ho96 MG botany.pdf

HO-96 Basic Botany By Ann Marie VanDerZanden, former Extension Master Gardener state coordinator, Oregon State University. Adapted for Kentucky by Richard Durham, consumer horticulture Extension specialist and state Master Gardener coordinator, University of Kentucky....

HO-96 Basic Botany By Ann Marie VanDerZanden, former Extension Master Gardener state coordinator, Oregon State University. Adapted for Kentucky by Richard Durham, consumer horticulture Extension specialist and state Master Gardener coordinator, University of Kentucky. distinguishing characteristics. For example, monocots (e.g., In this chapter: grasses) produce only one seed leaf, while dicots (broadleaf plants) have two. The vascular systems, flowers, and leaves Plant Life Cycles 01 of the two types of plants also differ (Table 1.1). These differ- ences will be important in our discussion of plant growth and Botany Terminology 02 development. Internal Plant Parts 02 External Plant Parts 02 Plant Life Cycles Stem Terminology 05 Based on its life cycle, a plant is classified as either an annual, Plant Growth and Development 15 biennial, or perennial. Environmental Factors Affecting Growth 16 An annual, such as a zinnia, completes its life cycle in one year. Annuals are said to go from seed to seed in one year or Plants in Communities 20 growing season. During this period, they germinate, grow, Plant Hormones and Growth Regulators 21 mature, bloom, produce seeds, and die. Summer annuals complete their life cycle during spring and summer; most For More Information 22 winter annuals complete their growing season during fall and winter. There are both winter and summer annual weeds, and P understanding a weed’s life cycle is important to controlling it. lants are essential to life on earth. Either directly or indi- A biennial requires all or part of two growing seasons to rectly, they are the primary food source for humans and complete its life cycle. During the first season, it produces veg- other animals. Additionally, they provide fuel, replenish etative structures (leaves) and food storage organs. The plant the earth’s oxygen supply, prevent soil erosion, slow down overwinters and then produces flowers, fruit, and seeds during wind movement, cool the atmosphere, provide wildlife habitat, its second season. Swiss chard, carrots, beets, sweet William, supply medicinal compounds, and beautify our surroundings. and parsley are examples of biennials. Many plants are familiar to us, and we can identify and Sometimes biennials go from seed germination to seed appreciate them based on their external structure. However, production in only one growing season. This situation occurs their internal structure and function often are overlooked. when extreme environmental conditions, such as drought or Understanding how plants grow and develop helps us capitalize temperature variation, cause the plant to pass rapidly through on their usefulness and make them part of our everyday lives. the equivalent of two growing seasons. This phenomenon is This chapter focuses on vascular plants—those that contain referred to as bolting. Sometimes bolting occurs when biennial water-, nutrient-, and food-conducting tissues called “xylem” plant starts are exposed to a cold spell before being planted in and “phloem.” Ferns and seed-producing plants fall into the garden. this category. Perennial plants live more than two years and are grouped In several cases, we will distinguish between monocotyledon- into two categories: herbaceous perennials and woody peren- ous and dicotyledonous plants. Sometimes called “monocots” nials. Herbaceous perennials have soft, nonwoody stems that and “dicots” for short, these plants have several important Table 1.1. Comparison of monocots and dicots. Structure Monocots Dicots Seed leaves (cotyledons) one two Vascular system Xylem and phloem are paired in bundles, Xylem and phloem form rings inside the stem. The phloem forms which are dispersed throughout the stem. an outer ring, the xylem an inner ring. In long-lived woody perenni- als, yearly concentric rings are produced. Floral parts Usually in threes or multiples of three. Usually in multiples of four or five. Leaves Often parallel-veined. Generally net-veined. CHAPTER 01 Basic Botany generally die back to the ground each winter. New stems grow from the plant’s crown each spring. Trees and shrubs, on the Botany Terminology Anther—The pollen sac on a male flower. other hand, have woody stems that withstand cold winter tem- peratures. They are referred to as woody perennials. Apex—The tip of a shoot or root. Apical dominance—The tendency of an apical bud to produce hormones that suppress growth of buds below it on the stem. Internal Plant Parts Axil—The location where a leaf joins a stem. Cells are the basic structural and physiological units of plants. Cambium—A layer of growing tissue (meristem) that separates the xylem and phloem and produces new xylem and phloem cells. Most plant reactions (cell division, photosynthesis, respiration, etc.) occur at the cellular level. Plant tissues (meristems, xylem, Chlorophyll—The green pigment in leaves that is responsible for captur- ing light energy from the sun. phloem, etc.) are large, organized groups of similar cells that work together to perform a specific function. Chloroplast—A specialized component (organelle) of certain cells; con- tains chlorophyll and is responsible for photosynthesis. A unique feature of plant cells is that they are readily toti- Cortex—Cells that make up the primary tissue of the root and stem. potent. In other words, almost all plant cells retain all of the genetic information (encoded in DNA) necessary to develop Cotyledon—The first leaf that appears on a seedling. Also called a seed leaf. into a complete plant. This characteristic is the main reason Cuticle—A relatively impermeable surface layer on the epidermis of that vegetative (asexual) reproduction works. E.g., the cells of a leaves and fruits. small leaf cutting from an African violet have all of the genetic Dicot—Having two cotyledons or seed leaves. information necessary to generate a root system, stems, more leaves, and ultimately flowers. Epidermis—The outermost layer of plant cells. Specialized groups of cells called meristems are a plant’s Guard cell—Epidermal cells that open and close to let water, oxygen, and carbon dioxide pass through the stomata. growing points. Meristems are the site of rapid, almost continu- ous cell division. These cells either continue to divide or begin Internode—The space between nodes on a stem. to differentiate into other tissues and organs. How they divide Meristem—Specialized groups of cells that undergo cell division and are a plant’s growing points. and whether they ultimately become a tissue or an organ are controlled by a complex array of internal plant hormones but Mesophyll—A leaf’s inner tissue, located between the upper and lower epidermis; contains the chloroplasts and other specialized cellular also can be influenced by environmental conditions. In many parts (organelles). cases, you can manipulate meristems to make a plant do some- Monocot—Having one cotyledon or seed leaf. thing you want, such as change its growth pattern, flower, alter Node—An area on a stem where a leaf, stem, or flower bud is located. its branching habit, or produce vegetative growth. Ovary—The part of a female flower where eggs are located. Also, the base of the pistil. External Plant Parts Petiole—The stalk that attaches a leaf to a stem. External plant structures such as leaves, stems, roots, flow- Phloem—Photosynthate-conducting tissue. ers, fruits, and seeds are known as plant organs. Each organ is Photosynthate—A food product (sugar or starch) created through photosynthesis. an organized group of tissues that work together to perform a specific function. These structures can be divided into two Photosynthesis—The process in green plants of converting carbon dioxide and water into food (sugars and starches) using energy from groups: sexual (reproductive) and vegetative. Sexual or repro- sunlight. ductive parts produce seed; they include flower buds, flowers, Pistil—The female flower part; consists of a stigma, style, and ovary. fruit, and seeds. Vegetative parts (Figure 1.1) include roots, Respiration—The process of converting sugars and starches into energy. stems, shoot buds, and leaves; they are not directly involved in sexual reproduction. Vegetative parts often are used in asexual Stamen—The male flower part; consists of an anther and a supporting filament. forms of reproduction such as cuttings, budding, or grafting. Stigma—The top of a female flower (pistil) part; collects pollen. Stoma (pl. stomates, stomata)—Tiny openings in the epidermis that allow Roots water, oxygen, and carbon dioxide to pass into and out of a plant. Often roots are overlooked, probably because they are less Style—The part of the female flower (pistil) that connects the stigma visible than the rest of the plant. However, it’s important to to the ovary. Pollen travels down the style to reach the ovary, where fertilization occurs. understand plant root systems because they have a pronounced Transpiration—The process of losing water (in the form of vapor) effect on a plant’s size and vigor, method of propagation, through stomata. adaptation to soil types, and response to cultural practices Turgor—Cellular water pressure; responsible for keeping cells firm. and irrigation. Vascular tissue—Water-, nutrient-, and photosynthate-conducting tissue (xylem and phloem). Xylem—Water- and nutrient-conducting tissue. 1-02  Basic Botany CHAPTER 01 Roots typically originate from the lower portion of a plant The zone of maturation is directly beneath the stem. Here, or cutting. They have a root cap but lack nodes and never bear cells become specific tissues such as epidermis, cortex, or leaves or flowers directly. Their principal functions are to absorb vascular tissue. nutrients and moisture, anchor the plant in the soil, support A root’s epidermis is its outermost layer of cells (Figure 1.3). the stem, and store food. In some plants, they can be used These cells are responsible for absorbing water and minerals for propagation. dissolved in water. Cortex cells are involved in moving water from the epidermis to the vascular tissue (xylem and phloem) Structure and in storing food. Vascular tissue is located in the center of Internally, there are three major parts of a root (Figure 1.2): the root and conducts food and water. The meristematic zone is at the tip and produces new cells; Externally, there are two areas of importance: the root it is an area of cell division and growth. cap and the root hairs (Figure 1.2). The root cap is the root’s Behind the meristem is the zone of elongation. In this area, outermost tip. It consists of cells that are sloughed off as the cells increase in size through food and water absorption. As root grows through the soil. Its function is to protect the root they grow, they push the root through the soil. meristem. Root hairs are delicate, elongated epidermal cells that occur in a small zone just behind the root’s growing tip. They gener- leaf primordia ally appear as fine down to the naked eye. Their function is to shoot apex increase the root’s surface area and absorptive capacity. Root hairs usually live one or two days. When a plant is transplanted, they are easily torn off or may dry out. Many roots have a naturally occurring symbiotic (mutually beneficial) relationship with mycorrhizae fungi, which improves leaf the plant’s ability to absorb water and nutrients. Types of Roots There are two major types of roots: primary and lateral roots. bud A primary root originates at the lower end of a seedling’s embryo (Figure 1.2). If the primary root continues to elongate downward, becomes the central feature of the root system, and has limited secondary branching, it is called a taproot (Figure 1.4a). Hickory and pecan trees, as well as carrots, have taproots. stem A lateral, or secondary, root is a side or 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 1.4b). These lateral roots branch repeatedly to form the network of feeding roots found on most plants. soil line vascular tissue Some plants, such as grasses, naturally produce a fibrous root system. In other cases, severing a plant’s taproot by undercutting it can encourage the plant to produce a fibrous root system. Nurseries use this technique with trees that naturally produce a taproot, because trees with a compact, fibrous root system lateral root are transplanted more successfully. primary root How Roots Grow During early development, a seedling absorbs nutrients and moisture from the soil around the sprouting seed. A band of fertilizer several inches to each side and slightly below newly planted seeds helps early growth of most row crops. As a plant becomes well established, the quantity and distribution of its roots strongly influence its ability to absorb moisture and nutrients. For most plants, the majority of the absorbing (feeder) roots are located in the top 12 inches of soil. Figure 1.1. Principal parts of a vascular plant. (Adapted with per- The soil environment in this region generally is best for root mission from Plant Physiology, The Benjamin/Cummings Publishing Company, Inc., 1991.) growth, with a good balance of fertility, moisture, and air spaces. 1-03 CHAPTER 01 Basic Botany The following factors are important in root growth: often extend well beyond a plant’s drip line (edge of foliage Roots in water-saturated soil do not grow well and ultimately or canopy). Keep this extensive root system in mind when may die due to lack of oxygen. disturbing the soil around existing trees and shrubs. Roots penetrate much deeper in loose, well-drained soil than in heavy, poorly drained soil. Roots as Food A dense, compacted soil layer can restrict or terminate root An enlarged root is the edible portion of several vegetable growth. crops. Sweet potatoes are a swollen tuberous root; and carrots, Container plants not only have a restricted area for root parsnips, salsify, and radishes are elongated taproots. growth, but they are susceptible to cold damage because the limited amount of soil surrounding their roots may not Stems provide adequate insulation. Dark-colored containers may Stems support buds and leaves and serve as conduits for car- also absorb solar radiation in summer, and the heat generated rying water, minerals, and food (photosynthates). The vascular may also damage root systems. system inside the stem forms a continuous pathway from the In addition to growing downward, roots grow laterally and root, through the stem, and finally to the leaves. It is through this system that water and food products move. Structure lateral root Vascular system—This system consists of xylem, phloem, and vascular cambium. It can be thought of as a plant’s plumbing. primary root zone of maturation Xylem tubes conduct water and dissolved minerals; phloem tubes carry food such as sugars. The cambium is a layer of root hairs meristematic tissue that separates the xylem and phloem and produces new xylem and phloem cells. This new tissue is zone of elongation responsible for a stem’s increase in girth. root tip The vascular cambium is important to gardeners. E.g., the meristematic zone root cap cambial tissues on a grafted scion and rootstock need to line up. In addition, careless weed trimming can strip the bark off Figure 1.2. Root structure. (Adapted with permission from Plant Physiology, The Benjamin/Cummings Publishing Company, Inc., 1991.) a tree, thus injuring the cambium and causing the tree to die. The vascular systems of monocots and dicots differ (Figure 1.5). Although both contain xylem and phloem, these structures epidermis are arranged differently in each. In a monocot, the xylem and phloem are paired in bundles, which are dispersed throughout cortex the stem. In a dicot, the vascular system is said to be continu- pericycle ground tissues ous because it forms rings inside the stem. The phloem forms endodermis the outer ring just under the bark in mature woody stems. The xylem forms the inner ring and may be divided into the xylem vascular sapwood and heartwood. Individual rings may be evident in phloem tissue the xylem that correspond to growth events. In temperate zones or climates with pronounced wet and dry seasons, these individual rings can be used to discern the plant’s age and the Figure 1.3. Cross section of a root. (Adapted with permission from Plant environmental conditions that may have caused differing rates Physiology, The Benjamin/Cummings Publishing Company, Inc., 1991.) of yearly growth. Nodes—A node is an area on a stem where buds are located (Figure 1.6). It is a site of great cellular activity and growth where small buds develop into leaves, stems, or flowers. When pruning, it is important to locate a plant’s nodes. Generally, you want to make a pruning cut just above, but not too close to, a node. Pruning in this manner encourages the buds at that node to begin development and ultimately form new stems or leaves. The area between two nodes is called an internode. Its length depends on many factors, including genetics. Several other factors also can influence internode length: (a) taproot (b) fibrous root Figure 1.4. Taproot of a carrot (a) and fibrous root of grass (b). 1-04  Basic Botany CHAPTER 01 Reduced soil fertility decreases internode length, while an phloem phloem application of high-nitrogen fertilizer can greatly increase it. cambium Lack of light increases internode length and causes spindly stems. This situation is known as stretch, or etiolation, and xylem often occurs in seedlings started indoors and in houseplants that do not get enough sunlight. Internode length also varies with the season. Early-season xylem growth has long internodes, while late-season growth gener- ally has much shorter internodes. pith If a stem’s energy is divided among three or four side stems (a) monocot (b) dicot or is diverted into fruit growth and development, internode Figure 1.5. Cross sections of stems: (a) discontinuous vascular system length is shortened. of a monocot stem and (b) continuous vascular system of a woody Plant growth regulator substances and herbicides also can dicot stem. influence internode length. Types of Stems Stems may be long, with great distances between the leaves Stem Terminology and buds (e.g., branches of trees, runners on strawberries) or Shoot—A young stem (one year old or less) with leaves. compressed, with short distances between buds or leaves (e.g., Twig—A young stem (one year old or less) that is in the dormant crowns of strawberry plants, fruit spurs, and African violets). winter stage (has no leaves). Although stems commonly grow aboveground, they sometimes Branch—A stem that is more than one year old, typically with grow belowground in the form of rhizomes, tubers, corms, or lateral stems radiating from it. bulbs. All stems must have buds or leaves to be classified as Trunk—A woody plant’s main stem. stem tissue. Specialized aboveground stems—Some plants have special- ized aboveground stems known as crowns, spurs, or stolons (Figure 1.7). Crowns (on strawberries, dandelions, and African bud violets) are compressed stems with leaves and flowers on short internodes. Spurs are short, stubby side stems that arise from a main stem. They are the fruit-bearing stems on pear, apple, and cherry trees. If severe pruning is done too close to fruit-bearing spurs, they can revert to nonfruiting stems, thus eliminating the year’s node potential fruit crop. Stolons are fleshy or semiwoody, elongated, horizontal internode stems that often lie along the soil surface. Strawberry runners are stolons that have small leaves at the nodes. Roots develop from these nodes, and a daughter plant is formed. This type of vegetative reproduction is an easy way to increase the size of a strawberry patch. Spider plants also produce stolons, which ultimately can become entirely new plants. Figure 1.6. Stem structure. (Adapted with permission from Plant Physiology, The Benjamin/Cummings Publishing Company, Inc., 1991.) Specialized belowground stems—Potato tubers, iris rhizomes, and tulip bulbs are underground stems that store food for the plant (Figure 1.8). It sometimes is difficult to distinguish between roots and stems, but one sure way is to look for nodes. Stems have nodes; roots do not. In potato tubers, for example, the “eyes” are actually the stem’s spur stolon nodes, and each eye contains a cluster of buds. When growing potatoes from “seed” pieces (cut potatoes), it is important that crown each piece contains at least one eye and be about the size of a golf ball so there will be enough energy for early growth of shoots and roots. Figure 1.7. Diversified aboveground stem development. 1-05  CHAPTER 01 Basic Botany leaves leaves Corms are another kind of belowground stem. Although both bulbs and corms are composed of stem tissue, they are not the same. Corms are shaped like bulbs, but do not contain fleshy scales. A corm is a solid, swollen stem with dry, scalelike stem stem leaves. Gladiolus and crocuses produce corms. Other plants (e.g., dahlias and sweet potatoes) produce unicate bulb tunicate bulb underground storage organs called tuberous roots, which often nontunicate nontunicate bulb bulb are confused with bulbs and tubers. However, these are root tissue,tuberous stem and stem tuberous not stem tissue, have neither nodes nor internodes. Stems and Propagationlateral budlateral bud Stems often are usedinternode internode for vegetative plant propagation. Using sections of aboveground stemsnodethat contain node nodes and inter- nodes is an effective way to propagate many ornamental plants. These stem cuttings produce rootsold old corm andcorm eventually new plants. Belowground stems also are good propagative tissues. You can divide rhizomes into pieces, remove small bulblets or rhizome rhizome tuberoustubers stem tubers cormels fromcorm the parent,corm and cut tubers into pieces containing lateral bud eyes and nodes. All of these tissues will produce new plants. internode Types of Plants and Their Stems node Trees generally have one, but occasionally several, main trunks, which usually are more than 12 feet tall when mature. old corm In contrast, shrubs generally have several main stems, which usually are less than 12 feet tall when mature. Most fruit trees, ornamental trees, and shrubs have woody tuberous stem tubers corm stems. These stems contain relatively large amounts of hardened Figure 1.8. Diversified belowground stem development. xylem tissue in the central core (heartwood or sapwood). lateral bud Herbaceous or succulent stems contain only a little xylem internode tissue and usually live for only one growing season. In peren- Rhizomes resemble stolons because they grow horizon- tally from plant to plant. Some node rhizomes are compressed and nial plants, new herbaceous stems develop from the crown fleshy (e.g., iris), while others are slender and have elongated (root–stem interface) each year. internodes (e.g., bentgrass). oldJohnsongrass corm is an insidious weed Canes are stems with relatively large pith. They usually live principally because of the spreading capability of its rhizomes. only one or two years. Tulips, lilies, daffodils, and onions produce bulbs, which are Examples of plants with canes include roses, grapes, black- corm shortened, compressed underground stems surrounded by berries, and raspberries. For fruit production, it is important fleshy scales (leaves) that envelop a central bud at the tip of the to know which canes to prune, how to prune them, and when stem. In November, you can cut a tulip or daffodil bulb in half to prune them. and see all of the flower parts in miniature. A vine is a plant with long, trailing stems. Some vines grow After a bulb-producing plant flowers, its phloem transports along the ground, while others must be supported by another food reserves from its leaves to the bulb’s scales. When the plant or structure. Twining vines circle a structure for support. bulb begins growing in the spring, it uses the stored food. For Some circle clockwise (e.g., hops and honeysuckle), while others this reason, it is important not to remove the leaves from daf- circle counterclockwise (e.g., pole beans and Dutchman’s pipe fodils, tulips, and other bulb-producing plants until after they vine). Climbing vines are supported either by aerial roots (e.g., have turned yellow and withered. At that time, these plants English ivy and poison ivy), by slender tendrils that encircle a have finished producing the food that will be used for next supporting object (e.g., cucumbers, gourds, grapes, and pas- year’s flowering. sionflowers), or by tendrils with adhesive tips (e.g., Virginia There are two types of bulbs: tunicate and nontunicate and Japanese creeper). In temperate areas both woody and (Figure 1.8). Tunicate bulbs (e.g., daffodils, tulips, and onions) herbaceous trailing plants are called vines, but in the tropics, have concentric scales, actually modified leaves. It helps protect woody trailing plants are called “lianas.” the bulb from damage during digging and from drying out once it is out of the soil. Nontunicate, or scaly, bulbs (e.g., lilies) have individual scalelike modified leaves. They are very susceptible to damage and drying out, so handle them very carefully. 1-06  Basic Botany CHAPTER 01 Stems as Food requirement, varies for different plants. Forsythia, for example, The edible portion of several cultivated plants, such as requires a relatively short rest period and grows at the first sign asparagus and kohlrabi, is an enlarged, succulent stem. The of warm weather. Many peach varieties, on the other hand, edible parts of broccoli are composed of stem tissue, flower require 700 to 1,000 hours of temperatures below 45°F. During buds, and a few small leaves. The edible tuber of a potato is a rest, dormant buds can withstand very low temperatures, but fleshy underground stem. And, although the name suggests after the rest period is satisfied, they are more susceptible to otherwise, the edible part of cauliflower actually is proliferated damage by cold temperatures or frost. stem tissue. A leaf bud is composed of a short stem with embryonic leaves. Leaf buds often are less plump and more pointed than flower buds (Figure 1.9). Buds A flower bud is composed of a short stem with embryonic A bud is an undeveloped shoot from which leaves or flower flower parts. In the case of fruit crops, flower buds sometimes parts grow. The buds of temperate-zone trees and shrubs typi- are called fruit buds. This terminology is inaccurate, however; cally develop a protective outer layer of small, leathery scales. although flowers have the potential to develop into fruits, they Annual plants and herbaceous perennials have naked buds with may not do so because of adverse weather conditions, lack of green, somewhat succulent, outer leaves. pollination, or other unfavorable circumstances. Buds of many plants require exposure to a certain number of days below a critical temperature before resuming growth Location in the spring. This period, often referred to as rest or chilling Buds are named for their location on the stem (Figure 1.10). Terminal buds are located at the apex (tip) of a stem. Lateral (axillary) buds are located on the sides of a stem and usually leaf bud arise where a leaf meets a stem (an axil). In some instances, an axil contains more than one bud. flower bud Adventitious buds arise at sites other than the terminal or axillary position. They may develop from roots, a stem inter- node, the edge of a leaf blade, or callus tissue at the cut end of a stem or root. Adventitious buds allow stem, leaf, and root cuttings to develop into entirely new plants. Buds as Food Enlarged buds or parts of buds form the edible portion of some horticultural crops. Cabbage and head lettuce are examples of unusually large terminal buds. Succulent axillary buds are the edible part of Brussels sprouts. In the case of globe artichoke, the fleshy basal portion of the flower bud’s bracts is Figure 1.9. Elm leaf and flower buds. 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 associated with the flower terminal buds, are eaten. bud Leaves Function and Structure The principal function of leaves is to absorb sunlight to man- lateral ufacture plant sugars through a process called photosynthesis. twig (axillary) Leaf surfaces are flattened to present a large area for efficient light absorption. The blade, or lamina, is the expanded thin bud structure on either side of the midrib and usually is the largest, most conspicuous part of a leaf (Figure 1.11). A leaf is held away from its stem by a stemlike appendage called a petiole, and the base of the petiole is attached to the stem at a node. Petioles vary in length or may be lacking entirely, in which case the leaf blade is described as sessile, or stalkless. Figure 1.10. Bud location. 1-07 CHAPTER 01 Basic Botany The node where a petiole meets a stem is called a leaf axil. The cuticle is part of the epidermis. It produces a waxy layer The axil contains single buds or bud clusters, referred to as called cutin, which protects the leaf from dehydration and dis- axillary buds. They may be either active or dormant; under the ease. The amount of cutin on a leaf increases with increasing right conditions, they will develop into stems or leaves. light intensity. For this reason, when moving plants from shade A leaf blade is composed of several layers (Figure 1.12). On into full sunlight, do so gradually over a period of a few weeks. the top and bottom is a layer of thick, tough cells called the epi- This gradual exposure to sunlight allows the cutin layer to build dermis. Its primary function is to protect the other layers of leaf up and protect the leaves from rapid water loss or sunscald. tissue. The arrangement of epidermal cells determines the leaf ’s The waxy cutin also repels water. For this reason, many surface texture. Some leaves, such as those of African violets, pesticides contain a spray additive to help the product adhere have hairs (pubescence), which are extensions of epidermal cells to, or penetrate, the cutin layer. that make the leaves feel like velvet. Special epidermal cells called guard 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 leaf axil with dioxide into and out of the leaf through tiny openings called axillary bud stomata. In most species, the majority of the stomata are located on the underside of leaves. Conditions that would cause plants to lose a lot of water (high temperature, low humidity) stimulate guard cells to close. In mild weather, they remain open. Guard cells also close in the blade absence of light. Located between the upper and lower epidermis is the midrib mesophyll. It is divided into a dense upper layer (palisade meso- phyll) and a lower layer that contains lots of air space (spongy mesophyll). Located within the mesophyll cells are chloroplasts, petiole where photosynthesis takes place. node Types of Leaves There are many kinds of plant leaves. The most common stem and conspicuous leaves are referred to as foliage and are the primary location of photosynthesis. However, there are many Figure 1.11. Leaf parts. (Adapted with permission from Plant other types of modified leaves: Physiology, The Benjamin/Cummings Publishing Company, Inc., 1991.) Scale leaves (cataphylls) are found on rhizomes and buds, which they enclose and protect. cuticle Seed leaves (cotyledons) are found on embryonic plants. upper They store food for the developing seedling. epidermis Spines and tendrils, such as those on barberry and pea plants, palisade protect a plant or help support its stems. layer Storage leaves, such as those on bulbous plants and succu- lents, store food. Bracts often are brightly colored. For example, the showy structures on dogwoods and poinsettias are bracts, vascular not petals. bundle Venation spongy mesophyll The vascular bundles of xylem and phloem extend from the stem, through the petiole, and into the leaf blade as veins. The term venation refers to how veins are distributed in the intercellular blade. There are two principal types of venation: parallel-veined chamber and net-veined (Figure 1.13). lower In parallel-veined leaves, numerous veins run essentially epidermis parallel to each other and are connected laterally by minute, guard cells stoma straight veinlets. Parallel-veined leaves occur most often on Figure 1.12. Leaf cross section. (Reprinted with permission from monocotyledonous plants. The most common type of parallel Plant Science: Growth, Development, and Utilization of Cultivated Plants, veining is found in plants of the grass family, whose veins run Prentice Hall, 1988.) from the leaf ’s base to its apex. 1-08  Basic Botany CHAPTER 01 In net-veined leaves (also called reticulate-veined), veins branch from the main rib or ribs and subdivide into finer veinlets. These veinlets then unite in a complicated network. This system of enmeshed veins makes the leaf more resistant to tearing than does a parallel vein structure. Net-veined leaves occur on dicotyledonous plants. Net venation may be either pinnate or palmate. In pinnate lanceolate linear cordate elliptical (featherlike) venation, the veins extend laterally from the mid- rib to the edge (e.g., apples, cherries, and peaches). In palmate venation, the principal veins extend outward, like the ribs of a fan, from the base of the leaf blade (e.g., grapes and maples). Leaves as Plant Identifiers Leaves are useful for plant identification. A leaf ’s shape, base, apex, and margin can be important lanceolate linear identifying characteristics cordate elliptical ovate (Figures 1.14–1.16). Figure 1.14. Common leaf blade shapes. Leaf type (Figure 1.17) also is important for identification. There Lanceolate—Longer than wide and tapering toward the apex and base. are two types of leaves: simple and compound. In simple leaves, Linear—Narrow, several times longer than wide and of approximately the leaf blade is a single, continuous unit. Compound leaves are the same width throughout. composed of several separate leaflets arising from the same petiole. Some leaves are doubly compound. Leaf type can be confusing Cordate (heart-shaped)—Broadly ovate, tapering to an acute apex, with the base turning in and forming a notch where the petiole is because a deeply lobed simple leaf may look like a compound leaf. attached. Leaf arrangement along a stem also is used in plant identifi- Elliptical—About two or three times as long as wide, tapering to an cation (Figure 1.18). There are four types of leaf arrangement: acute or rounded apex and base. Opposite leaves are positioned across the stem from each other, with two leaves at each node. Ovate—Egg-shaped, basal portion wide, tapering toward the apex. Alternate (spiral) leaves are arranged in alternate steps along the stem, with only one leaf at each node. Whorled leaves are arranged in circles along the stem. pinnate pinnate entire entire palmate entirecrenatecrenate crenate dentatedentate dentate serrateserrate se palmate (a) parallel-veined (b) net-veined (a) parallel-veined (b) net-veined pinnate entire entireentire crenate crenate crenate dentate dentate dentate serrate serrateserrate incised incisedincised lobed lobedlobed Figure 1.15. Common leaf margin shapes. Entire—Having a smooth edge with no teeth or notches. Crenate—Having rounded teeth. palmate 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 (b) net-veined incisions. Figure 1.13. Types of venation: (a) parallel-veined; (b) net-veined. Lobed—Having incisions that extend less than halfway to the midrib. 1-09  CHAPTER 01 Basic Botany acute acuminate obtuse cuneate obtuse cordate apex shapes base shapes opposite opposite alternate alternate whorled whor use cuneate oppositeopposite obtuse cordate alternatealternate whorledwhorled rosulaterosulate Figure 1.18. Leaf arrangement. Rosulate leaves are arranged in a rosette around a stem with base shapes extremely short nodes. Figure 1.16. Common leaf apex and base shapes. Leaves as Food Acute—Ending in an acute angle, with a sharp, but not acuminate, point. The leaf blade is the principal edible part of several horticul- tural crops, including chives, collards, endive, kale, leaf lettuce, Acuminate—Tapering to a long, narrow point. mustard, parsley, spinach, Swiss chard, and other greens. The Obtuse—Tapering to a rounded edge. edible part of leeks, onions, and Florence fennel is a cluster Cuneate—Wedge-shaped; triangular with the narrow end at the point of fleshy leaf bases. The petiole is the edible product in celery of attachment. and rhubarb. Cordate (heart-shaped)—Turning in and forming a notch. Flowers Flowers, which generally are the showiest part of a plant, have sexual reproduction as their sole function. Their beauty and fragrance have evolved not to please humans but to ensure continuance of the species. Fragrance and color attract pollina- tors (insects, birds, or other animals), which play an important role in the reproductive process. Flowers are important for plant classification. The system of plant nomenclature we use today was developed by Carl von Linné (Linnaeus) and is based on flowers and/or reproductive simple palmate compound parts of plants. pinnate compound One pinnate double reason compound his system is successful is because flowers are the plant part least influenced by environmental changes. Thus, knowledge of flowers and their parts is essential for anyone interested in plant identification. Structure As a plant’s reproductive part, a flower contains a stamen (male flower part) and/or pistil (female flower part), plus acces- sory parts such as sepals, petals, and nectar glands (Figure 1.19). The stamen is the male reproductive organ. It consists of a compound compound pinnate pinnatecompound compound double doublepinnate pinnatecompound compound pollen sac (anther) and a long supporting filament. This filament Figure 1.17. Leaf types. 1-10  Basic Botany CHAPTER 01 holds the anther in position, making the pollen available for fruit, male and female plants must be planted close enough dispersal by wind, insects, birds, or other animals. together for pollination to occur. In some instances (e.g., holly), The pistil is a plant’s female part. It generally is shaped like a the fruit is desirable. In the case of ginkgo, however, the fruit bowling pin and is located in the flower’s center. It consists of generally is not desirable due to its putrid smell when ripe. a stigma, style, and ovary. The stigma is located at the top and Kiwis are complicated because they may have one plant with is connected by the style to the ovary. The ovary contains eggs, bisexual flowers and another plant with only male flowers. The which reside in ovules. If an egg is fertilized, the ovule develops plant world isn’t all absolutes! into a seed. Sepals are small, green, leaf-like structures located at the Types of Inflorescences base of a flower. They protect the flower bud. Collectively, the Some plants bear only one flower per stem, which is called a sepals are called a calyx. solitary flower. Other plants produce an inflorescence—a cluster Petals generally are the highly colored portions of a flower. of flowers. Each flower in an inflorescence is called a floret. Like nectar glands, petals may produce fragrance. Collectively, Most inflorescences belong to one of two groups: racemes the petals are called a corolla. The number of petals on a flower and cymes. In the racemose group, the florets start blooming often is used to help identify plant families and genera. Flowers from the bottom of the stem and progress toward the top. In a of dicots typically have four or five sepals and/or petals or mul- cyme, the top floret opens first and blooms progress downward tiples thereof. In monocots, these floral parts typically come in along the stem. Detailed discussions of flower types are found threes or multiples of three. in many botany textbooks. (See “For More Information” at the end of this chapter.) Types of Flowers If a flower has a stamen, pistil, petals, and sepals, it is called How Seeds Form a complete flower (Figure 1.19). Roses are an example. If one of Pollination is the transfer of pollen from an anther to a these parts is missing, the flower is called incomplete. stigma, either by wind or by pollinators. Species pollinated The stamen and pistil are the essential parts of a flower by insects, animals, or birds often have brightly colored or and are involved in seed production. If a flower contains both patterned flowers that contain fragrance or nectar. While functional stamens and pistils, it is called a perfect flower, even searching for nectar, pollinators transfer pollen from flower to if it does not contain petals and sepals. If either stamens or flower, either on the same plant or on different plants. Plants pistils are lacking, the flower is called imperfect (Figure 1.20). evolved this ingenious mechanism in order to ensure their spe- Pistillate (female) flowers possess a functional pistil or pistils cies’ survival. Wind-pollinated flowers often lack showy floral but lack stamens. Staminate (male) flowers contain stamens parts and nectar because they don’t need to attract pollinators. but no pistils. A chemical in the stigma stimulates pollen to grow a long Plants with imperfect flowers are further classified as mon- tube down the style to the ovules inside the ovary. When pollen oecious or dioecious. Monoecious plants have perfect flowers of reaches the ovules, it releases sperm, and fertilization typically separate male and female flowers on the same plant (e.g., corn occurs. Fertilization is the union of a male sperm nucleus from and pecans). Some monoecious plants bear only male flowers a pollen grain with a female egg. If fertilization is successful, the at the beginning of the growing season but later develop both ovule develops into a seed. It is important to remember that sexes (e.g., cucumbers and squash). pollination is no guarantee that fertilization will occur. Dioecious species have separate male and female plants. Examples include holly, ginkgo, and pistachio. In order to set stigma stigma style style anther petal petal filament ovary stamen ovary pistil pistil sepal sepal ovules ovules pedicel pedicel Figure 1.19. Complete flower structure. Figure 1.20. Imperfect (pistillate) flower structure. 1-11  CHAPTER 01 Basic Botany (a) (a) simple fruits (a) simple simple fruits (a) simple fruits fruits (a) simple fruits Cross-fertilization combines genetic material from two par- hard hard stone hardhard stone stone stone encases seed encases encases encases seed seedseed ent plants. The resulting seed has a broader genetic base, which hard stone encases seed seed seed may enable the population to survive under a wider range of fleshy seed fleshy overlay fleshy overlay overlay fleshy overlay seed environmental conditions. seed fleshy overlay seeds seeds seeds apple (pome) seeds apple (pome) apple (pome) seeds peach peach (drupe) peach (drupe) (drupe) maple maple (samara, maple (samara, dry) (samara, dry) dry) Fruit apple (pome) peach (drupe) maple (samara, dry) apple (pome) peach (drupe) maple (samara, dry) Structure Fruit consists of fertilized, mature ovules (seeds) plus the ovary wall, which may be fleshy, as in a peach. pineapple pineapple The only part of the fruit that contains genes from both the pineapple berry berry berry male and female flowers is the seed. The rest of the fruit arises pineapple from the maternal plant and is genetically identical to it. pineapple berry Types of Fruit berry (b) Fruits are classified as simple, aggregate, or multiple (Figure (b) aggregate fruits (b) aggregate aggregate fruits fruits (c) (c) multiple multiple(c)fruit multiple fruit fruit 1.21). Simple fruits develop from a single flower and a single Figure 1.21. Types of fruit: (a) Simple fruits (apple, peach, and ovary. They include fleshy fruits such as cherries and peaches maple) and (b) aggregate fruits (berry and cone) and (c) multiple fruit (drupe), pears and apples (pome), and tomatoes (berries). (b) aggregate fruits (pineapple). (c) multiple fruit (b) aggregate fruits (c) multiple fruit Although generally referred to as a vegetable, tomato is techni- cally a fruit because it develops from a flower. Squash, cucum- plumule hypocotyl bers, and eggplants also develop from a single ovary and are radicle embryo classified botanically as fruits. seed coat endosperm Other types plumule of simple fruit are dry. Their wall is either papery (sheathed) plumule micropyle seed coat or leathery and hard, as opposed to the fleshy examples just hypocotyl cotyledon endosperm mentioned. Examples are peanuts (legume), poppies (capsule), perisperm hypocotyl seed coat cotyledon maples (samara), and walnuts (nut). radicle radicle An aggregate fruit develops from a single flower with many mule ovaries. (c) onion Examples are strawberries, raspberries, and blackber- cotyledons hypocotyl (a) beet (b) bean embryo ries. The flower has one corolla, one calyx, and one stem, but it radicle plumule (sheathed) has many pistils and ovaries. Each ovary is fertilized separately. micropyle seed coat If some ovules are not pollinated successfully, the fruit will cotyledon endosperm be misshapen. hypocotyl seed coat cotyledon Multiple fruits are derived from a tight cluster of separate, radicle independent flowers borne on a single structure. Each flower (c) onion has its own calyx and corolla. Pineapples and figs are examples. ) bean Figure 1.22. Parts of a seed: (a) beet; (b) bean; (c) onion. In bean, the cotyledons replace the endosperm in providing food for the germi- nating embryo. cotyledons cotyledon hypocotyl hypocotyl radicle radicle (a) germination of a bean (dicot) (b) germination of an onion (monocot) Figure 1.23. Germination of a dicot (a) and a monocot (b). 1-12  Basic Botany CHAPTER 01 light energy starch or sugar storage organ C6H12O6 (sugars, starch) sugars CO2 O2 H2O vapor photosynthesis, H2O respiration, and photorespiration sugars starch or sugar storage organ O2 CO2 H2O and minerals enter through root hairs Figure 1.24. Schematic representation of photosynthesis, respiration, leaf water exchange, and translocation of sugar (photosynthate) in a plant. (Reprinted with permission from Plant Science: Growth, Development, and Utilization of Cultivated Plants, Prentice Hall, 1988.) 1-13 CHAPTER 01 Basic Botany Seeds Because seeds are reproductive structures and thus impor- tant to a species’ survival, plants have evolved many mecha- A seed contains all of the genetic information needed to nisms to ensure seed survival. One such mechanism is seed develop into an entire plant. As shown in Figure 1.22, it is made dormancy. Dormancy comes in two forms: seed coat dormancy up of three parts: and embryo dormancy. The embryo is a miniature plant in an arrested state of devel- In seed coat dormancy, a hard seed coat does not allow water opment. It will begin to grow when conditions are favorable. to penetrate. Redbud, locust, and many other ornamental trees The endosperm (and in some species the cotyledons) is a and shrubs exhibit this type of dormancy. built-in food supply (although orchids are an exception A process called scarification is used to break or soften the where the seed contains no endosperm), which can be made seed coat. In nature, scarification is accomplished by means up of proteins, carbohydrates, or fats. such as the heat of a forest fire, digestion of the seed by a bird The seed coat, a hard outer covering, protects the seed from or mammal, or partial breakdown of the seed coat by fungi or disease and insects. It may also prevent water from entering insects. The breakdown can be done mechanically by nicking the seed and initiating germination before the proper time. the seed coat with a file or chemically by softening the seed Germination coat with sulfuric acid. In either instance, it is important to not damage the embryo. Germination is a complex process whereby a seed embryo Embryo dormancy is common in ornamental plants, includ- goes from a dormant state to an active, growing state (Figure ing elm and witch hazel. These seeds must go through a chilling 1.23). Before any visible signs of germination appear, the seed period before germinating. To break this type of dormancy, must absorb water through its seed coat. It also must have stratification is used. This process involves storing seeds in a enough oxygen and a favorable temperature. Some species, such moist medium (potting soil or paper towels) at temperatures as celery, also require light. Others require darkness. between 32°F and 50°F. The length of time required varies If these requirements are met, the radicle is the first part by species. of the seedling to emerge from the seed. It develops into the Even when environmental requirements for seed germina- primary root and grows downward in response to gravity. From tion are met and dormancy is broken, other factors also affect this primary root, root hairs and lateral roots develop. Between germination: the radicle and the first leaflike structure is the hypocotyl, which The seed’s age greatly affects its viability (ability to germi- grows upward in response to light. nate). Older seed generally is less viable than young seed, and The seed leaves, or cotyledons, encase the embryo. They if older seed does germinate, the seedlings are less vigorous usually are shaped differently than the leaves produced by the and grow more slowly. mature plant. Monocots produce one cotyledon, while dicots The seedbed must be properly prepared and made up of produce two. loose, fine-textured soil. high water vapor content low CO2 guard cell stoma stoma guard cell water vapor CO2 low water vapor content high CO2 Figure 1.25. Stomata open to allow carbon dioxide (CO2) to enter a leaf and water vapor to leave. (Reprinted with permission from Plant Physiology, The Benjamin/Cummings Publishing Company, Inc., 1991.) 1-14  Basic Botany CHAPTER 01 Seeds must be planted at the proper depth. If they are too Light shallow, they may wash away with rain or watering; if they Photosynthesis depends on the availability of light. Generally, are too deep, they won’t be able to push through the soil. as sunlight intensity increases, so does photosynthesis. Seeds must have a continual supply of moisture; however, if However, for each plant species, there is a maximum level of overwatered, they will rot. light intensity above which photosynthesis does not increase. Many weed seeds are able to germinate quickly and under Many garden crops, such as tomatoes, respond best to maxi- less-than-optimal conditions. This is one reason they make such mum sunlight. Tomato production decreases drastically as light formidable opponents in the garden. intensity drops, and few tomato varieties produce any fruit under minimal sunlight conditions. Plant Growth and Development Water Photosynthesis, respiration, and transpiration are the three Water is one of the raw materials for photosynthesis. It is major functions that drive plant growth and development taken up into the plant by the roots and moved upward through (Figure 1.24). All three are essential to a plant’s survival. How the xylem. Anything that hinders water movement in the plant, well a plant is able to regulate these functions greatly affects its such as physical injury or insect/disease damage, will impact ability to compete and reproduce. photosynthesis. Drought conditions that limit water availability may also cause stomata guard cells to close, limiting CO2 uptake Photosynthesis and slowing photosynthesis. One of the major differences between plants and animals Carbon dioxide is plants’ ability to manufacture their own food. This process Photosynthesis also requires carbon dioxide (CO2), which is called photosynthesis, which literally means “to put together enters a plant through its stomata (Figure 1.25). In most plants, with light.” To produce food, a plant requires energy from the photosynthesis fluctuates throughout the day as stomata open sun, carbon dioxide from the air, and water from the soil. The and close. Typically, they open in the morning, close down formula for photosynthesis can be written as follows: at midday, reopen in late afternoon, and shut down again in carbon dioxide + water + sunlight the evening. = sugar + oxygen Carbon dioxide is plentiful in the air, so it is not a limit- ing factor in plant growth. However, it is consumed rapidly or during photosynthesis and is replenished very slowly in the 6CO2 + 6H20 > C6H1206 + 602 atmosphere. Tightly sealed greenhouses may not allow enough outside air to enter and thus may lack adequate carbon diox- After producing carbohydrates, a plant either uses them ide for plant growth. Carbon dioxide generators are used to as energy, stores them as starch, or builds them into complex replenish or supplement CO2 in commercial greenhouses for energy compounds such as oils and proteins. All of these food crops such as roses, carnations, and tomatoes. In smaller home products are called ph