growth and development ✿See Chapter 3, such as the heat of a forest fire, digestion Propagation. Photosynthesis, respiration, and tran- of the seed by a bird or mammal, or spiration are the three major functions partial breakdown of the seed coat by that drive plant growth and development fungi or insects. It can be done mechani- (Figure 24). All three are essential to a cally by nicking the seed coat with a file, plant’s survival. How well a plant is able or chemically by softening the seed coat to regulate these functions greatly affects with sulfuric acid. In either instance, it is its ability to compete and reproduce. important to not damage the embryo. Embryo dormancy is common in orna- Photosynthesis mental plants, including elm and witch hazel. These seeds must go through a One of the major differences between chilling period before germinating. To plants and animals is plants’ ability to break this type of dormancy, stratification manufacture their own food. This process is used. This process involves storing is called photosynthesis, which literally seeds in a moist medium (potting soil or means “to put together with light.” To paper towels) at temperatures between produce food, a plant requires energy 32° and 50°F. The length of time required from the sun, carbon dioxide from the air, varies by species. and water from the soil. During photosyn- Even when environmental requirements thesis, it splits carbon dioxide into carbon for seed germination are met and and oxygen, adds water, and forms carbohydrates (starches and sugars). Oxygen is a by-product. Chapter 1—Botany Basics • 23

The formula for photosynthesis can be written as follows: Carbon dioxide + Water + Sunlight = Light Sugar + Oxygen energy or

6CO2 + 6H20 > C6H1206 + 602 After producing carbohydrates, a plant Starch or sugar either uses them as energy, stores them, C6H12O6 or builds them into complex energy Photosynthesis, (sugars storage organ starch) compounds such as oils and proteins. All respiration, and of these food products are called photo- photorespiration Sugars synthates. The plant uses them when light CO is limited, or transports them to its roots 2 O or developing fruits. 2 H20 Photosynthesis occurs only in the vapor H 0 mesophyll layers of plant leaves and, in 2 some instances, in mesophyll cells in the stem. Mesophyll cells are sandwiched Sugars between the leaf’s upper and lower epidermis (Figure 12) and contain numerous chloroplasts, where photosynthesis takes Starch place. Chloroplasts are incredibly small. or sugar One square millimeter, about the size of a storage period on a page, would contain 400,000 organ chloroplasts.

Chlorophyll, the pigment that makes leaves green, is found in the chloroplasts. ▼ It is responsible for trapping light energy Respiration, no photorespiration H20 and from the sun. Often chloroplasts are minerals O2 arranged perpendicular to incoming sun enter through CO2 rays so they can absorb maximum sun- root hairs light. If any of the ingredients for photosyn- Figure 24.—Schematic representation of photosynthesis, thesis—light, water, and carbon dioxide— respiration, leaf water exchange, and translocation of sugar is lacking, photosynthesis stops. If any (photosynthate) in a plant. (Reprinted by permission from factor is absent for a long period of time, Plant Science: Growth, Development, and Utilization of a plant will die. Each of these factors is Cultivated Plants, Prentice Hall, 1988.) described below. Light sunlight. Tomato production decreases Photosynthesis depends on the avail- drastically as light intensity drops, and ability of light. Generally, as sunlight only a few tomato varieties produce any intensity increases, so does photosynthe- fruit under minimal sunlight conditions. sis. However, for each plant species, there Water is a maximum level of light intensity Water is one of the raw materials for above which photosynthesis does not photosynthesis. It is taken up into the increase. Many garden crops, such as plant by the roots and moved upward tomatoes, respond best to maximum through the xylem. 24 • Botany Basics—Chapter 1

Respiration Carbohydrates made during photosynthesis are of value to a plant when they are converted to energy. This energy is High water used for cell growth and building new vapor content tissues. The chemical process by which Low CO 2 sugars and starches are converted to energy is called oxidation and is similar to the burning of wood or coal to produce heat. Controlled oxidation in a living cell Guard cell Stoma is called respiration and is shown by this Stoma Guard cell equation: Water vapor C H O + 6O > 6CO + 6H O + Energy CO2 6 12 6 2 2 2 Low water vapor content This equation is essentially the oppo- High CO2 site of photosynthesis. Photosynthesis is Figure 25.—Stomata open to allow carbon dioxide (CO ) to 2 a building process, while respiration is a enter a leaf and water vapor to leave. (Reprinted by permis- breaking-down process (Table 2). Unlike sion from Plant Physiology, The Benjamin/Cummings Pub- photosynthesis, respiration does not lishing Company, Inc., 1991.) depend on light, so it occurs at night as well as during the day. Respiration occurs Carbon dioxide in all life forms and in all cells. Photosynthesis also requires carbon dioxide (CO ), which enters a plant 2 Transpiration through its stomata (Figure 25). In most plants, photosynthesis fluctuates through- When a leaf’s guard cells shrink, its out the day as stomata open and close. stomata open, and water vapor is lost. Typically, they open in the morning, close This process is called transpiration. In down at midday, reopen in late afternoon, turn, more water is pulled through the and shut down again in the evening. plant from the roots. The rate of transpira- Carbon dioxide is plentiful in the air, so tion is directly related to whether stomata it is not a limiting factor in plant growth. are open or closed. Stomata account for However, it is consumed rapidly during only 1 percent of a leaf’s surface but photosynthesis and is replenished very 90 percent of the water transpired. slowly in the atmosphere. Tightly sealed Transpiration is a necessary process greenhouses may not allow enough out- and uses about 90 percent of the water side air to enter and thus may lack that enters a plant’s roots. The other adequate carbon dioxide for plant growth. 10 percent is used in chemical reactions Carbon dioxide generators are used to and in plant tissues. Water moving via the

generate CO2 in commercial greenhouses for crops such as roses, carnations, and Table 2.—Photosynthesis and respiration. tomatoes. In smaller home greenhouses, Photosynthesis Respiration dry ice is an effective source of CO . 2 • Produces food • Uses food Temperature • Stores energy • Releases energy • Uses water • Produces water Although not a direct component in • Uses carbon • Produces carbon photosynthesis, temperature is important. dioxide dioxide Photosynthesis occurs at its highest rate • Releases oxygen • Uses oxygen between 65° and 85°F and decreases at • Occurs in sunlight • Occurs in darkness higher or lower temperatures. as well as in light Chapter 1—Botany Basics • 25

transpiration stream is responsible for Environmental factors several things: • Transporting minerals from the soil affecting growth throughout the plant. Plant growth and geographic distribu- • Cooling the plant through evaporation. tion are greatly affected by the environ- • Moving sugars and plant chemicals. ment. If any environmental factor is less • Maintaining cell firmness. than ideal, it limits a plant’s growth and/or The amount and rate of water loss distribution. For example, only plants depends on factors such as temperature, adapted to limited amounts of water can humidity, and wind or air movement. live in deserts. Transpiration often is greatest in hot, dry Either directly or indirectly, most plant (low relative humidity), windy weather. problems are caused by environmental stress. In some cases, poor environmental A balancing act conditions (e.g., too little water) damage a In order for a plant to grow and develop plant directly. In other cases, environmen- properly, it must balance photosynthesis, tal stress weakens a plant and makes it respiration, and transpiration. Left to more susceptible to disease or insect their own devices, plants do a good job of attack. managing this intricate balance. If a plant Environmental factors that affect plant photosynthesizes at a high rate, but its growth include light, temperature, water, respiration rate is not high enough to humidity, and nutrition. It is important to break down the photosynthates pro- understand how these factors affect plant duced, photosynthesis will either slow growth and development. With a basic down or stop. On the other hand, if respi- understanding of these factors, you may ration is much more rapid than photosyn- be able to manipulate plants to meet your thesis, the plant won’t have adequate needs, whether for increased leaf, flower, photosynthates to produce energy for or fruit production. By recognizing the ✿See Chapter 16, growth. Hence, growth either will slow roles of these factors, you also will be Diagnosing down or stop altogether. better able to diagnose plant problems Problems. When stomata are open, transpiration caused by environmental stress. occurs, sometimes at a very high rate. A corn plant may transpire 50 gallons of Light water per season, but a large tree may Three principal characteristics of light move 100 gallons per day! Plants have affect plant growth: quantity, quality, and problems if they lose too much water, so duration. stomata close during hot, dry periods when transpiration is highest. However, Quantity Light quantity refers to the intensity, or CO2, which is needed for photosynthesis, also enters the plant through open sto- concentration, of sunlight. It varies with mata. Thus, if stomata stay closed a long the seasons. The maximum amount of light is present in summer, and the mini- time to stop water loss, not enough CO2 will enter for photosynthesis. As a result, mum in winter. Up to a point, the more photosynthesis and respiration will slow sunlight a plant receives, the greater its down, in turn reducing plant growth. capacity for producing food via photosyn- Many herb plants produce lots of high- thesis. energy oils, which help them survive in You can manipulate light quantity to the dry landscapes where they evolved. achieve different plant growth patterns. These oils help them survive extended Increase light by surrounding plants with periods of stomatal closure. reflective materials, a white background, 26 • Botany Basics—Chapter 1

or supplemental lights. Decrease it by is high in the blue wavelength. It encour- shading plants with cheesecloth or woven ages leafy growth and is excellent for shade cloth. starting seedlings. Incandescent light is high in the red or orange range, but Quality generally produces too much heat to be a Light quality refers to the color (wave- valuable light source for plants. Fluores- length) of light. Sunlight supplies the cent grow-lights attempt to imitate sun- complete range of wavelengths and can be light with a mixture of red and blue broken up by a prism into bands of red, wavelengths, but they are costly and orange, yellow, green, blue, indigo, and generally no better than regular fluores- violet. cent lights. Blue and red light, which plants absorb, have the greatest effect on plant growth. Duration Blue light is responsible primarily for Duration, or photoperiod, refers to the vegetative (leaf) growth. Red light, when amount of time a plant is exposed to light. combined with blue light, encourages Photoperiod controls flowering in many flowering. Plants look green to us because plants (Figure 26). Scientists initially they reflect, rather than absorb, green thought the length of light period trig- light. gered flowering and other responses Knowing which light source to use is within plants. Thus, they describe plants important for manipulating plant growth. as short-day or long-day, depending on For example, fluorescent (cool white) light what conditions they flower under. We now know that it is not the length of the Short-day (long-night) plants light period, but rather the length of Long, continuous Short, continuous Interrupted uninterrupted darkness, that is critical to dark period dark period dark period floral development. Plants are classified into three categories: short-day (long-night), long-day (short-night), or day-neutral, depending on their response to the duration of light or darkness. Short-day plants form flowers only when day length is less than about 12 hours. Many spring and fall-flowering 24 hours 24 hours 24 hours plants, such as chrysanthemums, poinsettias, and Christmas cactus, are in this Long-day (short-night) plants category. Long, continuous Short, continuous Interrupted In contrast, long-day plants form dark period dark period dark period flowers only when day length exceeds 12 hours. Most summer-flowering plants (e.g., rudbeckia, California poppies, and asters), as well as many vegetables (beets, radishes, lettuce, spinach, and potatoes), are in this category. Day-neutral plants form flowers regardless of day length. Examples are tomatoes, 24 hours 24 hours 24 hours corn, cucumbers, and some strawberry Figure 26.—Periodicity of plants. Short-day (long-night) plants cultivars. Some plants do not fit into any require a long period of uninterrupted darkness to flower. category, but may respond to combina- Long-day (short-night) plants require a short period of tions of day lengths. Petunias, for uninterrupted darkness to flower. Chapter 1—Botany Basics • 27

example, flower regardless of day length, manipulate flowering. For example, a but flower earlier and more profusely with Christmas cactus forms flowers as a result long days. of short days and low temperatures You can easily manipulate photoperiod (Figure 26). To encourage a Christmas to stimulate flowering. For example, cactus to bloom, place it in a room with chrysanthemums normally flower in the more than 12 hours of darkness each day short days of spring or fall, but you can and a temperature of 50° to 55°F until get them to bloom in midsummer by flower buds form. covering them with a cloth that com- If temperatures are high and days are pletely blocks out light for 12 hours each long, cool-season crops such as spinach day. After several weeks of this treatment, will flower (bolt). However, if tempera- the artificial dark period no longer is tures are too cool, fruit will not set on needed, and the plants will bloom as if it warm-season crops such as tomatoes. were spring or fall. This method also is used to make poinsettias flower in time Crop quality for Christmas. Low temperatures reduce energy use To bring a long-day plant into flower and increase sugar storage. Thus, leaving when day length is less than 12 hours, crops such as ripe winter squash on the expose the plant to supplemental light. vine during cool, fall nights increases their After a few weeks, flower buds will form. sweetness. Adverse temperatures, however, cause Temperature stunted growth and poor-quality vegetables. For example, high temperatures Temperature influences most plant cause bitter lettuce. processes, including photosynthesis, transpiration, respiration, germination, Photosynthesis and respiration and flowering. As temperature increases Thermoperiod refers to daily tempera- (up to a point), photosynthesis, transpira- ture change. Plants grow best when tion, and respiration increase. When daytime temperature is about 10° to combined with day length, temperature 15° higher than nighttime temperature. also affects the change from vegetative Under these conditions, plants photosyn- (leafy) to reproductive (flowering) growth. thesize (build up) and respire (break Depending on the situation and the spe- down) during optimum daytime tempera- cific plant, the effect of temperature can tures and then curtail respiration at night. either speed up or slow down this transi- However, not all plants grow best under tion. the same nighttime and daytime temperatures. For example, snapdragons grow Germination best at nighttime temperatures of 55°F; The temperature required poinsettias, at 62°F. for germination varies by Temperatures higher than needed species. Generally, cool- ✿See Chapter 7, Vegetables. increase respiration, sometimes above the season crops (e.g., spinach, rate of photosynthesis. Thus, photosyn- radishes, and lettuce) germinate best at thates are used faster than they are 55° to 65°F, while warm-season crops produced. For growth to occur, photosyn- (e.g., tomatoes, petunias, and lobelias) thesis must be greater than respiration. germinate best at 65° to 75°F. Daytime temperatures that are too low Flowering often produce poor growth by slowing Sometimes horticulturists use tempera- down photosynthesis. The result is ture in combination with day length to reduced yield (e.g., fruit or grain production). 28 • Botany Basics—Chapter 1

Breaking dormancy It is worth noting that the tops of hardy Some plants that grow in cold regions plants are much more cold-tolerant than need a certain number of days of low the roots. Plants that normally are hardy temperature (dormancy). Knowing the to 10°F may be killed if they are in contain- period of low temperature required by a ers and the roots are exposed to 20°F. plant, if any, is essential in getting it to Winter injury also may occur because of grow to its potential. desiccation (drying out) of plant tissues. Peaches are a prime example; most People often forget that plants need water varieties require 700 to 1,000 hours even during winter. When the soil is between 32° and 45°F before breaking frozen, water movement into a plant is their rest period and beginning growth. severely restricted. On a windy winter Lilies need 6 weeks of temperatures at or day, broadleaf evergreens can become slightly below 33°F before blooming. water-deficient in a few minutes, and the Daffodils can be forced to flower by leaves or needles then turn brown. To storing the bulbs at 35° to 40°F in October. minimize the risk of this type of injury, The cold temperature allows the bulbs to make sure your plants go into the winter mature. When transferred to a greenhouse well watered. in midwinter, they begin to grow, and flowers are ready to cut in 3 to 4 weeks. Water and humidity Hardiness Most growing plants contain about Plants are classified as hardy or 90 percent water. Water plays many roles nonhardy depending on their ability to in plants. It is: withstand cold temperatures. Hardy • A primary component in photosynthe- plants are those that are adapted to the sis and respiration cold temperatures of their growing envi- • Responsible for turgor pressure in cells ronment. (Like air in an inflated balloon, water Woody plants in the temperate zone is responsible for the fullness and have very sophisticated means for sens- firmness of plant tissue. Turgor is ing the progression from fall to winter. needed to maintain cell shape and Decreasing day length and temperature ensure cell growth.) trigger hormonal changes that cause • A solvent for minerals and carbohy- leaves to stop photosynthesizing and to drates moving through the plant ship nutrients to twigs, buds, stems, and • Responsible for cooling leaves as it roots. An abscission layer forms where evaporates from leaf tissue during each petiole joins a stem, and the leaves transpiration eventually fall off. Changes within the • A regulator of stomatal opening and trunk and stem tissues over a relatively closing, thus controlling transpiration, short period of time “freeze-proof” the and, to some degree, photosynthesis plant. • The source of pressure to move roots Winter injury to hardy plants may occur through the soil when temperatures drop too quickly in • The medium in which most biochemi- the fall before a plant has progressed to cal reactions take place full dormancy. In other cases, a plant may Relative humidity is the ratio of water break dormancy in mid- or late winter if vapor in the air to the amount of water the weather is unseasonably warm. If a the air could hold at the current tempera- sudden, severe cold snap follows the ture and pressure. Warm air can hold ✿See Chapter 9, warm spell, otherwise hardy plants can be more water vapor than cold air. Relative Woody Plants. seriously damaged. Chapter 1—Botany Basics • 29

humidity (RH) is expressed by the follow- environment around a plant. A lot must ing equation: happen before a chemical element in a RH = Water in air ÷ Water air could hold fertilizer can be used by a plant. (at constant temperature and pressure) ✿See Chapter 2, Plants need 16 elements for normal Soils and Relative humidity is given as a percent. growth. Three of them—carbon, hydro- Fertilizers, and For example, if a pound of air at 75°F gen, and oxygen—are found in air and Chapter 7, could hold 4 grams of water vapor, and water. The rest are found in the soil. Vegetable there are only 3 grams of water in the air, Three soil elements are called primary Gardening. then the relative humidity (RH) is: nutrients because they are used in rela- 3 ÷ 4 = 0.75 = 75% tively large amounts by plants. They are Water vapor moves from an area of high nitrogen, phosphorus, and potassium. relative humidity to one of low relative Calcium, magnesium, and sulfur are called humidity. The greater the difference in secondary nutrients because they are used humidity, the faster water moves. This in moderate amounts. Often, primary and factor is important because the rate of secondary nutrients are collectively called water movement directly affects a plant’s macronutrients (Table 3). transpiration rate. Seven other soil elements are used in The relative humidity in the air spaces much smaller amounts and are called between leaf cells approaches 100 per- micronutrients or trace elements (Table 4). cent. When a stoma opens, water vapor They are iron, boron, zinc, copper, manga- inside the leaf rushes out into the sur- nese, molybdenum, and chlorine. rounding air (Figure 25), and a bubble of Most of the nutrients a plant needs are high humidity forms around the stoma. By dissolved in water and then absorbed by saturating this small area of air, the its roots. In fact, 98 percent are absorbed bubble reduces the difference in relative from the soil-water solution, and only humidity between the air spaces within about 2 percent are actually extracted the leaf and the air adjacent to the leaf. As from soil particles. a result, transpiration slows down. If wind blows the humidity bubble away, Fertilizers however, transpiration increases. Thus, Fertilizers are materials containing transpiration usually is at its peak on hot, plant nutrients that are added to the dry, windy days. On the other hand, environment around a plant. Generally, transpiration generally is quite slow when they are added to the water or soil, but temperatures are cool, humidity is high, some can be sprayed on leaves. This and there is no wind. method is called foliar fertilization. It Hot, dry conditions generally occur should be done carefully with a dilute during the summer, which partially solution, because a high fertilizer concen- explains why plants wilt quickly in the tration can injure leaf cells. The nutrient, summer. If a constant supply of water is however, does need to pass not available to be absorbed by the roots through the thin layer of wax and moved to the leaves, turgor pressure (cutin) on the leaf surface. is lost and leaves go limp. Fertilizers are not plant food! Plants produce their own food Plant nutrition from water, carbon dioxide, and solar energy through photosyn- Plant nutrition often is confused with thesis. This food (sugars and fertilization. Plant nutrition refers to a carbohydrates) is combined with plant’s need for and use of basic chemical plant nutrients to produce elements. Fertilization is the term used proteins, enzymes, vitamins, and when these materials are added to the other elements essential to growth. continues on page 32 30 • Botany Basics—Chapter 1

Table 3.—Plant macronutrients. Leachability in soil/ Element Absorbed as Mobility in plants Signs of excess Signs of deficiency Notes - Nitrogen NO3 Leachable, Succulent growth; Reduced growth, In general, the best (N) (nitrate), especially NO -. dark green color; yellowing (chlorosis). NH +:NO - ratio is 1:1. + 3 4 3 NH4 Mobile in plants. weak, spindly Reds and purples Under low-sugar condi- (ammonium) growth; few fruits. may intensify in tions (low light), high + May cause brittle some plants. NH4 can cause leaf curl. growth, especially Reduced lateral Uptake is inhibited by under high bud breaks. high P levels. The N:K temperatures. Symptoms appear ratio is extremely first on older growth. important. Indoors, the best N:K ratio is 1:1 unless light is extremely high. In soils with a high C:N ratio, more N should be supplied. - - Phosphorus H2PO4 , HPO4 Normally not Shows up as Reduced growth. Rapidly bound (fixed) (P) (phosphate) leachable, but micronutrient Color may intensify. on soil particles. Under may leach from deficiency Browning or acid conditions, fixed soil high in bark of Zn, Fe, or Co. purpling of foliage in with Fe, Mg, and Al or peat. Not some plants. Thin (aluminum). Under alka- readily mobile in stems, reduced line conditions, fixed plants. lateral bud breaks, with Ca. Important for loss of lower leaves, young plant and seedling reduced flowering. growth. High P interferes with micronutrient absorption and N absorption. Used in relatively small amounts when compared to N and K. Potassium K+ Can leach in sandy Causes N Reduced growth, N:K balance is (K) soils. Mobile in deficiency in shortened inter- important. High N:low K plants. plant and may nodes. Marginal burn favors vegetative affect the uptake or scorch (brown growth; low N:high K of other positive leaf edges), necrotic promotes reproductive ions. (dead) spots in growth (flowers, fruit). leaves. Reduction of lateral bud breaks, tendency to wilt readily. Magnesium Mg++ Leachable. Mobile Interferes with Ca Reduction in growth. Mg commonly is defi- (Mg) in plants. uptake. Marginal chlorosis, cient in foliage plants interveinal chlorosis because it is leached (yellow between the and not replaced. Epsom veins) in some salts at a rate of 1 tea- species (may occur spoon per gallon may on middle or lower be used two times per leaves). Reduction in year. Mg also can be seed production, absorbed by leaves if cupped leaves. sprayed in a weak solution. Dolomitic limestone can be applied in outdoor situations to correct a deficiency.

continues on next page Chapter 1—Botany Basics • 31

Table 3.—Plant macronutrients (continued). Leachability in soil/ Element Absorbed as Mobility in plants Signs of excess Signs of deficiency Notes Calcium Ca++ Normally not High Ca usually Inhibition of bud Ca is important to pH (Ca) leachable. causes high pH, growth, death of control and rarely is Moderately limited which then root tips. Cupping of deficient if the correct mobility in plants. precipitates many maturing leaves, pH is maintained. Interferes with Mg micronutrients so weak growth. Water stress (too much absorption. that they become Blossom-end rot or too little) can affect unavailable to of many fruits, pits Ca relations within plants. on root vegetables. plants, causing deficiency in the location where Ca was needed at the time of stress.

- Sulfur SO4 Leachable. Not Sulfur excess General yellowing of S often is a carrier or (S) (sulfate) mobile in plants. usually is in the affected leaves or impurity in fertilizers form of air the entire plant. and rarely is deficient. pollution. It also may be absorbed from the air and is a by- product of combustion.

Table 4.—Plant micronutrients. Element Absorbed as Signs of excess Signs of deficiency Notes Iron Fe++, Fe+++ Rare except on flooded Soil high in Ca, Mn, P, Add Fe in the chelate form. (Fe) soils. Interveinal chlorosis, or heavy metals (Cu, Zn); The type of chelate primarily on young tissue, high pH; poorly drained needed depends on which eventually may turn soil; oxygen-deficient soil pH. white. soil; nematode attack on roots. - Boron BO3 Blackening or death of Failure to set seed, internal (B) (borate) tissue between veins. breakdown, death of apical buds. Zinc Zn++ Shows up as Fe deficiency. “Little leaf” (reduction in (Zn) Also interferes with Mg leaf size), short internodes, absorption. distorted or puckered leaf margins, interveinal chlorosis. Copper Cu++, Cu+ Can occur at low pH. New growth small, mis- May be found in some peat (Cu) Shows up as Fe deficiency. shapen, wilted. soils.

Manganese Mn++ Reduction in growth, brown Interveinal chlorosis of Found under acid (Mn) spotting on leaves. Shows leaves followed by brown conditions. up as Fe deficiency. spots, producing a checkered effect. - Molybdenum MoO4 Interveinal chlorosis on (Mo) (molybdate) older or midstem leaves, twisted leaves (whiptail). Chlorine Cl- Salt injury, leaf burn. May Leaves wilt, then become (Cl) increase succulence. bronze, then chlorotic, then die; club roots. 32 • Botany Basics—Chapter 1

Nutrient absorption them, either by removing those that have Anything that reduces or stops sugar outgrown their space or by selective production in leaves can lower nutrient pruning and thinning. Often, understory absorption. Thus, if a plant is under stress plants that did well when the landscape because of low light or extreme tempera- was young must be replaced with plants tures, nutrient deficiency may develop. that are more shade-tolerant. This process A plant’s developmental stage or rate of is a kind of plant succession, dictated by growth also may affect the amount of the changing light and moisture environ- nutrients absorbed. Many plants have a ment and carried out by the owner. rest (dormant) period during part of the A lawn also is a changing landscape. It year. During this time, few nutrients are starts out as a mix of several adapted absorbed. Plants also may absorb differ- grass species on bare ground. Other ent nutrients as flower buds begin to plants (which we often call weeds) sprout develop than they do during periods of from seed reserves in the soil. Additional rapid vegetative growth. seeds and plants move in and grow if conditions are right. Our most competitive lawn weed is a grass called annual Plants in communities bluegrass. It prospers in the winter when The preceding discussion focused on desired grasses are less vigorous. Broad- the structure and physiology of individual leaf weeds also may find niches. Moss plants. Interactions among plants also are begins to take over where the lawn is thin, important for gardeners. The study of a common problem in semishaded areas. these interactions is called plant or land- These changes are another example of scape ecology. plant succession. In ornamental gardens, we generally aim To manage invasive plants, keep your to develop a stable community of plants lawn grasses competitive by using proper that complement each other in form, cultural practices, periodically color, leaf characteristics, and bloom. We overseeding, and using herbicides in must pay attention to the differing certain situations. In spite of your best requirements of plants within this commu- efforts, however, plant succession may nity. occur. A garden’s framework often is Gardeners who plant wildflower defined by large shrubs or mixtures often discover that trees, which cast differ- there is much more variety ing amounts of in flowers the first year shade over the than in succeeding course of the year. years. Some species do When choosing very well, and others plants to grow simply cannot com- under or near large pete. Again, plant framework speci- succession occurs. mens, be sure their The most short-term needs match the assemblage of plants in ✿See Chapter 21, a garden occurs in Landscape available light and Design, moisture. annual vegetable and Chapter 12, As trees and flower beds. Here there Lawns, and shrubs grow and is no attempt to create Chapter 17, mature, you may a community that will Weeds. need to manipulate last more than one season. Chapter 1—Botany Basics • 33

Since many of the most competitive transport between species through symbi- weeds thrive in recently disturbed soil, it otic fungi called mycorrhizae. These is a challenge to give desired annual crop relationships are quite complex, and many plants an advantage. The plant that are not well understood. They are the captures light first will grow and suppress subject of active research and offer much plants beneath it. Early weed competition to think about for thoughtful gardeners. can have a devastating impact on crop growth. Consistent weeding, mulching, and the use of transplants improve the Plant hormones and odds for annual vegetable and flower growth regulators crops. Another type of relationship between Plant hormones and growth regulators plants is called allelopathy. In this phe- are chemicals that affect flowering; aging; nomenon, some plants produce com- root growth; distortion and killing of pounds in their leaves, roots, or both that leaves, stems, and other parts; prevention inhibit the growth of other plants. Black or promotion of stem elongation; color walnut is the most notorious example. Its enhancement of fruit; prevention of roots can suppress many common veg- leafing and/or leaf fall; and many other etable plants, and its leaves, if mulched on conditions (Table 5). Very small concen- a vegetable garden over the winter, can trations of these substances produce affect many annual crops like an herbicide major growth changes. the following spring. Some of the worst Hormones are produced naturally by weeds show allelopathic traits and pre- plants, while plant growth regulators are vent desired ornamental or vegetable applied to plants by humans. Plant growth species from growing. regulators may be synthetic compounds Finally, there are relationships between (e.g., IBA and Cycocel) that mimic natu- plants that involve pollinators, animals, rally occurring plant hormones, or they birds, pests, predators, and even nutrient may be natural hormones that were extracted from plant tissue (e.g., IAA).

Table 5.—Common growth-affecting materials. Compound Effect/Use Hormones Gibberellic acid (GA) Stimulates cell division and elongation, breaks dormancy, speeds germination.

Ethylene gas (CH2) Ripening agent; stimulates leaf and fruit abscission. Indoleacetic acid (IAA) Stimulates apical dominance, rooting, and leaf abscission. Plant growth regulators Indolebutyric acid (IBA) Stimulates root growth. Naphthalene acetic acid (NAA) Stimulates root growth, slows respiration (used as a dip on holly). Growth retardants (Alar, B-9, Cycocel, Arest) Prevent stem elongation in selected crops (e.g., chrysanthemums, poinsettias, and lilies). Herbicides (2,4-D, etc.) Distort plant growth; selective and nonselective materials used for killing unwanted plants. 34 • Botany Basics—Chapter 1

Applied concentrations of these sub- Ethylene is unique in that it is found stances usually are measured in parts per only in the gaseous form. It induces million (ppm) and in some cases parts per ripening, causes leaves to droop (epi- billion (ppb). These growth-regulating nasty) and drop (abscission), and pro- substances most often are applied as a motes senescence. Plants often increase spray to foliage or as a liquid drench to ethylene production in response to stress, soil around a plant’s base. Generally, their and ethylene often is found in high con- effects are short lived, and they may need centrations within cells at the end of a to be reapplied in order to achieve the plant’s life. The increased ethylene in leaf desired effect. tissue in the fall is part of the reason There are five groups of plant growth- leaves fall off of trees. Ethylene also is regulating compounds: auxin, gibberellin used to ripen fruit (e.g., green bananas). (GA), cytokinin, ethylene, and abscisic Abscisic acid (ABA) is a general plant- acid (ABA). For the most part, each group growth inhibitor. It induces dormancy; contains both naturally occurring hor- prevents seeds from germinating; causes mones and synthetic substances. abscission of leaves, fruits, and flowers; Auxin causes several responses in and causes stomata to close. High concen- plants: trations of ABA in guard cells during • Bending toward a light source (photo- periods of drought stress probably play a tropism) role in stomatal closure. • Downward root growth in response to gravity (geotropism) • Promotion of apical dominance For more information • Flower formation Capon, B. Botany for Gardeners: An Intro- • Fruit set and growth duction and Guide (Timber Press, • Formation of adventitious roots Portland, OR, 1990). Auxin is the active ingredient in most Salisbury, F.B., and C.W. Ross. Plant Physi- rooting compounds in which cuttings are ology, 4th edition (Wadsworth Publish- dipped during vegetative propagation. ing Company, Belmont, CA, 1992). Gibberellins stimulate cell division and Williams Rice, L., and R.P. Rice, Jr. Practical elongation, break seed dormancy, and Horticulture, 4th edition (Prentice Hall, speed germination. The seeds of some Upper Saddle River, NJ, in press). species are difficult to germinate; you can soak them in a GA solution to get them started. Unlike other hormones, cytokinins are found in both plants and animals. They stimulate cell division and often are included in the sterile media used for growing plants from tissue culture. If a medium’s mix of growth-regulating compounds is high in cytokinins and low in auxin, the tissue culture explant (small plant part) will produce numerous shoots. On the other hand, if the mix has a high ratio of auxin to cytokinin, the explant will produce more roots. Cytokinins also are used to delay plant aging and death (senescence).