Botany Basics ❂1 lants are essential to life on earth. Either directly or indirectly, they are the primary food source for humans and other animals. Additionally, they P ❂ OPICS IN THIS CHAPTER provide fuel, replenish the earth’s oxygen supply, prevent T soil erosion, slow down wind movement, cool the atmo- sphere, provide wildlife habitat, supply medicinal com- ■ Plant life cycles pounds, and beautify our surroundings. ■ Internal plant parts Many plants are familiar to us, and we can identify and ■ External plant parts appreciate them based on their external structures. However, their internal structures and functions often are ■ Plant growth and overlooked. Understanding how plants grow and develop development helps us capitalize on their usefulness and make them ■ Environmental factors part of our everyday lives. affecting growth This chapter focuses on vascular plants—those that contain water-, nutrient-, and food-conducting tissues ■ Plants in communities called xylem and phloem. Ferns and seed-producing ■ Plant hormones and plants fall into this category. growth regulators In several cases, we will distinguish between monocoty- ledonous and dicotyledonous plants. Sometimes called monocots and dicots for short, these plants have several important distinguishing characteristics. For example, monocots (e.g., grasses) produce only one seed leaf, while dicots (broadleaf plants) have two. The vascular systems, flowers, and leaves of the two types of plants By Ann Marie VanDerZanden, Extension also differ (Table 1). These differences will be important master gardener state coordinator, Oregon in our discussion of plant growth and development. State University. Table 1.—Comparison between monocots and dicots. Structure Monocots Dicots Seed leaves one two Vascular system Xylem and phloem are paired in bundles, Xylem and phloem form rings inside the stem. which are dispersed throughout the stem. The phloem forms an outer ring, the xylem an inner ring. Floral parts Usually in threes or multiples of three. Usually in multiples of four or five. Leaves Often parallel-veined. Generally net-veined. 4 • Botany Basics—Chapter 1 Botany terminology Monocot—Having one seed leaf. Anther—The pollen sac on a male flower. Node—An area on a stem where a leaf, Apex—The tip of a shoot or root. stem, or flower bud is located. Apical dominance—The tendency of an Ovary—The part of a female flower apical bud to produce hormones that where eggs are located. suppress growth of buds below it on Petiole—The stalk that attaches a leaf to the stem. a stem. Axil—The location where a leaf joins a Phloem—Photosynthate-conducting stem. tissue. Cambium—A layer of growing tissue that Photosynthate—A food product (sugar separates the xylem and phloem and or starch) created through photo- continuously produces new xylem and synthesis. phloem cells. Photosynthesis—The process in green Chlorophyll—The green pigment in plants of converting carbon dioxide leaves that is responsible for trapping and water into food (sugars and light energy from the sun. starches) using energy from sunlight. Chloroplast—A specialized component of Pistil—The female flower part; consists of certain cells; contains chlorophyll and a stigma, style, and ovary. is responsible for photosynthesis. Respiration—The process of converting Cortex—Cells that make up the primary sugars and starches into energy. tissue of the root and stem. Stamen—The male flower part; consists Cotyledon—The first leaf that appears on of an anther and a supporting filament. a seedling. Also called a seed leaf. Stigma—The top of a female flower part; Cuticle—A relatively impermeable collects pollen. surface layer on the epidermis of leaves Stoma (pl. stomates, stomata)—Tiny and fruits. openings in the epidermis that allow Dicot—Having two seed leaves. water, oxygen, and carbon dioxide to Epidermis—The outermost layer of plant pass into and out of a plant. cells. Style—The part of the female flower that Guard cell—Epidermal cells that open connects the stigma to the ovary. and close to let water, oxygen, and Pollen travels down the style to reach carbon dioxide pass through the the ovary, where fertilization occurs. stomata. Transpiration—The process of losing Internode—The space between nodes on water (in the form of vapor) through a stem. stomata. Meristem—Specialized groups of cells Turgor—Cellular water pressure; that are a plant’s growing points. responsible for keeping cells firm. Mesophyll—A leaf’s inner tissue, located Vascular tissue—Water-, nutrient-, and between the upper and lower photosynthate-conducting tissue epidermis; contains the chloroplasts (xylem and phloem). and other specialized cellular parts Xylem—Water- and nutrient-conducting (organelles). tissue. Chapter 1—Botany Basics • 5 Plant life cycles grow from the plant’s crown each spring. Trees and shrubs, on the Based on its life cycle, a other hand, have woody stems plant is classified as either that withstand cold winter an annual, biennial, or temperatures. They are perennial. referred to as woody perenni- An annual, such as a als. zinnia, completes its life cycle in 1 year. Annuals are said to go Internal plant parts from seed to seed in 1 year Cells are the basic structural or growing season. During and physiological units of plants. this period, they germinate, Most plant reactions (cell division, grow, mature, bloom, produce photosynthesis, respiration, etc.) occur at seeds, and die. Summer annuals the cellular level. Plant tissues (meristems, complete their life cycle during spring and xylem, phloem, etc.) are large, organized summer; most winter annuals complete groups of similar cells that work together their growing season during fall and to perform a specific function. winter. There are both winter and summer A unique feature of plant cells is that annual weeds, and understanding a they are readily totipotent. In other words, weed’s life cycle is important in control- almost all plant cells retain all of the ling it. See Chapter 3, genetic information (encoded in DNA) ✿ A biennial requires all or part of 2 years Propagation. necessary to develop into a complete to complete its life cycle. During the first plant. This characteristic is the main season, it produces vegetative structures reason that vegetative (asexual) reproduc- (leaves) and food storage organs. The tion works. For example, the cells of a plant overwinters and then produces small leaf cutting from an African violet flowers, fruit, and seeds during its second have all of the genetic information neces- season. Swiss chard, carrots, beets, sweet sary to generate a root system, stems, William, and parsley are examples of more leaves, and ultimately flowers. biennials. Specialized groups of cells called Sometimes biennials go from seed meristems are a plant’s growing points. germination to seed production in only Meristems are the site of rapid, almost one growing season. This situation occurs continuous cell division. These cells when extreme environmental conditions, either continue to divide or begin to such as drought or temperature variation, differentiate into other tissues and organs. cause the plant to pass rapidly through How they divide, and whether they ulti- the equivalent of two growing seasons. mately become a tissue or an organ, is This phenomenon is referred to as bolting. controlled by a complex array of internal Sometimes bolting occurs when biennial plant hormones but also can be influ- plant starts are exposed to a cold spell enced by environmental conditions. In before being planted in the garden. many cases, you can manipulate mer- Perennial plants live more than 2 years istems to make a plant do something you and are grouped into two categories: want, such as change its growth pattern, herbaceous perennials and woody peren- flower, alter its branching habit, or pro- nials. Herbaceous perennials have soft, duce vegetative growth. nonwoody stems that generally die back to the ground each winter. New stems 6 • Botany Basics—Chapter 1 External plant parts vegetative. Sexual reproductive parts produce seed; they include flower buds, External plant structures such as flowers, fruit, and seeds. Vegetative parts leaves, stems, roots, flowers, fruits, and (Figure 1) include roots, stems, shoot seeds are known as plant organs. Each buds, and leaves; they are not directly organ is an organized group of tissues that involved in sexual reproduction. Vegeta- work together to perform a specific tive parts often are used in asexual forms function. These structures can be divided of reproduction such as cuttings, budding, into two groups: sexual reproductive and or grafting. Leaf primordia Roots Shoot apex Often roots are overlooked, probably because they are less visible than the rest of the plant. However, it’s important to understand plant root systems because Leaf they have a pronounced effect on a plant’s size and vigor, method of propagation, adaptation to soil types, and response to cultural practices and irrigation. Bud Roots typically originate from the lower portion of a plant or cutting. They have a root cap, but lack nodes and never bear leaves or flowers directly. Their principal functions are to absorb nutrients and moisture, anchor the plant in the soil, Stem support the stem, and store food. In some plants, they can be used for propagation. Structure Internally, there are three major parts of Soil Vascular a root (Figure 2): line tissue • The meristematic zone is at the tip and manufactures new cells; it is an area of cell division and growth. • Behind the meristem is the zone of elongation. In this area, cells increase in size through food and water absorp- Lateral root tion. As they grow, they push the root Primary through the soil. root • The zone of maturation is directly beneath the stem. Here, cells become specific tissues such as epidermis, cortex, or vascular tissue. A root’s epidermis is its outermost layer of cells (Figure 3). These cells are respon- sible for absorbing water and minerals Figure 1.—Principal parts of a vascular plant.