Botany Basics ❂1
lants are unique and essential to life on earth. Unlike most living things, plants make their own food from ❂ Topics in this chapter P sunlight and water. Either directly or indirectly, they are the primary food source for humans and other animals. ❂❂ Plant life cycles Additionally, they provide fuel, replenish the earth’s oxygen ❂ supply, prevent soil erosion, slow down wind movement, ❂ Internal plant parts cool the atmosphere, provide wildlife habitat, supply me- ❂❂ External plant parts dicinal compounds and beautify our surroundings. ❂❂ Plant growth and development Many plants are familiar to us, and we can identify and ❂❂ Environmental factors appreciate them based on their external structures. How- affecting growth ever, their internal structures and functions often are over- ❂❂ Plants in communities looked. Understanding how plants grow and develop helps ❂❂ Plant hormones and growth us capitalize on their usefulness and make them part of our regulators everyday lives. This chapter focuses on vascular plants. Vascular plants contain xylem and phloem, which are the water, nutrient and food conducting tissues. Ferns and seed-producing By Ann Marie VanDerZanden, Extension Master plants fall into this category. We will distinguish between Gardener State Coordinator, Oregon State monocotyledonous and dicotyledonous plants. Sometimes University. called monocots and dicots for short, these plants have im- Adapted by Jay Moore, former Land Resources portant distinguishing characteristics. For example, mono- Agent, Cooperative Extension Service, University cots (e.g., grasses) produce only one seed leaf (cotyledon) of Alaska Fairbanks.
Table 1.—Comparison between monocots and dicots. Structure Monocots Dicots Seed leaves One Two (cotyledons) 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 Ovary—The part of a female flower where Anther—The pollen sac on a male flower. eggs are located. Apex—The tip of a shoot or root. Petiole—The stalk that attaches a leaf to a Apical dominance—The tendency of an api- stem. cal bud to produce hormones that suppress Phloem—Photosynthate-conducting tissue. growth of buds below it on the stem. Photosynthate—A food product (sugar or Axil—The location where a leaf joins a stem. starch) created through photosynthesis. Cambium—A layer of growing tissue that Photosynthesis—The process in green plants separates the xylem and phloem and con- of converting carbon dioxide and water tinuously produces new xylem and phloem into food (sugars and starches) using en- cells. ergy from sunlight. Chlorophyll—The green pigment in leaves Pistil—The female flower part; consists of a that is responsible for trapping light energy stigma, style, and ovary. from the sun. Respiration—The process of converting sug- Chloroplast—A specialized component of ars and starches into energy. certain cells; contains chlorophyll and is Stamen—The male flower part; consists of an responsible for photosynthesis. anther and a supporting filament. Cortex—Cells that make up the primary tis- Stigma—The top of a female flower part; col- sue of the root and stem. lects pollen. Cotyledon—The first leaf that appears on a Stoma (pl. stomates, stomata)—Tiny open- seedling. Also called a seed leaf. ings in the epidermis that allow water, Cuticle—A relatively impermeable surface oxygen and carbon dioxide to pass into layer on the epidermis of leaves and fruits. and out of a plant. Dicot—Having two seed leaves. Style—The part of the female flower that Epidermis—The outermost layer of plant connects the stigma to the ovary. Pollen cells. travels down the style to reach the ovary, Guard cell—Epidermal cells that open and where fertilization occurs. close to let water, oxygen and carbon diox- Transpiration—The process of losing water ide pass through the stomata. (in the form of vapor) through stomata. Internode—The space between nodes on a Turgor—Cellular water pressure; responsible stem. for keeping cells firm. Meristem—Specialized groups of cells that Vascular tissue—Water-, nutrient- and pho- are a plant’s growing points. tosynthate-conducting tissue (xylem and Mesophyll—A leaf’s inner tissue, located phloem). between the upper and lower epidermis; Xylem—Water- and nutrient-conducting contains the chloroplasts and other special- tissue. ized cellular parts (organelles). Monocot—Having one seed leaf. Node—An area on a stem where a leaf, stem or flower bud is located. Chapter 1—Botany Basics • 5
while dicots (broadleaf plants) have lengths. This is one reason why spin- two. The vascular systems, flow- ach, radish and beets can be hard ers and leaves of the two types crops to grow in Alaska. These of plants also differ (Table 1). plants bolt instead of produc- These differences will be im- ing a good crop of leaves portant in our discussion of or roots. This situation can plant growth and develop- also occur when plants are ment. exposed to extreme envi- ronmental conditions such as temperature variation and Plant life cycles drought. A plant is classified as either an Perennial plants live more than 2 annual, biennial or perennial based on years and are grouped into two categories: its life cycle or how many years it takes herbaceous perennials and woody perennials. to produce flowers and seeds. Herbaceous perennials have soft, non- An annual, such as a marigold, completes woody stems that generally die back to the its life cycle in 1 year. Annuals go from ground each winter. New stems grow from seed to seed in 1 year or growing season. the plant’s crown each spring. A delphinium During this period, they germinate, grow, is an example of an herbaceous plant. Trees mature, bloom, produce seeds and die. and shrubs, on the other hand, have woody Summer annuals complete their life cycle stems that withstand cold winter tempera- during spring and summer; most winter an- tures. They are referred to as woody peren- nuals complete their growing season during nials. fall and winter. There are both winter and summer annual weeds, and understanding a weed’s life cycle is important in controlling Internal plant parts it. Of course, in most locations in Alaska Cells are the basic structural and physi- the temperature does not allow for winter ological units of plants. Most plant reac- See Chapter 4, annuals. Some plants that are winter annu- tions (cell division, photosynthesis, respira- Plant Propagation. als in warmer climates act as summer annu- tion, etc.) occur at the cellular level. Plant als in Alaska. tissues are organized groups of similar cells A biennial completes its life cycle in 2 that work together to perform a specific years. During the first season, it produces function. An example would be the xylem, vegetative structures (leaves) and food stor- which functions to move the water through age organs. The plant overwinters and then a plant or the phloem that moves food. produces flowers, fruit and seeds during its Plant cells are totipotent. In other words, second season. Swiss chard, carrots, beets, plant cells retain all of the genetic informa- sweet William and parsley are examples of tion (encoded in DNA) necessary to devel- biennials. op into a complete plant. This characteristic Biennials can sometimes go from seed is the reason vegetative (asexual) reproduc- germination to seed production in one grow- tion works. It is why it is so easy to share ing season. This phenomenon is referred houseplants with friends. For example, the to as bolting. It is a common occurrence in cells of a small leaf cutting from an African Alaska due to the midnight sun, or long day violet have the genetic information neces- 6 • Botany Basics—Chapter 1
sary to generate a root system, stems, leaves Leaf primordia and ultimately flowers. This characteristic Shoot apex also allow growers to utilize tissue culture to propagate disease-free potatoes, orchids, etc. Specialized groups of cells or tissue Leaf called meristems/meristematic tissues are a plant’s growing points. Meristems are the site of rapid, almost continuous cell divi- Bud sion. These cells either continue to divide or differentiate into tissues and organs. How they divide is controlled by a complex ar- ray of plant hormones and environmental conditions. Meristems can be manipulated Stem to change plant growth patterns, flowering, branching or vegetative growth. Vascular tissue External plant parts Soil line External plant structures such as leaves, stems, roots, flowers, fruits and seeds are Lateral root known as plant organs. Each organ is an or- Primary ganized group of tissues that work together root to perform a specific function. These struc- tures can be divided into two groups: sexual reproductive and vegetative. Sexual repro- ductive parts produce seed; they include flower buds, flowers, fruit and seeds. Veg- etative parts (Figure 1) include roots, stems, Figure 1.—Principal parts of a vascular plant. shoots, nodes, buds and leaves; they are not directly involved in sexual reproduction. Roots typically originate from the lower Vegetative parts often are used in asexual portion of a plant or cutting. They have forms of reproduction such as cuttings, bud- a root cap, but lack nodes and never bear ding or grafting. leaves or flowers directly. Their principal functions are to absorb nutrients and mois- Roots ture, anchor the plant in the soil, support the Often roots are overlooked, probably stem and store food. In some plants, roots because they are less visible than the rest of can be used for propagation. the plant. However, it’s important to under- Structure stand plant root systems because they have a pronounced effect on a plant’s size and Internally, there are three major parts of a vigor, method of propagation, adaptation to root (Figure 2): y soil types and response to cultural practices y The meristematic zone is at the tip and and irrigation. manufactures new cells; it is an area of cell division and growth. Chapter 1—Botany Basics • 7
yy Behind the meristem is the zone of elon- gation. In this area, cells increase in size Lateral through food and water absorption. As root they grow, they push the root through Primary Zone of the soil. root maturation yy The zone of maturation is directly be- Root neath the stem. Here, cells become spe- hairs cific tissues such as epidermis, cortex or vascular tissue. A root’s epidermis is its outermost layer of cells (Figure 3). These cells are respon- sible for absorbing water and minerals dissolved in water. Cortex cells are involved Zone of elongation in moving water from the epidermis to the Root tip vascular tissue (xylem and phloem) and in Meristematic zone Root cap storing food. Vascular tissue, located in the center of the root, conducts food and water. Figure 2.—Root structure. Externally, there are two areas of impor- tance: the root cap and the root hairs (Figure 2). The root cap is the root’s outermost tip. It consists of cells that are sloughed off as Epidermis the root grows through the soil. Its function Cortex Ground tissues is to protect the root meristem. Root hairs are delicate, elongated epi- Endodermis dermal cells that occur in a small zone Xylem just behind the root’s growing tip. They Vascular tissue generally resemble fine down to the naked Phloem eye. Their function is to increase the root’s surface area and absorptive capacity. Root Figure 3.—Cross section of a root. hairs usually live 1 or 2 days. When a plant is transplanted, they are easily torn off or There are two types of mycorrhizal fungi, may dry out in the sun. ectotrophic mycorrhizae and vesicular- Many roots have naturally occurring sym- arbuscular mycorrhizae. Ectotrophic typi- biotic (mutually beneficial) relationships cally form a thick sheath of fungal hyphae with certain fungi. Fungi grow inside plant around the roots with some of the fungal roots and in the surrounding soil helping to mycelium growing between cortical cells. facilitate mineral uptake. The fungi hyphae The ectotrophic fungi do not penetrate the (threadlike cells of the fungus) growing cells. The vesicular-arbuscular mycorrhizae through the soil increase the area available do not produce a fungal sheath around the for the absorption of water and nutrients. roots but grow in clusters outside the root Plants provide carbohydrates (the products and penetrate individual cells of the cortex. from photosynthesis) or food for the fungi. Some plants require mycorrhizae to survive. This beneficial association is calledmycor - Legumes and a few plants such as alder rhizae (fungus + root). produce root nodules, which contain nitro- 8 • Botany Basics—Chapter 1
How roots grow During early development, a seedling absorbs nutrients and moisture from the soil around the sprouting seed. A band of fertil- izer several inches to each side and slightly below newly planted seeds helps early growth of most row crops.
(a) Taproot (b) Fibrous root As a plant becomes established, the quan- tity and distribution of its roots strongly Figure 4.—Taproot of a carrot (a) and fibrous root of grass (b). influence its ability to absorb moisture and nutrients. For most plants, the majority of gen-fixing bacteria. These bacteria can fix the absorbing (feeder) roots are located in atmospheric nitrogen into a form that can be the top 6–12 inches of soil. In Alaska used by plants. Garden seeds such as peas, plants growing on permafrost are close beans and clover are inoculated with nitro- to the surface. The cold soils reduce gen fixing bacteria when planted. Because deep root development. See Chapter 3, inoculums are living organisms they must Soils and The following factors are important Fertilizers. be fresh and handled properly. in root growth: y Types of roots y Roots in water-saturated soil do not grow well and may die due to lack of There are two major types of roots: pri- oxygen. mary and lateral roots. yy Roots penetrate deeper in loose, well- A primary root originates at the lower drained soil than in heavy, poorly end of a seedling’s embryo (Figure 2). A drained soil. primary root is called a taproot if it continues yy Dense, compacted soil layers can re- to elongate downward, has limited secondary strict or terminate root growth. branching and becomes the central feature of yy Container plants not only have a re- the root system (Figure 4a). Dandelions and stricted area for root growth, but also are carrots have taproots. susceptible to cold damage because the A lateral, or secondary, root is a side limited amount of soil surrounding roots root that arises from another root. A fibrous reduces insulation. root system is formed if the primary root yy In addition to growing downward, roots ceases to elongate and produces numerous grow laterally and often extend well lateral roots (Figure 4b). These lateral roots beyond a plant’s drip line. Keep this branch repeatedly to form the network of extensive root system in mind when feeding roots found on most plants. Some disturbing the soil around existing trees plants, such as grasses, naturally produce a and shrubs. fibrous root system. In other cases, sever- ing a plant’s taproot by undercutting it can Roots as food encourage the plant to produce a fibrous Several vegetable crops are enlarged root system. Nurseries use this technique roots. Sweet potatoes are swollen tuberous with trees that naturally produce a taproot roots; and carrots, beets, rutabaga, parsnips, because trees with a compact fibrous root turnips and radishes are elongated taproots. system transplant more successfully. Chapter 1—Botany Basics • 9
Stems Phloem Phloem Cambium Stems, support buds and leaves and serve Xylem as conduits for carrying water, minerals and food (photosynthates). Stems place the leaves in favorable positions for exposure to Xylem light. The vascular system inside the stem Pith forms a continuous pathway from the root, (a) Monocot (b) Dicot through the stem, and finally to the leaves. It is through this system that water and food Figure 5.—Cross sections of stems: (a) Discontinuous vascular products move. system of a monocot stem; (b) continuous vascular system of a woody dicot stem. Structure used to remove grass from around the trunk Vascular system of trees. Stem boring insects are attracted to The vascular system consists of xylem, cuts made by weed trimmers. phloem and vascular cambium. It can be The vascular systems of monocots and thought of as a plant’s plumbing. Xylem dicots differ (Figure 5). Although both con- tubes conduct water and dissolved miner- tain xylem and phloem, these structures are als; phloem tubes carry food such as sugars. arranged differently in each. In a monocot, The cambium is a layer of meristematic the xylem and phloem are paired in bundles tissue that separates the xylem and phloem dispersed throughout the stem. The vascular and continuously produces new xylem and system in dicots is said to be continuous phloem cells. This new tissue is responsible because it forms rings inside the stem. The for a stem’s increase in girth. phloem forms the outer ring and eventu- The vascular cambium is important to ally becomes part of the bark in mature gardeners in several ways. Xylem and phloem woody stems. The xylem forms the inner tissues on a grafted scion (top wood) and ring. Sapwood is newer xylem that is still rootstock need to line up, if the graft is to conducting and heartwood is older xylem take. Careless weed trimming can strip the that is crushed in the center of the stem and bark off a tree, injuring the cambium and is no longer conducting. Heartwood is filled girdling the tree and causing it to die. This with waste materials such as gums, resins, often occurs when string weed trimmers are tannins and oils. The difference in the vascular systems of Stem terminology monocots and dicots is of practical inter- Shoot—A young stem (1 year old or est to gardeners because some herbicides less) with leaves. affect only one group. For example, 2, 4-D Twig—A young stem (1 year old or less) kills only plants with a continuous vascular that is in the dormant winter stage system (dicots). Nonselective herbicides, on (has no leaves). the other hand (e.g., glyphosate), kill plants Branch—A stem that is more than regardless of their type of vascular system. 1 year old, typically with lateral Nodes stems radiating from it. A node is an area on a stem where buds Trunk—A woody plant’s main stem. are located (Figure 6). It is a site of great 10 • Botany Basics—Chapter 1
Bud yy Plant growth regulator substances and herbicides also can influence internode length. Types of stems Stems may be long with great distances Node between nodes (e.g., branches of trees, Internode runners on strawberries) or compressed, with short distances between nodes (e.g., crowns of strawberry plants, fruit spurs, and African violets). Although stems commonly grow above ground, they sometimes grow below ground in the form of rhizomes, Figure 6.—Stem structure. tubers, corms or bulbs. All stems must have buds or leaves. cellular activity and growth where small Specialized aboveground stems buds develop into leaves, stems or flowers. Some plants have specialized above- When pruning, it is important to locate a ground stems known as crowns, spurs or plant’s nodes. Generally, you want to cut stolons (Figure 7). Crowns (on strawber- See Chapter 5, Pruning. just above, but not too close to, a node. ries, dandelions and African violets) are Pruning in this manner encourages the buds compressed stems with leaves and flowers at that node to begin development and ulti- on short internodes. Spurs are short, stubby mately form new stems or leaves. side stems that arise from a main stem. The area between nodes is called the They are the fruit-bearing stems on pear, internode. Its length depends on many fac- apple and cherry trees. If severe pruning is tors, including genetics. Other factors also done too close to fruit-bearing spurs, they can influence internode length: can revert to nonfruiting stems, thus elimi- yy Reduced soil fertility decreases inter- nating the year’s potential fruit crop. node length, while an application of Stolons are fleshy or semiwoody, elongat- high-nitrogen fertilizer can increase it. ed, horizontal stems that often lie along the yy Lack of light increases internode length soil surface. Strawberry runners are stolons causing spindly stems. This situation that have small leaves at the nodes. Roots is known as stretch, or etiolation, and develop from these nodes, and a daughter often occurs in seedlings started indoors plant is formed. This type of vegetative and in houseplants under low light con- reproduction is an easy way to increase the ditions size of a strawberry patch. Spider plants yy Internode length varies seasonally. also produce stolons, which ultimately can Early-season growth has long inter- become entirely new plants. nodes; late-season growth has shorter Specialized belowground stems internodes. Potato tubers, iris rhizomes and tulip yy Plant energy divided among three or bulbs are underground stems that store four side stems, or diverted into fruit food for the plant (Figure 8). It sometimes growth and development, shortens inter- is difficult to distinguish between roots node length. Chapter 1—Botany Basics • 11
Figure 7.—Diversified aboveground stem development. and stems, but one sure way is to look for begins growing in the spring, it utilizes the nodes. Stems have nodes; roots do not. stored food. For this reason, it is important In potato tubers, the “eyes” are the stem’s not to remove the leaves from lilies and nodes, and each eye contains a cluster of other bulb producing plants until after they buds. When growing potatoes from seed have turned yellow or had sufficient time to pieces, it is important that each piece con- grow. In late summer and early fall, these tain at least one eye and be at least 2 ounces plants store food for next year’s flowering. or 2½ inches in diameter, so there will be There are two types of bulbs: tunicate and enough energy for growth of new plant. nontunicate (Figure 8). Tunicate bulbs (e.g., Rhizomes resemble stolons because they tulips and onions) have a thin, papery cov- grow horizontally from plant to plant. Some ering, which actually is a modified leaf. It rhizomes are compressed and fleshy (e.g., helps protect the bulb from damage during iris), while others are slender and have digging and from drying out once it is out elongated internodes (e.g., bluegrass). Some of the soil. Nontunicate bulbs (e.g., lilies) grasses are insidious weeds principally do not have this papery covering. They are because of the spreading capability of their very susceptible to damage and drying out, rhizomes. and must be handled carefully. Tulips, lilies and onions produce bulbs, Corms are another kind of belowground which are shortened, compressed under- stem. Although both bulbs and corms are ground stems surrounded by fleshy scales composed of stem tissue, they are not the (leaves) that envelop a central bud at the same. Corms are shaped like bulbs, but do tip of the stem. In late August you can cut not contain fleshy scales. A corm is a solid, a tulip or lily bulb in half and see all of the swollen stem with dry, scale-like leaves. flower parts in miniature. Gladiolus and crocuses produce corms. After a bulb-producing plant flowers, its Some plants (e.g., tuberous begonias and phloem transports food reserves from the cyclamen) produce a modified underground leaves to the bulb’s scales. When the bulb stem called a tuberous stem. These stems 12 • Botany Basics—Chapter 1
Stem Tuberous stem
Tunicate bulb Nontunicate bulb Lateral bud
Tubers Corm Rhizome
Figure 8.—Diversified below-ground stem development.
are short, flat and enlarged. Buds and shoots Types of plants and their stems arise from the top (crown), and fibrous roots Trees generally have one main trunk, grow from the bottom. usually more than 12 feet tall when mature. Other plants (e.g., dahlias and sweet In contrast, shrubs have several main stems, potatoes) produce underground storage or- less than 12 feet tall when mature. gans called tuberous roots, which often are Most fruit trees, ornamental trees and confused with bulbs and tubers. These have shrubs have woody stems. These stems con- neither nodes nor internodes. tain relatively large amounts of hardened xylem tissue (heartwood) in the central Stems and propagation core. The sapwood is the light colored liv- Stems often are used for vegetative plant ing xylem near the outside just under the propagation. Using sections of aboveground bark of a tree. stems that contain nodes and internodes is Herbaceous or succulent stems contain See Chapter 4, an effective way to propagate many orna- only a little xylem tissue and usually live Plant Propagation. mental plants. Aboveground stem cuttings for only one growing season. In perennial produce roots and, eventually, new plants. plants, new herbaceous stems develop from Belowground stems also are good propa- the crown (root-stem interface) each year. gative tissues. You can divide rhizomes into Canes are stems with relatively large pith pieces; remove small bulblets or cormels or central strength-giving tissues. from their parent; and cut tubers into pieces They usually live only 1 or 2 years. containing eyes and nodes. All of these tis- Examples of plants with canes sues will produce new plants. See Chapter 13, include roses, blackberries and Berry Crops. raspberries. For fruit production, it Chapter 1—Botany Basics • 13
is important to know which canes to prune, Leaf bud how to prune them, and when to prune Flower bud them. For example, two-year old raspberry canes are pruned at the ground. One-year old canes are left to produce the following season. A vine is a plant with long, trailing stems. Some vines grow along the ground, while others must be supported by another plant or structure. Twining vines circle a structure for support. Some circle clockwise (e.g., hops and honeysuckle), while others circle counterclockwise (e.g., pole beans). Climb- ing vines are supported either by aerial Figure 9.—Elm leaf and flower buds. roots (e.g., English ivy and poison ivy), by slender tendrils that encircle a supporting on the other hand, require 700 to 1,000 object (e.g., cucumbers, gourds, grapes and hours of temperatures below 45°F. During passionflowers) or by tendrils with adhesive rest, dormant buds can withstand very low tips (e.g., Virginia and Japanese creeper). temperatures, but after the rest period is satisfied, they are more susceptible to dam- Stems as food age by cold temperatures or frost. The rest The edible tuber of a potato and the period protects the plant from coming out of Jerusalem artichoke are both fleshy under- dormancy too soon and being damaged by ground stems. Asparagus and kohlrabi are early spring frost. enlarged, succulent stems. A ginger “root” A leaf bud is composed of a short stem is actually a rhizome or underground stem. with embryonic leaves. Leaf buds often are Sugarcane and bamboo shoots are stems. less plump and more pointed than flower buds (Figure 9). Buds Terminal bud A flower bud is composed A bud is an undeveloped shoot from of a short stem with embry- which leaves or flowers grow. The buds of onic flower parts. In fruit crops, temperate-zone trees and shrubs typically flower buds sometimes are called develop a protective outer layer of small, fruit buds. This terminology is leathery scales. Annual plants and herba- inaccurate, however; although ceous perennials have naked buds with flowers have the potential to green succulent outer leaves. Twig develop into fruits, they may not Lateral Buds of many plants require exposure do so because of adverse weather (axillary) to a certain number of days below a criti- conditions, lack of pollination, or bud cal temperature before resuming growth in other unfavorable circumstances. the spring. This period, often referred to as the rest period, varies for different plants. Location Forsythia, for example, requires a relatively Buds are named for their loca- short rest period and grows at the first sign tion on the stem (Figure 10). of warm weather. Many peach varieties, Terminal buds are located at the Figure 10.—Bud location. 14 • Botany Basics—Chapter 1
apex (tip) of a stem. Lateral (axillary) buds
are located on the sides of a stem and arise Leaf axil with in the axil where a leaf meets a stem. In axillary bud some instances, an axil contains more than one bud. Adventitious buds arise at sites other than the terminal or axillary position. They may Blade develop from roots, a stem internode, the edge of a leaf blade or callus tissue at the Midrib cut end of a stem or root. Adventitious buds allow stem, leaf and root cuttings to devel- Petiole op into entirely new plants. Node Buds as food Stem Enlarged buds or parts of buds form the Figure 11.—Leaf parts. edible portion of some horticultural crops. Cabbage and head lettuce are examples of side of the midrib and usually is the largest, unusually large terminal buds. Succulent most conspicuous part of a leaf (Figure 11). axillary buds are the edible part of Brus- A leaf is held away from its stem by a sels sprouts. In the case of globe artichoke, stem-like appendage called a petiole, and the fleshy basal portion of the flower bud’s the base of the petiole is attached to the bracts is eaten, along with its solid stem. stem at a node. Petioles vary in length or Broccoli and cauliflower are important may be lacking entirely, in which case the horticultural plants with edible flower buds. leaf blade is described as sessile or stalk- In this case, portions of the stem associated less. A celery stalk is a leaf petiole. with the flower buds are eaten. The node where a petiole meets a stem is called a leaf axil. The axil contains single Leaves buds or bud clusters, referred to as axillary Function and struc- ture Cuticle Upper The principal func- epidermis tion of leaves is to Palisade absorb sunlight need- layer ed to manufacture Vascular plant sugars through bundle a process called photosynthesis. Leaf Spongy mesophyll surfaces are flattened to present a large Lower epidermis area for efficient light absorption. The blade Stoma Intercellular chamber is the expanded thin Guard cells structure on either Figure 12.—Leaf cross section. Chapter 1—Botany Basics • 15
buds. They may be either active or dormant. mild weather, they remain open. Guard cells Under the right conditions, they will de- also close in the absence of light. velop into stems or leaves. Located between the upper and lower A leaf blade is composed of several lay- epidermis is the mesophyll. It is divided into ers (Figure 12). On the top and bottom is a a dense upper layer (palisade mesophyll) layer of thick, tough cells called the epider- and a lower layer that contains lots of air mis. Its primary function is to protect the space (spongy mesophyll). Located within other layers of leaf tissue. The arrangement the mesophyll cells are chloroplasts, where of epidermal cells determines the leaf’s photosynthesis takes place. surface texture. Some leaves, such as those Types of leaves of African violets, have hairs (pubescence), which are extensions of epidermal cells that There are many kinds of leaves. The make the leaves feel like velvet. most common and conspicuous leaves are The cuticle is part of the epidermis. It referred to as foliage and are the primary produces a waxy layer called cutin, which location of photosynthesis. However, there protects the leaf from dehydration and dis- are many other types of modified leaves: y ease. The amount of cutin on a leaf increas- y Scale leaves (cataphylls) are found on es with increasing light intensity. For this rhizomes and buds, which they enclose reason, when moving plants from shade into and protect. y full sunlight, do so gradually over a period y Seed leaves (cotyledons) are found on of a few weeks. This gradual exposure to embryonic plants. They store food for sunlight is referred to as hardening off. It al- the developing seedling. y lows the cutin layer to build up and protect y Spines and tendrils, such as those on the leaves from rapid water loss or sun- barberry and pea plants, protect a plant scald. Transplants produced indoors should or help support its stems. y be hardened off before planting outdoors. y Storage leaves, such as those on bul- The waxy cutin also repels water. For bous plants and succulents, store food. y this reason, many pesticides contain a spray y Bracts often are brightly colored modi- additive (sticker, spreader, etc.) to help the fied leaves. The showy structures on product adhere to, or penetrate, the cutin dogwoods and poinsettias are bracts, not layer. petals. Special epidermal cells called guard cells Venation open and close in response to environmen- The vascular bundles of xylem and tal stimuli such as changes in weather and phloem extend from the stem, through the light. They regulate the passage of water, petiole and into the leaf blade as veins. oxygen and carbon dioxide into and out of The term venation refers to how veins are the leaf through tiny openings called sto- distributed in the blade. There are two prin- mata. In most species, the majority of the cipal types of venation: parallel-veined and stomata are located on the undersides of net-veined (Figure 13). leaves. In parallel-veined leaves, numerous veins Conditions that would cause plants to run parallel to each other and are con- lose a lot of water (high temperature, low nected laterally by minute, straight veinlets. humidity) stimulate guard cells to close. In Parallel-veined leaves occur most often on 16 • Botany Basics—Chapter 1
(a) Parallel-veined (b) Net-veined Figure 13.—Types of venation: (a) Parallel-veined; (b) net-veined.
monocotyledonous plants. The most com- leaves, the leaf blade is a single, continu- mon type of parallel veining is found in ous unit. Compound leaves are composed plants of the grass family, whose veins run of several separate leaflets arising from the from the leaf’s base to its apex. Another same petiole. Some leaves are doubly com- type of parallel venation is found in plants pound. Leaf type can be confusing because such as banana, calla and pickerelweed. a deeply lobed simple leaf may look like a Their veins run laterally from the midrib. 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 veinlets. There are four types of leaf arrangement: These veinlets then unite in a complicated yy Opposite leaves are positioned across network. This system of enmeshed veins the stem from each other, with two makes the leaf more resistant to tearing than leaves at each node. does a parallel vein structure. Net-veined yy Alternate (spiral) leaves are arranged in leaves occur on dicotyledonous plants. alternate steps along the stem, with only Net venation may be either pinnate or one leaf at each node. palmate. In pinnate venation, the veins yy Whorled leaves are arranged in circles extend laterally from the midrib to the along the stem. edge (e.g., apples, cherries and peaches). In yy Rosulate leaves are arranged in a rosette palmate venation, the principal veins extend around a stem with extremely short outward, like the ribs of a fan, from the base nodes. of the leaf blade (e.g., grapes and maples). Leaves as food Leaves as plant identifiers The leaf blade is the edible part of several Leaves are useful for plant identifica- horticultural crops, including chives, col- tion. A leaf’s shape, base, apex and margin lards, endive, kale, leaf lettuce, mustard, can be important identifying characteristics parsley, spinach, Swiss chard and other (Figures 14–16). greens. The edible part of leeks, onions, and Leaf type (Figure 17) also is important Florence fennel is a cluster of fleshy leaf for identification. There are two types of bases. The petiole is the edible product in leaves: simple and compound. In simple celery and rhubarb. Chapter 1—Botany Basics • 17
Leaves and plant identification (blade and margin forms)
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 Lanceolate Linear Cordate ovate, tapering to an acute apex, 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, Elliptical Ovate tapering toward the apex. 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 Entire Crenate Dentate sharp, deep, irregular teeth or inci- sions. Lobed—Having incisions that extend less than halfway to the midrib.
Serrate Incised Lobed
Figure 15.—Common leaf margin shapes. 18 • Botany Basics—Chapter 1
Leaves and plant identification (apex and base shapes)
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.
Leaf types and arrangement (Figure 17-18)
Simple Palmate compound Pinnate compound Double pinnate compound Figure 17.—Leaf types.
Opposite Alternate Whorled Rosulate
Figure 18.—Leaf arrangement. Chapter 1—Botany Basics • 19
Flowers holds the anther in position, making the pollen available for dispersement by wind, Flowers, which are the insects, or birds. showiest part of a plant, have The pistil is a plant’s female part. It sexual reproduction as their See Chapter 2, generally is shaped like a bowling pin and Plant sole function. Their beauty and Identification. is located in the flower’s center. It consists fragrance have evolved not to of a stigma, style and ovary. The stigma is please humans but to ensure continuance located at the top and is connected by the of the species. Fragrance and color attract style to the ovary. The ovary contains eggs, pollinators (insects or birds) that play an which reside in ovules. If an egg is fertil- important role in the reproductive process. ized, the ovule develops into a seed. Flowers are important for plant classifica- Sepals are small, green, leaf-like struc- tion. The system of plant nomenclature we tures located at the base of a flower. They use today was developed by Carl von Linné protect the flower bud. Collectively, the (Linnaeus) and is based on flowers and/or sepals are called a calyx. reproductive parts of plants. One reason his Petals are the highly colored portions of system is successful is because flowers are a flower. Like nectar glands, petals may the plant part least influenced by environ- contain perfume. Collectively, the petals are mental changes. Thus, knowledge of flow- called a corolla. The number of petals on a ers and their parts is essential for anyone flower is used to help identify plant families interested in plant identification. and genera. Flowers of dicots typically have Structure four or five sepals and/or petals, or multiples Flowers contain a stamen (male flower thereof. In monocots, these floral parts typi- part) and/or pistil (female flower part), plus cally come in threes or multiples of three. accessory parts such as sepals, petals and Types of flowers nectar glands (Figure 19). If a flower has a stamen, pistil, petals and The stamen is the male reproductive or- sepals, it is called a complete flower (Figure gan. It consists of a pollen sac (anther) and 19). Roses are an example. If one of these a long supporting filament. This filament parts is missing, the flower is called incomplete. Petal The stamen and pistil are the essential parts of a flower and are involved in seed pro- Stigma duction. If a flower contains Anther Style both functional stamens and Pistil Stamen pistils, it is called a perfect Ovary Filament flower, even if it does not contain petals and sepals. If either stamens or pistils are Sepal lacking, the flower is called Pedicel imperfect (Figure 20). Pistil- Figure 19.—Complete flower structure. (Reprinted by late (female) flowers possess permission from Plant Science: Growth, Development, and a functional pistil or pistils, Utilization of Cultivated Plants, Prentice Hall, 1988.) 20 • Botany Basics—Chapter 1
Most inflorescences belong to one of two groups: racemes and cymes. In the race- Petal Anther mose group, the florets start blooming from the bottom of the stem and progress toward Stamen the top. In a cyme, the top floret opens first Filament and blooms progress downward along the stem. Detailed discussions of flower types are found in many botany textbooks. (See Sepal “For more information” at the end of this chapter.) Pedicel How seeds form Figure 20.—Imperfect (staminate) flower structure. Pollination is the transfer of pollen from an anther to a stigma, either by wind or but lack stamens. Staminate (male) flowers pollinators. Species pollinated by insects, contain stamens, but no pistils. animals or birds often have brightly colored Plants with imperfect flowers are fur- or patterned flowers that contain fragrance ther classified as monoecious or dioecious. or nectar. While searching for nectar, polli- Monoecious plants have separate male and nators transfer pollen from flower to flower, female flowers on the same plant (e.g., corn either on the same plant or on different and pecans). Monoecious is sometimes re- plants. Plants evolved this ingenious mech- ferred to as one house for both sexes. Some anism to ensure species survival. monoecious plants bear only male flowers In Alaska, cool-wet conditions can reduce at the beginning of the growing season, but the flight of pollinators. This can be the later develop both sexes (e.g., cucumbers cause of a small blueberry crop, which de- and squash). pends on insect pollination. Zucchini flow- Dioecious species have separate male and ers, which are normally pollinated by bees, female plants. Examples include kiwi, gink- may have to be hand pollinated in cool wet gos and cottonwood. In order to set fruit, summers. Wind-pollinated flowers often male and female plants must be planted lack showy floral parts and nectar because close enough together for pollination to they don’t need to attract pollinators. occur. In some instances the fruit is desir- A chemical in the stigma stimulates able. In the case of ginkgos, the fruit is not pollen to grow a long tube down the style desirable due to its putrid smell when ripe. to the ovules inside the ovary. The stigma Kiwis are complicated because they may recognizes pollen, which has a chemical have one plant with bisexual flowers and specific to that type of plant. The wrong another plant with only male flowers. The pollen will not germinate because it lacks plant world doesn’t always have absolutes! the chemical signature. Types of inflorescences When pollen reaches the ovules, it releas- Some plants bear only one flower per es sperm and fertilization occurs. Fertiliza- stem, which is called a solitary flower. Oth- tion is the union of a male sperm nucleus er plants produce an inflorescence, a cluster from a pollen grain with a female egg. If of flowers. Each flower in an inflorescence fertilization is successful, the ovule devel- is called a floret. ops into a seed. Pollination does not guar- Chapter 1—Botany Basics • 21
antee that fertilization will occur. Moisture Other types of simple fruit are dry. Their and temperature stresses can cause a lack wall is either papery or leathery and hard, of fertilization. High temperature in green- as opposed to the fleshy examples just house settings can reduce fruit set in toma- mentioned. Examples are peanuts (legume), toes and cucumbers. poppies (capsule), maples (samara) and Cross-fertilization combines genetic walnuts (nut). material from two similar parent plants from An aggregate fruit develops from a single the same species. The resulting seed has a flower with many ovaries. Examples are broader genetic base, which may enable the strawberries, raspberries and blackber- population to survive under a wider range of ries. The flower is a simple flower with environmental conditions. Cross-pollinated one corolla, one calyx and one stem, but plants usually are more successful than self- it has many pistils or ovaries. Each ovary pollinated plants. Consequently, more plants is fertilized separately. If some ovules are reproduce by cross than by self-pollination. not pollinated successfully, the fruit will be misshapen. Fruit Multiple fruits are derived from a tight cluster of separate, independent flowers Structure borne on a single structure. Each flower has Fruit consists of fertilized, mature ovules its own calyx and corolla. Pineapples and (seeds) plus the ovary wall. Fruit may be figs are examples. fleshy, as in an apple, or dry and hard, as in an acorn. In some fruits, the seeds are Fruit as food enclosed within the ovary (e.g., apples, Some foods that we call vegetables are peaches, oranges, squash and cucumbers). actually fruit. A ripened, mature ovary that In others, seeds are situated on the outside contains seeds is a fruit. Vegetable can also of fruit tissue (e.g., corn and strawberries). be a human dietary term for some fruit. The only part of the fruit that contains So, botanically speaking, tomatoes, pep- genes from both the male and female flow- pers, cucumbers, squash, green beans and ers is the seed. The rest of the fruit arises eggplants are fruit. Other important fruits from the maternal plant and is genetically include apple, pear, peach, orange, lemon, identical to it. lime, banana, pineapple, strawberry, vanilla, coconut and date. Types of fruit Fruits are classified as simple, aggre- Seeds gate or multiple (Figure 21). Simple fruits A seed contains all of the genetic infor- develop from a single ovary. They include mation needed to develop into an entire fleshy fruits such as cherries and peaches plant. It is made up of three parts (Figure (drupe), pears and apples (pome) and toma- 22). The embryo is a miniature plant in an toes (berries). Although generally referred arrested state of development. It begins to to as a vegetable, tomatoes technically are grow when conditions are favorable. The a fruit because they develop from a flower. endosperm or cotyledon is the seed’s built- Squash, cucumbers and eggplants also de- in food supply and is made up of proteins, velop from a single ovary and are classified carbohydrates or fats (orchids are an excep- botanically as fruits. 22 • Botany Basics—Chapter 1
(a) Simple fruits
Hard stone enclosing 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).
tion). The third part is the seed coat, a hard and grows downward in response to gravity. outer covering that protects the seed from From this primary root, root hairs and lateral disease and insects. It also prevents water roots develop. Between the radicle and the from entering the seed and initiating germi- first leaf-like structure is thehypocotyl , which nation before the proper time. grows upward in response to light. The seed leaves, or cotyledons, encase Seed germination the embryo. They usually are shaped differ- Germination is a complex process where- ently than the leaves the mature plant will by a seed embryo goes from a dormant produce. Monocots produce one cotyledon, state to an active, growing state (Figure 23). while dicots produce two. Before any visible signs of germination ap- Seeds are reproductive structures, im- pear, the seed must absorb water through its portant to survival, and have evolved many seed coat. It also must have enough oxygen mechanisms to ensure their survival. One and a favorable temperature. Some species, such mechanism is seed dormancy. Dor- such as celery, also require light. Others mancy comes in two forms: seed coat dor- require darkness. mancy and embryo dormancy. If these requirements are met, the radicle In seed coat dormancy, a hard seed coat is the first part of the seedling to emerge from does not allow water to penetrate. Redbud, the seed. It develops into the primary root locust and many other ornamental trees and Chapter 1—Botany Basics • 23
Plumule Embryo Hypocotyl Radicle Seed coat Endosperm Plumule Micropyle Seed coat Plumule (sheathed) Hypocotyl Cotyledon Endosperm Perisperm Hypocotyl Seed coat Radicle Cotyledon Radicle 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).
shrubs exhibit this type of dormancy. between 32° and 50°F. The length of time A process called scarification is used to required varies by species. break or soften the seed coat. In nature, Even when environmental requirements scarification is accomplished by means such for seed germination are met and dormancy is as the heat of a forest fire, digestion of the broken, other factors also affect germination: seed by a bird or mammal or partial break- yy Age affects seed viability (ability to ger- down of the seed coat by fungi or insects. minate). Older seed is less viable than It can be done mechanically by nicking young seed, and if it does germinate, the seed coat with a file, or chemically by the seedlings are less vigorous and grow softening the seed coat with sulfuric acid. In more slowly. either instance, it is important to not dam- yy The seedbed must be properly prepared age the embryo. and made up of loose, fine-textured soil. Embryo dormancy is common in ornamen- yy Seeds must have a continual supply of See Chapter 4, tal plants, including lupine and apple. These moisture; however, if over-watered, Plant Propagation. seeds must go through a chilling period they rot. before germinating. To break this type of yy Seeds must be planted at the proper dormancy, stratification is used. This process depth and the right temperature. involves storing seeds in a moist medium The temperature required for germination (potting soil or paper towels) at temperatures varies by species. Generally, cool-season 24 • Botany Basics—Chapter 1
crops (e.g., spinach, radishes and lettuce) germinate best at 55° to Light energy 65°F, while warm-season crops (e.g., tomatoes, petunias and squash) germinate best at 65° to 75°F. Starch or sugar storage organ Many weed seeds are able to Photosynthesis, respiration, and germinate quickly and under less C6H12O6 photorespiration (sugars than optimal conditions. This is starch) Sugars one reason they make such for- CO2 midable opponents in the garden. O2 H20 vapor Plant growth and H20
development Sugars Photosynthesis, respiration and transpiration are the three major physiological functions Starch or sugar that drive plant growth and storage development (Figure 24). All organ three are essential to a plant’s survival. How well a plant is
able to regulate these functions Respiration, ▼
affects its ability to compete and no photorespiration O2 reproduce. H20 and CO2 minerals enter through Photosynthesis root hairs One of the major differences between plants and animals is Figure 24.—Schematic representation of photosynthesis, plants’ ability to manufacture respiration, leaf water exchange and translocation of sugar (photosynthate) in a plant. their own food. This process is called photosynthesis, which lit- The formula for photosynthesis can be erally means “to put together with light.” To written as follows: produce food, a plant requires energy from Carbon dioxide + Water + Sunlight = the sun, carbon dioxide from the air and wa- Sugar + Oxygen ter transported from the soil through the xy- or lem. During photosynthesis, it splits carbon 6CO2 + 6H20 = C6H1206 + 602 dioxide into carbon and oxygen, adds water The produced carbohydrates will be and forms carbohydrates (starches and used, stored or built into complex energy sugars). Carbohydrates are used by the cells compounds such as oils and proteins. All or transported through the phloem to other of these food products are called photosyn- parts of the plant. Oxygen is a by-product thates. The plant uses stored products when that exits the plant through the stomata. Chapter 1—Botany Basics • 25
light is limited, or transports them to its roots or developing fruits. Photosynthesis occurs only in the me- sophyll layers of plant leaves and, in some instances, in mesophyll cells in the stem. Mesophyll cells are sandwiched between the leaf’s upper and lower epidermis (Fig- High water Low ure 12) and contain numerous chloroplasts, vapor content CO2 where photosynthesis takes place. Chloro- plasts are incredibly small. One square mil- limeter, about the size of a period on a page, would contain 400,000 chloroplasts. Stoma Guard cell Guard Chlorophyll, the pigment that makes Stoma CO2 Water vapor cell leaves green, is found in the chloroplasts. It is responsible for trapping light energy from Low water vapor content High CO2 the sun. Chloroplasts are arranged per- Figure 25.—Stomata open to allow carbon dioxide (CO2) to enter a pendicular to incoming sun rays to absorb leaf and water vapor to leave. maximum sunlight. Photosynthesis stops if any of the ingre- dients for photosynthesis — light, water and when given 10 to 12 hours of suitable light. carbon dioxide — are lacking. If any factor In the Arctic, photosynthesis can potentially is absent for a long period of time, a plant occur for 18 to 20 hours a day. will die. Two of these factors are described Carbon dioxide below. Photosynthesis requires carbon dioxide
Light (CO2), which enters a plant through its Photosynthesis depends on the quantity, stomata (Figure 25). Photosynthesis fluctu- quality and duration of light. In general, as ates throughout the day as stomata open and intensity increases, so does photosynthesis. close. Typically, they open in the morning, However, for each plant species, there is close down at midday, reopen in late after- a maximum level of light intensity above noon and shut down again in the evening. which photosynthesis does not increase. Carbon dioxide is plentiful in the air, so Many garden crops, such as tomatoes, it is not a limiting factor in plant growth. respond best to maximum sunlight. Tomato However, it is consumed rapidly during production decreases drastically as light in- photosynthesis and is replenished very tensity drops, and only a few tomato variet- slowly in the atmosphere. Tightly sealed ies produce any fruit under minimal sunlight greenhouses with poor ventilation may lack conditions. Plants do not use all the spectrum adequate carbon dioxide for plant growth. or wavelengths of light for photosynthesis. Carbon dioxide generators are used to
Photosynthesis occurs predominately in the generate CO2 in commercial greenhouses orange-red region and in the blue-violet re- for high cash crops such as roses, carnations gion of the spectrum. This corresponds with and tomatoes. In smaller home greenhouses, the wavelength of light that the chlorophylls ventilation is the best method for insuring absorb. As a general rule, plants do best that adequate CO2 is present. 26 • Botany Basics—Chapter 1
Respiration count for only 1 percent of a leaf’s surface but 90 percent of the water transpired. Carbohydrates made during photosynthe- Transpiration is a necessary process and sis are of value to a plant when converted to uses about 90 percent of the water that energy. This energy is used for cell growth enters a plant’s roots. The other 10 percent and building new tissues. The chemical is used in chemical reactions and in plant process by which sugars and starches are tissues. Water moving via the transpiration converted to energy is called oxidation. It stream is responsible for several things: is similar to the burning of wood or coal yy Transporting minerals from the soil to produce heat. Controlled oxidation in a throughout the plant living cell is called respiration and is shown yy Cooling the plant through evaporation by this equation: yy Moving sugars and plant chemicals
C6H12O6 + 6O2 = 6CO2 + 6H2O + Energy yy Maintaining cell firmness This equation is essentially the oppo- The amount and rate of water loss de- site of photosynthesis. Photosynthesis is pends on factors such as temperature, a building process, while respiration is a humidity and wind or air movement. Tran- breaking-down process (Table 2). Unlike spiration is greatest in hot, dry (low relative photosynthesis, respiration does not depend humidity), windy weather. on light, so it occurs at night as well as during the day. Respiration occurs in all life A balancing act forms and in all cells. In order for a plant to grow and develop properly, it must balance photosynthesis, Transpiration respiration and transpiration. Left to their When a leaf’s guard cells shrink, the own devices, plants do a good job of man- stomata open and water vapor is lost. This aging this intricate balance. If a plant photo- process is called transpiration. Evaporating synthesizes at a high rate, but its respiration water causes a negative water pressure in rate is not high enough to break down the the plant and more water is pulled up from photosynthates produced, photosynthesis the roots. Dissolved nutrients are pulled in will either slow down or stop. On the other with the water from the roots. The rate of hand, if respiration is much more rapid transpiration is directly related to whether than photosynthesis, the plant won’t have stomata are open or closed. Stomata ac- adequate photosynthates to produce energy for growth. Hence, growth either will slow down or stop altogether. When stomata are open, transpiration Table 2.—Photosynthesis and respiration. occurs, sometimes at a very high rate. A Photosynthesis Respiration corn plant may transpire 50 gallons of water • Produces food. • Uses food. per season, but a large tree may move 100 • Stores energy. • Releases energy. gallons per day! Plants have problems if • Uses water. • Produces water. • Uses carbon • Produces carbon they lose too much water, so stomata close dioxide. dioxide. during hot, dry periods when transpiration • Releases oxygen. • Uses oxygen. • Occurs in sunlight. • Occurs in dark as is highest. However, CO2, needed for photo- well as in light. synthesis, also enters the plant through open Chapter 1—Botany Basics • 27
stomata. Thus, if stomata stay closed a long Quantity time, not enough CO2 will enter for photo- Light quantity is the intensity, or con- synthesis. As a result, photosynthesis and centration, of sunlight. It varies with the See Chapter 19, Diagnosing respiration will slow down, reducing plant seasons. The maximum amount of light is Plant Problems. growth. present in summer, and the minimum in Many herb plants produce high-energy oils winter. Up to a point, the more sunlight a to help them survive in the dry landscapes plant receives, the greater its capacity for where they evolved. Oils help plants survive producing food via photosynthesis. extended periods of stomatal closure. Light quantity can be manipulated to achieve different plant growth patterns. Environmental factors affecting Increase light by surrounding plants with reflective materials, a white background or growth supplemental lights. Decrease it by shad- The environment affects plant growth and ing plants with cheesecloth or woven shade geographic distribution. If any environmen- cloth. tal factor is less than ideal, it limits a plant’s growth and/or distribution. For example, only plants adapted to limited amounts of water can live in deserts. Either directly or indirectly, most plant Short-day (long-night) plants problems are caused by environmental Long, continuous Short, continuous Interrupted stress. In some cases, poor environmental dark period dark period dark period conditions (e.g., too little water) damage a plant directly. In other cases, environmental stress weakens a plant and makes it more susceptible to disease or insect attack. Environmental factors that affect plant growth include light, temperature, water, humidity and nutrition. It is important to 24 hours 24 hours 24 hours understand how these factors affect plant growth and development. With a basic Long-day (short-night) plants understanding of these factors, a plant can Long, continuous Short, continuous Interrupted dark period be manipulated to meet your needs for in- dark period dark period creased leaf, flower or fruit production. By recognizing the roles of these factors, you can better diagnose plant problems caused by environmental stress. Light Three principal characteristics of light af- 24 hours 24 hours 24 hours fect plant growth: quantity, quality and dura- Figure 26.—Periodicity of plants. Short-day (long-night) plants tion (how much, what type and how long). require a long period of uninterrupted darkness to flower. Long- day (short-night) plants require a short period of uninterrupted darkness to flower. 28 • Botany Basics—Chapter 1
Quality response to the duration of light or dark- Light quality refers to the color (wave- ness. Short-day plants form flowers only length) of light. Sunlight supplies the when day length is less than about 12 hours. complete range of wavelengths and can be Many spring and fall-flowering plants, such broken up by a prism into bands of red, or- as chrysanthemums, poinsettias and Christ- ange, yellow, green, blue, indigo and violet. mas cactus, are in this category. Blue and red light is absorbed by plants In contrast, long-day plants form flow- and has the greatest effect on growth. Blue ers only when day length exceeds 12 hours. light is responsible for vegetative (leaf) Most summer-flowering plants (e.g., rud- growth. Red light, when combined with beckia, California poppies and asters), as blue light, encourages flowering. Plants well as many vegetables (beets, radishes, look green because they reflect, rather than lettuce, spinach and potatoes), are in this absorb, green light. This means that plants category. Alaska summers are full of long- do not use green light for growth. day conditions. Knowing which light source to use is im- Day-neutral plants form flowers regard- portant for manipulating plant growth. For less of day length. Examples are tomatoes, example, fluorescent (cool white) light is corn, cucumbers and some strawberry culti- high in the blue wavelength. It encourages vars. Some plants do not fit into any cat- leafy growth and is excellent for starting egory, but may respond to combinations of seedlings. Incandescent light is high in the day lengths. Petunias, for example, flower red or orange range, but generally produces regardless of day length, they but flower too much heat to be a valuable light source earlier and more profusely with long days. for plants. Fluorescent grow lights attempt You can easily manipulate photoperiod to to imitate sunlight with a mixture of red and stimulate flowering. For example, chrysan- blue wavelengths, but they are costly and themums normally flower in the short days generally no better than regular fluorescent of spring or fall, but bloom in midsummer lights. when covered with a cloth that completely blocks out light for 12 hours each day. After Duration several weeks of this treatment, the artificial Duration, or photoperiod, refers to the dark period is no longer needed, and the amount of time a plant is exposed to light. plants will bloom as if it were spring or fall. Photoperiod controls flowering in many This method also is used to make poinset- plants (Figure 26). Scientists initially tias flower in time for Christmas. thought the length of light period triggered To bring a long-day plant into flower flowering and other responses within plants. when day length is less than 12 hours, ex- Thus, they describe plants as short-day or pose the plant to supplemental light. After a long-day, depending on what conditions few weeks, flower buds will form. they flower under. We now know that it is not the length of the light period, but rather Temperature the length of uninterrupted darkness, that is Temperature is a fundamental factor critical to floral development. that affects the rate of most biological Plants are classified into three categories: and chemical reactions. It influences most short-day (long-night), long-day (short- plant processes, including photosynthesis, night) or day-neutral, depending on their transpiration, respiration, germination and Chapter 1—Botany Basics • 29
flowering. As temperature increases (up to nighttime temperature. Under these condi- a point), photosynthesis, transpiration and tions, plants photosynthesize (build up) respiration increase. When combined with and respire (break down) during optimum day length, temperature also affects the daytime temperatures and then curtail respi- change from vegetative (leafy) to reproduc- ration at night. However, not all plants grow tive (flowering) growth. Depending on the best under the same nighttime and daytime situation and the specific plant, the effect temperatures. For example, snapdragons of temperature can either speed up or slow grow best at nighttime temperatures of down this transition. High temperatures can 55°F, poinsettias at 62°F. damage fruit production by killing the pol- Photosynthesis occurs at its highest rate len that is developing in pollinated flowers. between 65° and 85°F and decreases at Sometimes horticulturists use temperature higher or lower temperatures. Temperatures in combination with day length to manipu- higher than needed increase respiration, late flowering. For example, a Christmas sometimes above the rate of photosynthesis. cactus forms flowers as a result of short Thus, photosynthates are used faster than days and low temperatures (Figure 26). To they are produced. For growth to occur, pho- encourage a Christmas cactus to bloom, tosynthesis must be greater than respiration. place it in a room with more than 12 hours Daytime temperatures that are too low of- of darkness each day and a temperature of ten produce poor growth by slowing down 50° to 55°F until flower buds form. photosynthesis. The result is reduced yield If temperatures are high and days are (e.g., fruit or grain production). long, cool-season crops such as spinach will Breaking dormancy flower (bolt). However, if temperatures are too cool, fruit will not set on warm-season Some plants that grow in cold regions crops such as tomatoes. need a certain number of days of low tem- Low temperatures reduce energy use perature (dormancy). Knowing the period and increase sugar storage. Thus, leaving of low temperature required by a plant is es- crops such as ripe winter squash on the sential in getting it to grow to its potential. vine during cool, fall nights increases their Peaches are a prime example: most va- sweetness. This is one reason vegetables in rieties require 700 to 1,000 hours between Alaska are so sweet. Potatoes in cool stor- 32° and 45°F before breaking their rest age also become sweet over time and need period and beginning growth. Lilies need 6 to be brought up to room temperature in weeks of temperatures at or slightly below order for the sugars to be converted back to 33°F before blooming. starch. Daffodils can be forced to flower by stor- Adverse temperatures can cause stunted ing the bulbs at 35° to 40°F in October. The growth and poor-quality vegetables. For cold temperature allows the bulbs to ma- example, high temperatures cause bitter let- ture. When transferred to a greenhouse in tuce and cucumber. midwinter, they begin to grow, and flowers are ready to cut in 3 to 4 weeks. Photosynthesis and respiration Hardiness Thermoperiod refers to daily temperature change. Plants grow best when daytime Plants are classified as hardy or nonhardy temperature is about 10° to 15° higher than depending on their ability to withstand cold 30 • Botany Basics—Chapter 1
temperatures. Hardy plants are those that needles then turn brown. To minimize the are adapted to the cold temperatures of their risk of this type of injury, it is important that growing environment. plants, especially trees, go into the winter Woody plants in the temperate zone have well watered. very sophisticated means for sensing the progression from fall to winter. Decreas- Water and humidity ing day length and temperatures trigger Most growing plants contain about 90 hormonal changes that cause leaves to stop percent water. Water is taken up into the photosynthesizing and to ship nutrients to plant by roots and moved upward through twigs, buds, stems and roots. An abscission the xylem. Water plays many roles in plants. layer forms where each petiole joins a stem, It is: and the leaves eventually fall off. Changes yy a primary component in photosynthesis within the trunk and stem tissues over a and respiration. relatively short period of time “freeze- yy responsible for turgor pressure in cells proof” the plant. (Like air in an inflated balloon, water is Fall days are longer in Alaska than in the responsible for the fullness and firm- lower 48 states. Because of this, plants may ness of plant tissue. Turgor is needed not get the short day triggers necessary to to maintain cell shape and ensure cell harden off for winter. Consequently, plants growth.). that would normally withstand cold tem- yy a solvent for moving minerals and car- perature are not winter hardy in Alaska. bohydrates through the plant. Winter injury to hardy plants may oc- yy responsible for cooling leaves as it cur when temperatures drop too quickly evaporates from leaf tissue during tran- in the fall before a plant has progressed to spiration. full dormancy. In other cases, a plant may yy a regulator of stomatal opening and break dormancy in mid- or late winter if the closing, thus controlling transpiration weather is unseasonably warm. If a sudden, and photosynthesis. severe cold snap follows the warm spell, yy the source of pressure to move roots otherwise hardy plants can be seriously through the soil. damaged. yy the medium in which most biochemical It is worth noting that the tops of hardy reactions takes place. plants are much more cold tolerant than Relative humidity is the ratio of water the roots. Plants that normally are hardy to vapor in the air to the amount of water the 10°F may be killed if they are in containers air could hold at the current temperature and the roots are exposed to 20°F. and pressure. Warm air can hold more water Winter injury also may occur because of vapor than cold air. Relative humidity (RH) desiccation (drying out) of plant tissues. is expressed by the following equation: People often forget that plants need water RH = Water in air ÷ Water air could hold even during winter. In the spring, when the (at constant temperature and pressure) soil is frozen, water movement into a plant is severely restricted. These conditions are Relative humidity is given as a percent. For common in Alaska. On a windy winter day, example, if a pound of air at 75°F could broadleaf evergreens can become water- hold 4 grams of water vapor and there are deficient in a few minutes, and the leaves or Chapter 1—Botany Basics • 31
only 3 grams of water in the air, then the before a plant can use a chemical element in (RH) is: a fertilizer. 3 ÷ 4 = 0.75 = 75% Plants need 17 elements for normal growth. Three of them — carbon, hydrogen See Chapter 3, Water vapor moves from an area of high and oxygen — are found in air and water. Soils and relative humidity to one of low relative hu- Fertilizers. The rest are found in the soil. See Chapter 8, midity. The greater the difference in humid- Three soil elements are called primary Vegetable ity, the faster water moves. This factor is nutrients because they are used in relatively Gardening. important because the rate of water move- large amounts by plants. They are nitro- ment directly affects a plant’s transpiration gen, phosphorus and potassium. Calcium, rate. magnesium and sulfur are called secondary The relative humidity in the air spaces nutrients because they are used in moder- between leaf cells approaches 100 percent. ate amounts. Often, primary and secondary When a stoma opens, water vapor inside nutrients are collectively called macronutri- the leaf rushes out into the surrounding air ents (Table 3). (Figure 25), and a bubble of high humidity Seven other soil elements are used in forms around the stoma. By saturating this much smaller amounts and are called micro- small area of air, the bubble reduces the dif- nutrients or trace elements (Table 4). They ference in relative humidity between the air are iron, boron, zinc, copper, manganese, spaces within the leaf and the air adjacent molybdenum and chlorine. to the leaf. As a result, transpiration slows Most of the nutrients a plant needs are down. How does this affect nutrient uptake? dissolved in water and then absorbed by If wind blows the humidity bubble away, its roots. In fact, 98 percent are absorbed however, transpiration increases. Thus, tran- from the soil-water solution, and only about spiration usually is at its peak on hot, dry, 2 percent are actually extracted from soil windy days. On the other hand, transpira- particles. tion generally is quite slow when tempera- tures are cool, humidity is high and there is Fertilizers no wind. Fertilizers are materials containing plant Hot, dry conditions generally occur dur- nutrients that are added to the environment ing the summer, which partially explains around a plant. Generally, fertilizers are why plants wilt quickly in the summer. If a added to the water or soil, but some can be constant supply of water is not available to sprayed on leaves. This method is called fo- be absorbed by the roots and moved to the liar fertilization. It should be done carefully leaves, turgor pressure is lost and leaves go with a dilute solution, because limp. a high fertilizer concentration can injure leaf cells. The nutri- Plant nutrition ent, however, does need to pass Plant nutrition often is confused with fer- through the thin layer of wax tilization. Plant nutrition refers to a plant’s (cutin) on the leaf surface. need for and use of basic chemical ele- Fertilizers are not plant food! ments. Fertilization is the term used when Plants produce their own food these materials are added to the environ- from water, carbon dioxide and ment around a plant. A lot must happen solar energy through photosyn- 32 • 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 NO3 . dark green color; yellowing (chlorosis). NH4 :NO3 ratio is 1:1. + NH4 Mobile in plants. weak, spindly Reds and purples Under low-light condi- + (ammonium) growth; few fruits. may intensify in tions , high NH4 can May cause brittle some plants. 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 or Fe. 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. Chapter 1—Botany Basics • 33
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 defi- ciency 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. 34 • Botany Basics—Chapter 1
thesis. This food (sugars and carbohydrates) available light and moisture. is combined with plant nutrients to produce As trees and shrubs grow and mature, proteins, enzymes, vitamins and other ele- you may need to manipulate them, either ments essential to growth. by removing those that have outgrown their space or by selective pruning and thinning. Nutrient absorption Often, understory plants that did well when Anything that reduces or stops sugar pro- the landscape was young must be replaced duction in leaves can lower nutrient absorp- with plants that are more shade-tolerant. tion. If a plant is under stress because of This process is a kind of plant succession, low light or extreme temperatures, nutrient dictated by the changing light and moisture deficiency may develop. environment and carried out by the owner. A plant’s developmental stage or rate of A lawn also is a changing landscape. It growth also may affect the amount of nutri- starts out as a mix of several adapted grass ents absorbed. Many plants have a rest (dor- species on bare ground. Weeds sprout from mant) period during part of the year. Dur- seed reserves in the soil. Additional seeds ing this time, few nutrients are absorbed. and plants move in and grow if conditions Plants also may absorb different nutrients as are right. Broadleaf weeds also may find flower buds begin to develop than they do niches. Moss begins to take over where the during periods of rapid vegetative growth. lawn is thin, a common problem in semi- shaded areas. These changes are another Plants in communities example of plant succession. To manage invasive plants, keep your The preceding discussion focused on lawn grasses competitive by using proper the structure and physiology of individual nutrition and cultural practices (mowing, plants. Interactions among plants also are thatch, aerating etc.) and by periodically important for gardeners. The study of these overseeding. In spite of your best efforts, interactions is called plant or landscape however, plant succession may occur. ecology. Gardeners who plant wildflower mixtures In ornamental gardens, we generally aim often discover that there is much more vari- to develop a stable community of plants ety in flowers the first year than in succeed- that complement each other in form, color, ing years. Some species do very well, and leaf characteristics and bloom. We must pay others simply cannot compete. Again, plant attention to the differing requirements of succession occurs. plants within this community. The most short-term as- A garden’s framework semblage of plants in a garden often is defined by large See Chapter 15, occurs in annual vegetable Sustainable shrubs or trees, which cast and flower beds. Here there is Landscape differing amounts of sum- Design. no attempt to create a commu- mer shade and winter struc- See Chapter 14, nity that will last more than Lawns. tures over the course of the one season. See Chapter 20, year. When choosing plants Weed Since many of the most Management. to grow under or near large competitive weeds thrive in framework specimens, be recently disturbed soil, it is sure their needs match the a challenge to give desired Chapter 1—Botany Basics • 35
annual crop plants an advantage. The plant ships are quite complex, and many are not that captures light first will grow and sup- well understood. They are the subject of ac- press plants beneath it. Early weed competi- tive research and offer much to think about tion can have a devastating impact on crop for thoughtful gardeners. growth. Consistent weeding, mulching, and the use of transplants improve the odds for annual vegetable and flower crops. Plant hormones and growth Another type of relationship between regulators plants is called allelopathy. In this phenom- Plant hormones and growth regulators enon, some plants produce compounds in are chemicals that affect flowering; ag- their leaves, roots or both that inhibit the ing; root growth; distortion and killing of growth of other plants. Black walnut is the leaves, stems and other plant parts; preven- most notorious example. Its roots can sup- tion or promotion of stem elongation; color press many common vegetable plants, and enhancement of fruit; prevention of leaf- its leaves, if mulched on a vegetable garden ing and/or leaf fall; and many other condi- over the winter, can affect many annual tions (Table 5). Very small concentrations crops like an herbicide the following spring. of these substances produce major growth Some of the worst weeds show allelopathic changes. traits and prevent desired ornamental or Plants produce hormones naturally, while vegetable species from growing. humans apply plant growth regulators to Finally, there are relationships between plants. Plant growth regulators may be syn- plants that involve pollinators, animals, thetic compounds (e.g., IBA and Cycocel) birds, pests, predators and even nutrient that mimic naturally occurring plant hor- transport between species through symbiotic mones, or they may be natural hormones that fungi called mycorrhizae. These relation- 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., chrysan- themums, poinsettias and lilies) Herbicides (2,4-D, etc.) Distort plant growth; selective and nonselective materials used for killing unwanted plants 36 • Botany Basics—Chapter 1
Applied concentrations of these substanc- plant aging and death (senescence). es usually are measured in parts per million Ethylene is unique in that it is found (ppm) and in some cases parts per billion only in the gaseous form. It induces ripen- (ppb). These growth-regulating substances ing, causes leaves to droop (epinasty) and most often are applied as a spray to foliage drop (abscission) and promotes senescence. or as a liquid drench to soil around a plant’s Plants often increase ethylene production base. Generally, their effects are short lived, in response to stress, and ethylene often is and they may need to be reapplied in order found in high concentrations within cells at to achieve the desired effect. the end of a plant’s life. The increased eth- There are five groups of plant growth- ylene in leaf tissue in the fall is part of the regulating compounds: auxin, gibberellin reason leaves fall off of trees. Ethylene also (GA), cytokinin, ethylene and abscisic acid is used to ripen fruit (e.g., green bananas). (ABA). For the most part, each group con- Abscisic acid (ABA) is a general plant- tains both naturally occurring hormones and growth inhibitor. It induces dormancy; synthetic substances. prevents seeds from germinating; causes Auxin causes several responses in plants: abscission of leaves, fruits and flowers; and yy Bending toward a light source (photot- causes stomata to close. High concentra- ropism) tions of ABA in guard cells during periods yy Downward root growth in response to of drought stress probably play a role in gravity (geotropism) stomatal closure. yy Promotion of apical dominance yy Flower formation yy Fruit set and growth For more information yy Formation of adventitious roots Capon, B. 2010. Botany for Gardeners: An Auxin is the active ingredient in most Introduction and Guide, 3rd edition. rooting compounds in which cuttings are Portland, Oregon: Timber Press. dipped during vegetative propagation. Salisbury, F.B., and C.W. Ross. 1992. Gibberellins stimulate cell division and Plant Physiology, 4th edition. Belmont, elongation, break seed dormancy and speed California:Wadsworth Publishing Com- germination. The seeds of some species are pany. difficult to germinate; you can soak them in Williams Rice, L. and R.P. Rice, Jr. 2010. a GA solution to get them started. Practical Horticulture, 6th edition. Up- Unlike other hormones, cytokinins are per Saddle River, New Jersey: Prentice found in both plants and animals. They Hall. stimulate cell division and often are in- cluded 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