Campbell Biology - Vascular Plant Structure, Growth, & Development (2020) PDF

35 KEY CONCEPTS Vascular Plant Structure, Growth, and Development 35.1 Plants have a hierarchical organization consisting of organs, tissues, and cells p. 759 35.2 Different meristems generate new...

35 KEY CONCEPTS Vascular Plant Structure, Growth, and Development 35.1 Plants have a hierarchical organization consisting of organs, tissues, and cells p. 759 35.2 Different meristems generate new cells for primary and secondary growth p. 766 35.3 Primary growth lengthens roots and shoots p. 768 35.4 Secondary growth increases the diameter of stems and roots in woody plants p. 772 35.5 Growth, morphogenesis, and cell differentiation produce the plant body p. 775 Figure 35.1 There is beauty to behold at every level of plant organization: Every cell, every tissue, and every organ has a function, and the structure of each has Study Tip been molded by natural selection. Make a table: To help keep track of what different plant cells do, make the following table: How does structure fit function Type of How structure in vascular plants? plant cell What it does fits function At the organ level At the cellular level Leaves provide Photosynthetic cells surface area for are packed with chloroplasts absorbing sunlight and At the tissue level that convert exchanging gases. Dermal Vascular sunlight into tissue tissue chemical protects provides energy. organs. support and transports resources. Chloroplasts Go to Mastering Biology Stems support and Tube-shaped For Students (in eText and Study Area) elevate leaves, cells transport Get Ready for Chapter 35 maximizing resources. The cell BioFlix® Animation: Tour of a Plant Cell photosynthesis. shown here Figure 35.19 Walkthrough: Secondary carries water and minerals. Others Growth of a Woody Stem conduct sugars. For Instructors to Assign (in Item Library) Tutorial: Visualizing Primary and Leaf cross section Secondary Growth Cells with Activity: Primary and Secondary Growth Ground tissue root hairs includes cells that carry near the tips out photosynthesis and of roots store sugars. increase the surface area for absorbing Roots anchor the plant and water and absorb water and minerals. minerals. 758 Chapters 29 and 30 provided an overview of plant diversity,. Figure 35.2 An overview of a flowering plant. The plant including both nonvascular and vascular plants. In this chap- body is divided into a root system and a shoot system, connected by vascular tissue (purple strands in this diagram) that is continuous ter and throughout Unit Six, we’ll focus on vascular plants, throughout the plant. The plant shown is an idealized eudicot. especially angiosperms because flowering plants are the primary producers in many terrestrial ecosystems and are of Reproductive shoot (flower) great agricultural importance. This chapter mainly explores Apical bud nonreproductive growth—roots, stems, and leaves—and Node focuses primarily on the two main groups of angiosperms: Internode eudicots and monocots (see Figure 30.16). Later, in Chapter 38, we’ll examine angiosperm reproductive growth: flowers, Apical seeds, and fruits. bud Shoot system Vegetative CONCEPT 35.1 shoot Blade Plants have a hierarchical Leaf Petiole organization consisting of Axillary bud organs, tissues, and cells Stem Plants, like most animals, are composed of cells, tissues, Taproot and organs. A cell is the fundamental unit of life. A tissue is a group of cells consisting of one or more cell types that together perform a specialized function. An organ consists of Lateral Root several types of tissues that together carry out particular func- (branch) system tions. As you learn about plant structure, keep in mind how roots natural selection has produced plant forms that fit plant func- tion at all levels of structure. We begin by discussing plant organs because their structures are most familiar. Vascular Plant Organs: embryo, is the first root (and the first organ) to emerge from Roots, Stems, and Leaves a germinating seed. It soon branches to form lateral roots EVOLUTION The basic morphology, or shape, of vascular (see Figure 35.2) that can also branch, greatly enhancing the plants reflects their evolutionary history as terrestrial organ- ability of the root system to anchor the plant and to acquire isms that inhabit and draw resources from two very different resources such as water and minerals from the soil. environments—below the ground and above the ground. Tall, erect plants with large shoot masses generally have They must absorb water and minerals from below the ground a taproot system, consisting of one main vertical root, the surface and CO2 and light from above the ground surface. The taproot, which usually develops from the primary root. In ability to acquire these resources efficiently is traceable to the taproot systems, the role of absorption is restricted largely evolution of roots, stems, and leaves as the three basic organs. to the tips of lateral roots. A taproot, although energetically These organs form a root system and a shoot system, the expensive to make, facilitates the anchorage of the plant in latter consisting of stems and leaves (Figure 35.2). Vascular the soil. By preventing toppling, the taproot enables the plant plants, with few exceptions, rely on both systems for survival. to grow taller, thereby giving it access to more favorable light Roots are almost never photosynthetic; they starve unless conditions and, in some cases, providing an advantage for photosynthates, the sugars and the other carbohydrates pro- pollen and seed dispersal. Taproots can also be specialized for duced during photosynthesis, are imported from the shoot food storage. system. Conversely, the shoot system depends on the water Small vascular plants or those that have a trailing growth and minerals that roots absorb from the soil. habit are particularly susceptible to grazing animals that can uproot the plant and kill it. Such plants are most effi- Roots ciently anchored by a fibrous root system, a thick mat of slen- A root is an organ that anchors a vascular plant in the soil, der roots spreading out below the soil surface (see Figure absorbs minerals and water, and often stores carbohydrates 30.16). In plants that have fibrous root systems, including and other reserves. The primary root, originating in the seed most monocots, the primary root dies early on and does not CHAPTER 35 Vascular Plant Structure, Growth, and Development 759 c Figure 35.3 Root hairs of a radish seedling. Root hairs grow by the thousands just behind the tip of each root. By increasing the root’s surface area, they greatly enhance the absorption of water and minerals from the soil. Mastering Biology Video: Root Growth in a Radish Seedling m Prop roots. The aerial, adventitious roots of maize (corn) are prop roots, so named because they support tall, form a taproot. Instead, many small roots emerge from the top-heavy plants. All roots of a mature stem. Such roots are said to be adventitious, a term describing maize plant are adventitious whether they emerge above or below ground. a plant organ that grows in an unusual location, such as roots arising from stems or leaves. Each root forms its own lateral roots, which in turn form their own lateral roots. Because this mat of roots holds the topsoil in place, plants such as grasses that have dense fibrous root systems are especially good at preventing soil erosion. In most plants, the absorption of water and minerals occurs primarily near the tips of elongating roots, where vast numbers of root hairs, thin, finger-like extensions of root m Storage roots. Many plants, such epidermal cells, emerge and increase the surface area of the as the common root enormously (Figure 35.3). Most root systems also form beet, store food mycorrhizal associations, symbiotic interactions with soil fungi and water in their that increase a plant’s ability to absorb minerals (see Figure roots. 37.14). The roots of many plants are adapted for specialized functions (Figure 35.4).. Figure 35.4 Evolutionary adaptations of roots. m Pneumatophores. Also known as air roots, pneumatophores are produced by trees such as mangroves that inhabit tidal swamps. By projecting above the water’s surface at low tide, they enable the root system to obtain oxygen, which is lacking in the thick, waterlogged mud. c “Strangling” aerial roots. b Buttress roots. Because of moist Strangler fig seeds germi- conditions in the tropics, root systems nate in the crevices of tall of many of the tallest trees are trees. Aerial roots grow surprisingly shallow. Aerial roots that to the ground, wrapping look like buttresses, such as seen in around the host Gyranthera caribensis in Venezuela, tree and objects give architectural support to such as this the trunks of trees. Cambodian temple. Shoots grow upward and shade out the host tree, killing it. Stems Leaves A stem is a plant organ bearing leaves and buds. Its chief In most vascular plants, the leaf is the main photosyn- function is to elongate and orient the shoot in a way that thetic organ. In addition to intercepting light, leaves maximizes photosynthesis by the leaves. Another function of exchange gases with the atmosphere, dissipate heat, and stems is to elevate reproductive structures, thereby facilitating defend themselves from herbivores and pathogens. These the dispersal of pollen and fruit. Green stems may also per- functions may have conflicting anatomical and physiologi- form a limited amount of photosynthesis. Each stem consists cal requirements. For example, a dense covering of hairs of an alternating system of nodes, the points at which leaves may help repel herbivorous insects but may also trap air are attached, and internodes, the stem segments between near the leaf surface, thereby reducing gas exchange and, nodes (see Figure 35.2). Most of the growth of a young shoot consequently, photosynthesis. Because of these conflicting is concentrated near the growing shoot tip, or apical bud. demands and trade-offs, leaves vary extensively in form. In Apical buds are not the only types of buds found in shoots. In general, however, a leaf consists of a flattened blade and a the upper angle (axil) formed by each leaf and the stem is an stalk, the petiole, which joins the leaf to the stem at a node axillary bud, which can potentially form a lateral branch (see Figure 35.2). Grasses and many other monocots lack or, in some cases, a thorn or flower. petioles; instead, the base of the leaf forms a sheath that Some plants have stems with alternative functions, such as envelops the stem. food storage or asexual reproduction. Many of these modified Monocots and eudicots differ in the arrangement of veins, stems, including rhizomes, stolons, and tubers, are often mis- the vascular tissue of leaves. Most monocots have paral- taken for roots (Figure 35.5). lel major veins of equal diameter that run the length of the blade. Eudicots generally have a branched network of veins arising from a major vein (the midrib) that runs down the cen-. Figure 35.5 Evolutionary adaptations of stems. ter of the blade (see Figure 30.16). In identifying angiosperms according to structure, tax- b Rhizomes. The base of this iris onomists rely mainly on floral morphology, but they also use plant is an example of a rhizome, a horizontal shoot that grows just variations in leaf morphology, such as leaf shape, the branch- below the surface. Vertical shoots ing pattern of veins, and the spatial arrangement of leaves. Rhizome emerge from axillary buds on the Figure 35.6 illustrates a difference in leaf shape: simple versus rhizome. compound. Unlike leaves, the leaflets of compound leaves are not associated with axillary buds. Compound leaves may help confine invading pathogens to a single leaflet, rather than Root allowing them to spread to the entire leaf. c Stolons. Shown Stolon here on a straw-. Figure 35.6 Simple versus compound leaves. berry plant, stolons are horizontal Simple leaf shoots that grow along the surface. These “runners” A simple leaf has a single, enable a plant to undivided blade. Some reproduce asexually, simple leaves are deeply as plantlets grow lobed, as shown here. from axillary buds along each runner. Axillary Petiole bud Compound leaf b Tubers. Tubers, such Leaflet as these potatoes, are In a compound leaf, the enlarged ends of blade consists of multiple rhizomes or stolons leaflets. A leaflet has no axillary specialized for storing bud at its base. In some plants, food. The “eyes” of a each leaflet is further divided potato are clusters of into smaller leaflets. axillary buds. Axillary Petiole bud ? Which of these three examples has nodes? CHAPTER 35 Vascular Plant Structure, Growth, and Development 761. Figure 35.7 Evolutionary adaptations of leaves. Scientific Skills Exercise c Tendrils. The tendrils by which this pea plant clings to a support are modified leaves. After it has “lassoed” a support, a tendril forms a coil that brings the plant closer Using Bar Graphs to Interpret Data to the support. Tendrils Nature Versus Nurture: Why Are Leaves from are typically modified Northern Red Maples “Toothier” Than Leaves leaves, but some tendrils from Southern Red Maples? Not all leaves are modified stems, as in of the red maple (Acer rubrum) are the grapevines. same. The “teeth” along the margins of leaves growing in northern locations dif- fer in size and number from those of their southern counterparts. (The leaf seen here has an intermediate appearance.) Are these differences due to genetic differences be- tween northern and southern Acer rubrum b Spines. The spines of cacti, such populations, or do they arise from environmental as this prickly pear, are actually differences between northern and southern locations, such as leaves; photosynthesis is carried average temperature, that affect gene expression? out by the fleshy green stems. How the Experiment Was Done Seeds of Acer rubrum were collected from four latitudinally distinct sites: Ontario (Canada), b Storage leaves. Bulbs, Pennsylvania, South Carolina, and Florida. The seeds from the such as this cut onion, four sites were then grown in a northern location (Rhode Island) have a short underground and a southern location (Florida). After a few years of growth, stem and modified leaves leaves were harvested from the four sets of plants growing in the Plantlet that store food. two locations. The average area of single teeth and the average number of teeth per leaf area were determined. Storage leaves Data from the Experiment Stem Seed Average Area of a Number of Teeth per Collection Site Single Tooth (cm2) cm2 of Leaf Area b Reproductive leaves. The leaves of some succulents, such Grown Grown as Kalanchoë daigremontiana, in Rhode Grown in in Rhode Grown in produce adventitious plantlets, Island Florida Island Florida which fall off the leaf and take Ontario 0.017 0.017 3.9 3.2 root in the soil. (43.32°N) Pennsylvania 0.020 0.014 3.0 3.5 The shapes of leaves are often products of genetic (42.12°N) programs that are tweaked by environmental influences. South Carolina 0.024 0.028 2.3 1.9 Interpret the data in the Scientific Skills Exercise to explore (33.45°N) the roles of genetics and the environment in determining leaf Florida 0.027 0.047 2.1 0.9 (30.65°N) morphology in red maple trees. Almost all leaves are specialized for photosynthesis. Data from D. L. Royer et al., Phenotypic plasticity of leaf shape along a tempera- ture gradient in Acer rubrum, PLoS ONE 4(10):e7653 (2009). However, in some species evolution has resulted in additional INTERPRET THE DATA functions, such as support, protection, storage, or asexual reproduction (Figure 35.7). Some are sporophylls, leaves 1. Make a bar graph for tooth size and a bar graph for number of teeth. (For information on bar graphs, see the Scientific Skills highly specialized for sexual reproduction, such as carpels Review in Appendix D.) From north to south, what is the general and stamens in flowers (see Figure 30.12). trend in tooth size and number of teeth in leaves of Acer rubrum? 2. Based on the data, would you conclude that leaf tooth traits in the red maple are largely determined by genetic heritage (geno- Dermal, Vascular, and Ground Tissues type), by the capacity for responding to environmental change All three basic vascular plant organs—roots, stems, and within a single genotype (phenotypic plasticity), or by both? Make specific reference to the data in answering the question. leaves—are composed of three fundamental tissue types: 3. The “toothiness” of leaf fossils of known age has been used dermal, vascular, and ground tissues. Each of these general by paleoclimatologists to estimate past temperatures in a re- types forms a tissue system that is continuous through- gion. If a 10,000-year-old fossilized red maple leaf from South out the plant, connecting all the organs. However, specific Carolina had an average of 4.2 teeth per square centimeter characteristics of the tissues and the spatial relationships of of leaf area, what could you infer about the temperature of South Carolina 10,000 years ago compared with the tempera- tissues to one another vary in different organs (Figure 35.8). ture today? Explain your reasoning. Dermal tissue serves as the outer protective covering of the plant. Like our skin, it forms the first line of defense against phys- Instructors: A version of this Scientific Skills Exercise can be assigned in Mastering Biology. ical damage and pathogens. In nonwoody plants, it is usually 762 UNIT SIX Plant Form and Function. Figure 35.8 The three tissue systems. The dermal tissue. Figure 35.9 Trichome diversity on the surface of a leaf. system (blue) provides a protective cover for the entire body of Three types of trichomes are found on the surface of marjoram a plant. The vascular tissue system (purple), which transports (Origanum majorana). Spear-like trichomes help hinder the materials between the root and shoot systems, is also continuous movement of crawling insects, while the other two types of trichomes throughout the plant but is arranged differently in each organ. The secrete oils and other chemicals involved in defense (colorized SEM). ground tissue system (yellow), which is responsible for most of the metabolic functions, is located between the dermal tissue and the vascular tissue in each organ. Trichomes 300 om made (usually the leaves) to where they are needed or stored— usually roots and sites of growth, such as developing leaves and fruits. The vascular tissue of a root or stem is collectively called the stele (the Greek word for “pillar”). The arrangement of the stele varies, depending on the species and organ. In angio- sperms, for example, the root stele is a solid central vascular cylinder of xylem and phloem, whereas the stele of stems and leaves consists of vascular bundles, separate strands containing xylem and phloem (see Figure 35.8). Both xylem and phloem are composed of a variety of cell types, including cells that are highly specialized for transport or support. Tissue that is neither dermal nor vascular is ground tissue. Ground tissue that is internal to the vascular tissue is Dermal tissue known as pith, and ground tissue that is external to the vas- cular tissue is called cortex. Ground tissue is not just filler: It Ground includes cells specialized for functions such as storage, photo- tissue Vascular tissue synthesis, support, and short-distance transport. Common Types of Plant Cells a single tissue called the epidermis, a layer of tightly packed In a plant, as in any multicellular organism, cells undergo cell cells. In leaves and most stems, the cuticle, a waxy epidermal differentiation; that is, they become specialized in structure and coating, helps prevent water loss. In woody plants, protective tis- function during the course of development. Cell differentiation sues called periderm replace the epidermis in older regions of may involve changes both in the cytoplasm and its organelles stems and roots. In addition to protecting the plant from water and in the cell wall. Figure 35.10, on the next two pages, focuses loss and disease, the epidermis has specialized characteristics in on the major types of plant cells. Notice the structural adapta- each organ. In roots, water and minerals absorbed from the soil tions that make specific functions possible. You may also wish to enter through the epidermis, especially in root hairs. In shoots, review basic plant cell structure (see Figures 6.8 and 6.27). specialized epidermal cells called guard cells are involved in gaseous exchange. Another class of highly specialized epidermal Mastering Biology BioFlix® Animation: Tour of a Plant Cell cells found in shoots consists of outgrowths called trichomes. In some desert species, hairlike trichomes reduce water loss CONCEPT CHECK 35.1 and reflect excess light. Some trichomes defend against insects 1. How does the vascular tissue system enable leaves and roots through shapes that hinder movement or glands that secrete to function together in supporting growth and develop- sticky fluids or toxic compounds (Figure 35.9). ment of the whole plant? The two major functions of vascular tissue are to facilitate 2. WHAT IF? If humans were photoautotrophs, making food by capturing light energy for photosynthesis, how might the transport of materials through the plant and to provide our anatomy be different? mechanical support. Vascular tissues are of two types: xylem 3. MAKE CONNECTIONS Explain how central vacuoles and and phloem. Xylem conducts water and dissolved miner- cellulose cell walls contribute to plant growth (see Concepts als upward from roots into the shoots. Phloem transports 6.4 and 6.7). sugars, the products of photosynthesis, from where they are For suggested answers, see Appendix A. CHAPTER 35 Vascular Plant Structure, Growth, and Development 763. Figure 35.10 Exploring Examples of Differentiated Plant Cells Parenchyma Cells Mature parenchyma cells have primary walls that are relatively thin and flexible, and most lack secondary walls. (See Figure 6.27 to review primary and secondary cell walls.) When mature, parenchyma cells generally have a large central vacuole. Parenchyma cells perform most of the metabolic functions of the plant, synthesizing and storing various organic products. For example, photosynthesis occurs within the chloroplasts of paren- chyma cells in the leaf. Some parenchyma cells in stems and roots have colorless plastids called amyloplasts that store starch. The fleshy tissue of many fruits is composed mainly of parenchyma cells. Most parenchyma cells retain the ability to divide and differentiate into other types of plant cells under particular conditions—during wound repair, for example. It is even possible to grow an entire plant from a single parenchyma cell. Parenchyma cells in a 25 om privet (Ligustrum) leaf (LM) Collenchyma Cells Grouped in strands, collenchyma cells (seen here in cross section) help support young parts of the plant shoot. Collenchyma cells are generally elongated cells that have thicker primary walls than parenchyma cells, though the walls are unevenly thickened. Young stems and petioles often have strands of collenchyma cells just below their epidermis. Collenchyma cells provide flexible support without restraining growth. At maturity, these cells are living and flexible, elongating with the stems and leaves they support—unlike sclerenchyma cells, which we discuss next. Collenchyma cells in a 5 om common nettle (Urtica dioica) stem (LM) Sclerenchyma Cells 5 om Sclerenchyma cells also function as supporting elements in the plant but are much more rigid than collenchyma cells. In scleren- chyma cells, the secondary cell wall, produced after cell elonga- tion has ceased, is thick and contains large amounts of lignin, a relatively indigestible strengthening polymer that accounts for Sclereid cells (in pear) (LM) more than a quarter of the dry mass of wood. Lignin is present in all vascular plants but not in bryophytes. Mature sclerenchyma cells cannot elongate, and they occur in regions of the plant that 25 om have stopped growing in length. Sclerenchyma cells are so spe- cialized for support that many are dead at functional maturity, but they produce secondary walls before the protoplast (the liv- ing part of the cell) dies. The rigid walls remain as a “skeleton” that supports the plant, in some cases for hundreds of years. Cell wall Two types of sclerenchyma cells, known as sclereids and fibers, are specialized entirely for support and strengthening. Sclereids, which are boxier than fibers and irregular in shape, have very thick, lignified secondary walls. Sclereids impart the hardness to nutshells and seed coats and the gritty texture to pear fruits. Fibers, which are usually grouped in strands, are long, slender, and tapered. Some are used commercially, such as hemp fibers for making rope and flax fibers for weaving into linen. Fiber cells (cross section from ash tree) (LM) 764 UNIT SIX Plant Form and Function  Vessel Tracheids Water-Conducting Cells of the Xylem The two types of water-conducting cells, tracheids and 100 om vessel elements, are tubular, elongated cells that are dead and lignified at functional maturity. Tracheids occur in the xylem of all vascular plants. In addition to tracheids, most angiosperms, as well as a few gymnosperms and a few seedless vascular plants, have vessel elements. When the living cellular contents of a tra- cheid or vessel element disintegrate, the cell’s thickened walls remain behind, forming a nonliving conduit through which water can flow. The secondary walls of tracheids and vessel elements are often interrupted by pits, thinner regions where only primary walls are present (see Figure 6.27 to review primary and secon- dary walls). Water can migrate laterally between neighboring cells through pits. Tracheids are long, thin cells with tapered ends. Water moves Pits from cell to cell mainly through the pits, where it does not have to cross thick secondary walls. Tracheids and vessels Vessel elements are generally wider, shorter, thinner walled, and (colorized SEM) Perforation less tapered than the tracheids. They are aligned end to end, form- plate ing long pipes known as vessels that in some cases are visible with the naked eye. The end walls of vessel elements have perforation plates that enable water to flow freely through the vessels. The secondary walls of tracheids and vessel elements are Vessel hardened with lignin. This hardening provides support and prevents element collapse under the tension of water transport. Pits Vessel elements, with perforated end walls Tracheids Sugar-Conducting Cells of the Phloem 3 om Unlike the water-conducting cells of the xylem, the Sieve-tube elements: sugar-conducting cells of the phloem are alive at longitudinal view (LM) functional maturity. In seedless vascular plants and gymnosperms, sugars and other organic nutrients are transported through long, narrow cells called sieve cells. In the phloem of angiosperms, these nu- trients are transported through sieve tubes, which Kristina consist of chains of cells that are called sieve-tube NEED photo Sieve plate but can’t download elements, or sieve-tube members. Though alive, sieve-tube elements lack a nucleus, ri- Sieve-tube element (left) It’s a quicktime movie Companion Can you download? bosomes, a distinct vacuole, and cytoskeletal elements. and companion cell: cells This reduction in cell contents enables nutrients to cross section (TEM) pass more easily through the cell. The end walls be- tween sieve-tube elements, called sieve plates, have Sieve-tube pores that facilitate the flow of fluid from cell to cell elements along the sieve tube. Alongside each sieve-tube ele- ment is a nonconducting cell called a companion cell, which is connected to the sieve-tube element by nu- Plasmodesma merous plasmodesmata (see Figure 6.27). The nucleus 30 om and ribosomes of the companion cell serve not only that cell itself but also the adjacent sieve-tube element. Sieve In some plants, the companion cells in leaves also help plate load sugars into the sieve-tube elements, which then transport the sugars to other parts of the plant. Nucleus of companion cell 15 om Sieve-tube elements: longitudinal view Sieve plate with pores (LM) CHAPTER 35 Vascular Plant Structure, Growth, and Development 765  Apical bud Bud scale b Figure 35.12 CONCEPT 35.2 Axillary buds Three years’ Different meristems generate growth in a winter twig. new cells for primary and This year’s growth (one year old) Leaf secondary growth scar A major difference between plants and most animals is that Bud Node One-year-old plant growth is not limited to an embryonic or juvenile scar branch formed period. Instead, growth occurs throughout the plant’s life, Internode from axillary bud near shoot tip a process called indeterminate growth. Plants can keep growing because they have undifferentiated tissues called Last year’s growth meristems containing cells that can divide, leading to new (two years old) Leaf scar cells that elongate and become differentiated (Figure 35.11). Stem Except for dormant periods, most plants grow continuously. In contrast, most animals and some plant organs—such Bud scar as leaves, thorns, and flowers—undergo determinate growth; they stop growing after reaching a certain size. There are two main types of meristems: apical meristems Growth of two years ago and lateral meristems. Apical meristems, located at root and (three years old) Leaf scar shoot tips, provide cells that enable primary growth, growth in length. Primary growth allows roots to extend throughout the soil and shoots to increase exposure to light. In herbaceous (non- and internodes. On each growth segment, nodes are marked woody) plants, it produces all, or almost all, of the plant body. by scars left when leaves fell. Leaf scars are prominent in many Woody plants, however, also grow in circumference in the parts twigs. Above each scar is an axillary bud or a branch formed by of stems and roots that no longer grow in length. This growth in an axillary bud. Farther down are bud scars from whorls of scales thickness, known as secondary growth, is made possible by that enclosed the apical bud during the previous winter. In each lateral meristems: the vascular cambium and cork cambium. growing season, primary growth extends shoots, and secondary These cylinders of dividing cells extend along the length of growth increases the diameter of parts formed in previous years. roots and stems. The vascular cambium adds vascular tissue Although meristems enable plants to grow throughout their called secondary xylem (wood) and secondary phloem. Most of lives, plants do die, of course. Based on the length of their life the thickening is from secondary xylem. The cork cambium cycle, flowering plants can be categorized as annuals, biennials, replaces the epidermis with the thicker, tougher periderm. or perennials. Annuals complete their life cycle—from germina- Cells in apical and lateral meristems divide frequently tion to flowering to seed production to death—in a single year or during the growing season, generating additional cells. Some less. Many wildflowers are annuals, as are most staple food crops, new cells remain in the meristem and produce more cells, including legumes and cereal grains such as wheat and rice. while others differentiate and are incorporated into tissues Dying after producing seeds and fruits enables plants to transfer and organs. Cells that remain as sources of new cells have tra- the maximum amount of energy to reproduction. Biennials, ditionally been called initials but are increasingly being called such as turnips, generally require two growing seasons to com- stem cells to correspond to animal stem cells that also divide plete their life cycle, flowering and fruiting only in their second and remain functionally undifferentiated. year. Perennials live many years and include trees, shrubs, and Cells displaced from the meristem may divide several some grasses. Some buffalo grass of the North American plains is more times as they differentiate into mature cells. During thought to have been growing for 10,000 years from seeds that primary growth, these cells give rise to three tissues called sprouted at the close of the last ice age. primary meristems—the protoderm, ground meristem, CONCEPT CHECK 35.2 and procambium—that will produce, respectively, the three mature tissues of a root or shoot: the dermal, ground, and vas- 1. Would primary and secondary growth ever occur simultane- ously in the same plant? cular tissues. The lateral meristems in woody plants also have 2. Roots and stems grow indeterminately, but leaves do not. stem cells, which give rise to all secondary growth. How might this benefit the plant? The relationship between primary and secondary growth is 3. WHAT IF? After growing carrots for one season, a gardener seen in the winter twig of a deciduous tree. At the shoot tip is decides that the carrots are too small. Since carrots are bien- the dormant apical bud, enclosed by scales that protect its apical nials, the gardener leaves the crop in the ground for a sec- ond year, thinking the carrot roots will grow larger. Is this a meristem (Figure 35.12). In spring, the bud sheds its scales and good idea? Explain. begins a new spurt of primary growth, producing a series of nodes For suggested answers, see Appendix A. 766 UNIT SIX Plant Form and Function ▼ Figure 35.11 VISUALIZING PRIMARY AND SECONDARY GROWTH All vascular plants have primary growth: growth in length. Woody plants also have Mastering Biology Animation: secondary growth: growth in thickness. As you study the diagrams, visualize how Primary and Secondary Growth shoots and roots grow longer and thicker. Primary Growth (growth in length) Apical meristem cells are undifferentiated. When they divide, Primary growth is made possible by apical some daughter cells remain in the apical meristem, ensuring meristems at the tips of shoots and roots. a continuing population of undifferentiated cells. Other daughter cells become partly differentiated as primary Shoot apical Leaf primordia meristem cells. After dividing and growing in length, they meristem become fully differentiated cells in the mature tissues. Cell division in Youngest apical meristem differentiated Growth Daughter cell in Time cells Primary primary meristem meristems Cell division in Older primary meristem differentiated cells Growing cells in Mature primary meristem tissues The addition of elongated, differentiated cells Differentiated cells lengthens a stem (as shown here) or root. Dermal Ground Vascular (for example, vessel elements) 1. A thimble-like root cap protects each root apical meristem. Draw and label a simple Cutaway view of primary growth outline of a root divided into four sections: Root apical in a shoot tip root cap (bottom), root apical meristem, meristem primary meristems, and mature tissues. Direction of secondary growth Secondary Growth (growth in thickness) Secondary growth is made possible by two lateral Addition of secondary xylem Vascular cambium cell meristems extending along the length of a shoot or and phloem cells: When a root where primary growth has ceased. vascular cambium cell divides, X1 sometimes one daughter cell Time Vascular cambium becomes a secondary xylem cell X1 P1 (X) to the inside of the cambium The lateral meristems, or a secondary phloem cell (P) to X1 X2 P1 called the vascular the outside. Although xylem and cambium and cork phloem cells are shown being X1 X2 P2 P1 cambium, are cylinders added equally here, usually many of dividing cells that are more xylem cells are produced. one cell thick. Direction of secondary growth Cork cambium Addition of cork cells: Cork cambium cell When a cork cambium cell Time Increased circumference: divides, sometimes one daughter When a cambium cell C1 cell becomes a cork cell (C) to divides, sometimes both the outside of the cambium. daughter cells remain in C2 C1 the cambium and grow, Cell Cell increasing the cambium division growth circumference. 2. Draw the row of cells from the Completed primary growth When the vascular cambium and boxed area below and label the cork cambium become active in a vascular cambium cell (V), 5 xylem stem (or root), primary growth cells from oldest (X1) to youngest has ceased in that area. (X5), and 3 phloem cells (P1 to P3). Vascular cambium cell Show what happens after growth Cork cambium cell continues by drawing and labeling a row with twice as many xylem and phloem cells. How does the Direction of secondary growth A stem (or root) thickens as vascular cambium’s Lateral secondary xylem, secondary location change? meristems phloem, and cork cells are Youngest Youngest Cork added. Most of the cells are xylem cell phloem cell cells secondary xylem (wood). Instructors: Additional questions related to this Visualizing Figure can be assigned in Oldest Oldest Mastering Biology. 767 xylem cell phloem cell  CONCEPT 35.3 Typically, a few millimeters behind the tip of the root is the zone of elongation, where most of the growth occurs as root Primary growth lengthens cells elongate—sometimes to more than ten times their origi- nal length. Cell elongation in this zone pushes the tip farther roots and shoots into the soil. Meanwhile, the root apical meristem keeps add- Primary growth arises directly from cells produced by apical ing cells to the younger end of the zone of elongation. Even meristems. In herbaceous plants, almost the entire plant is before the root cells finish lengthening, many begin spe- created through primary growth, whereas in woody plants cializing in structure and function. As this occurs, the three only the nonwoody, more recently formed parts of the plant primary meristems—the protoderm, ground meristem, and represent primary growth. Although both roots and shoots procambium—become evident. In the zone of differentiation, lengthen as a result of cells derived from apical meristems, the or zone of maturation, cells complete their differentiation details of their primary growth differ in many ways. and become distinct cell types. The protoderm, the outermost primary meristem, gives rise to the epidermis, a single layer of cuticle-free cells cover- Primary Growth of Roots ing the root. Root hairs are the most prominent feature of The entire biomass of a primary root is derived from the the root epidermis. These modified epidermal cells function root apical meristem. The root apical meristem also makes in the absorption of water and minerals. Root hairs typically a thimble-like root cap, which protects the delicate apical only live a few weeks but together make up 70–90% of the meristem as the root pushes through the abrasive soil. The total root surface area. It has been estimated that a four- root cap secretes a polysaccharide slime that lubricates the month-old rye plant has about 14 billion root hairs. Laid soil around the tip of the root. Growth occurs just behind the end to end, the root hairs of a single rye plant would cover tip in three overlapping zones of cells at successive stages of 10,000 km, one-quarter the length of the equator. primary growth. These are the zones of cell division, elonga- Sandwiched between the protoderm and the procambium tion, and differentiation (Figure 35.13). is the ground meristem, which gives rise to mature ground The zone of cell division includes the stem cells of the root tissue. The ground tissue of roots, consisting mostly of paren- apical meristem and their immediate products. New root cells chyma cells, is found in the cortex, the region between the are produced in this region, including cells of the root cap. vascular tissue and epidermis. In addition to storing carbohy- drates, cells in the cortex transport water and salts from the. Figure 35.13 Primary growth of a eudicot root. In the root hairs to the center of the root. The cortex also allows for micrograph, mitotic cells in the apical meristem are revealed by extracellular diffusion of water, minerals, and oxygen from the staining for cyclin, a protein involved in cell division (LM). root hairs inward because there are large spaces between cells. Cortex Vascular cylinder The innermost layer of the cortex is called the endodermis, a cylinder one cell thick that forms the boundary with the Epidermis vascular cylinder. The endodermis is a selective barrier that Key regulates passage of substances from the soil into the vascular Zone of to labels cylinder (see Figure 36.9). Root hair differentiation Dermal The procambium gives rise to the vascular cylinder, which Ground consists of a solid core of xylem and phloem tissues sur- Vascular rounded by a cell layer called the pericycle. In most eudicot roots, the xylem has a starlike appearance in cross section, Primary meristems 70 om and the phloem occupies the indentations between the (elongating, partly arms of the xylem “star” (Figure 35.14a). In many monocot differentiated cells) Zone of elongation roots, the vascular tissue consists of a core of undifferentiated Protoderm parenchyma cells surrounded by a ring of alternating xylem and phloem tissues (Figure 35.14b). Ground Mitotic meristem cells By increasing the length of roots, primary growth facilitates their penetration and exploration of the soil. If a Procambium Zone of cell resource-rich pocket is located in the soil, the branching of division roots may be stimulated. Branching, too, is a form of primary (including Root apical meristem apical growth. Lateral (branch) roots arise from meristematically (undifferentiated cells) meristem) active regions of the pericycle, the outermost cell layer in the vascular cylinder, which is adjacent to and just inside the Root cap endodermis (see Figure 35.14). The emerging lateral roots 768 UNIT SIX Plant Form and Function. Figure 35.14 Organization of primary tissues in young. Figure 35.15 The formation of a lateral root. A lateral root roots. Parts (a) and (b) show cross sections of the roots of a originates in the pericycle, the outermost layer of the vascular Ranunculus (buttercup) species and Zea mays (maize), respectively. cylinder of a root, and destructively pushes through the outer These represent two basic patterns of root organization, of which tissues before emerging. In this light micrograph, the view of the there are many variations, depending on the plant species (all LMs). original root is a cross section, but the view of the lateral root is a longitudinal section (a view along the length of the lateral root). Epidermis Emerging lateral root Cortex Epidermis Endodermis Vascular cylinder Pericycle Xylem Vascular cylinder Phloem Pericycle 100 om 100 om Cortex (a) Root with xylem and phloem in the center (typical of eudicots). In the roots of typical gymnosperms and eudicots, as well as some monocots, the stele is a DRAW IT Draw what the original root and lateral root would look like vascular cylinder appearing in cross section as a lobed when viewed from the side, labeling both roots. core of xylem with phloem between the lobes. Endodermis disruptively push through the outer tissues until they emerge Key from the established root (Figure 35.15). to labels Pericycle Dermal Xylem Ground Primary Growth of Shoots Vascular The entire biomass of a primary shoot—all its leaves and stems— derives from its shoot apical meristem, a dome-shaped mass of Phloem dividing cells at the shoot tip (Figure 35.16). The shoot apical meristem is a delicate structure protected by the leaves of the api- 70 om cal bud. These young leaves are spaced close together because the Epidermis. Figure 35.16 The shoot tip. Leaf primordia arise from the flanks of the dome of the apical meristem. This is a longitudinal Cortex section of the shoot tip of Coleus (LM). Endodermis Leaf primordia Vascular Young leaf cylinder Pericycle Shoot apical meristem Core of parenchyma Protoderm cells Procambium 100 om Xylem Ground Phloem meristem (b) Root with parenchyma in the center (typical of Axillary bud monocots). The stele of many monocot roots meristems is a vascular cylinder with a core of parenchyma surrounded by a ring of xylem and a ring of phloem. 0.25 mm Mastering Biology Animation: Root Cross Sections CHAPTER 35 Vascular Plant Structure, Growth, and Development 769 internodes are very short. Shoot elongation is due to the length-. Figure 35.17 Organization of primary tissues in young stems. ening of internode cells below the shoot tip. As with the root api- Phloem Xylem cal meristem, the shoot apical meristem gives rise to three types of primary meristems in the shoot—the protoderm, ground Sclerenchyma Ground tissue meristem, and procambium. These three primary meristems in (fiber cells) connecting turn give rise to the mature primary tissues of the shoot. pith to cortex The branching of shoots, which is also part of primary growth, arises from the activation of axillary buds, each of which has its own shoot apical meristem. Because of chemi- cal communication by plant hormones, the closer an axillary bud is to an active apical bud, the more inhibited it is, a phe- nomenon called apical dominance. (The specific hormonal changes underlying apical dominance are discussed in Concept 39.2.) If an animal eats the end of the shoot or if shading results Pith in the light being more intense on the side of the shoot, the chemical communication underlying apical dominance is dis- rupted. As a result, the axillary buds break dormancy and start to grow. Released from dormancy, an axillary bud eventually Cortex Epidermis gives rise to a lateral shoot, complete with its own apical bud, Vascular leaves, and axillary buds. When gardeners prune shrubs and bundle pinch back houseplants, they are reducing the number of apical 1 mm buds a plant has, thereby allowing branches to develop and giv- (a) Cross section of stem with vascular bundles forming a ring (typical of eudicots). Ground tissue toward the ing the plants a fuller, bushier appearance. inside is called pith, and ground tissue toward the outside is called cortex (LM). Stem Growth and Anatomy The stem is covered by an epidermis that is usually one cell Key thick and covered with a waxy cuticle that prevents water to labels loss. Some examples of specialized epidermal cells in the stem include guard cells and trichomes. Dermal The ground tissue of stems consists mostly of paren- Ground chyma cells. However, collenchyma cells just beneath the Vascular epidermis strengthen many stems during primary growth. Sclerenchyma cells, especially fiber cells, also provide support Epidermis in those parts of the stems that are no longer elongating. Ground Vascular tissue runs the length of a stem in vascular tissue bundles. Unlike lateral roots, which arise from vascular tissue deep within a root and disrupt the vascular cylinder, cor- tex, and epidermis as they emerge (see Figure 35.15), lateral shoots develop from axillary bud meristems on the stem’s surface and do not disrupt other tissues (see Figure 35.16). Near the soil surface, in the transition zone between shoot and root, the bundled vascular arrangement of the stem con- verges with the solid vascular cylinder of the root. The vascular tissue of stems in most eudicot species consists of vascular bundles arranged in a ring (Figure 35.17a). The xylem in each bundle faces the pith, and the phloem in each Vascular bundle faces the cortex. In most monocot stems, the vascular bundles 1 mm bundles do not form a ring but have a more scattered arrange- (b) Cross section of stem with scattered vascular bundles ment in the ground tissue (Figure 35.17b). (typical of monocots). In such an arrangement, ground tissue is not partitioned into pith and cortex (LM). Leaf Growth and Anatomy VISUAL SKILLS Compare the locations of the vascular bundles in Figure 35.18 provides an overview of leaf anatomy. Leaves eudicot and monocot stems. Then explain why the terms pith and cortex are not used in describing the ground tissue of monocot stems. develop from leaf primordia (singular, primordium), projec- tions shaped like a cat’s ear that emerge along the sides of the Mastering Biology Animation: Stem Cross Sections 770 UNIT SIX Plant Form and Function. Figure 35.18 Leaf anatomy. Guard cells Key to labels Stomatal 50 om pore Dermal Epidermal Ground cell Cuticle Sclerenchyma Vascular fibers Stoma (b) Surface view of a spiderwort (Tradescantia) leaf (LM) Upper epidermis Palisade mesophyll Bundle- Spongy sheath mesophyll cell 100 om Lower epidermis Cuticle Xylem Phloem Vein Guard Vein Air spaces Guard cells (a) Cutaway drawing of leaf tissues cells (c) Cross section of a lilac (Syringa) leaf (LM) Mastering Biology Animation: Leaf Anatomy shoot apical meristem (see Figure 35.16). Unlike roots and CO2 and O2 circulate to and from the palisade layer. The air stems, secondary growth in leaves is minor or nonexistent. As spaces are particularly large in the vicinity of stomata, where with roots and stems, the three primary meristems give rise to CO2 is taken up from the outside air and O2 is released. the tissues of the mature organ. The vascular tissue of each leaf is continuous with the The leaf epidermis is covered by a waxy cuticle that greatly vascular tissue of the stem. Veins subdivide repeatedly and reduces water loss except where it is interrupted by stomata branch throughout the mesophyll. This network brings (singular, stoma), which allow exchange of CO2 and O2 xylem and phloem into close contact with the photosyn- between the surrounding air and the photosynthetic cells thetic tissue, which obtains water and minerals from the inside the leaf. In addition to regulating CO2 uptake for pho- xylem and loads its sugars and other organic products into tosynthesis, stomata are major avenues for the evaporative the phloem for transport to other parts of the plant. The vas- loss of water. The term stoma can refer to the stomatal pore or cular structure also functions as a framework that reinforces to the entire stomatal complex consisting of a pore flanked the shape of the leaf. Each vein is enclosed by a protective by the two specialized epidermal cells known as guard cells, bundle sheath, a layer of cells that regulates the movement of which regulate the opening and closing of the pore. (We will substances between the vascular tissue and the mesophyll. discuss stomata in detail in Concept 36.4.) Bundle-sheath cells are very prominent in leaves of species The leaf’s ground tissue, called the mesophyll (from the that carry out C4 photosynthesis (see Concept 10.5). Greek mesos, middle, and phyll, leaf), is sandwiched between the upper and lower epidermal layers. Mesophyll consists CONCEPT CHECK 35.3 mainly of parenchyma cells specialized for photosynthesis. 1. Contrast primary growth in roots and shoots. The mesophyll in many eudicot leaves has two distinct layers: 2. WHAT IF? A fossil leaf from a region that in the geological palisade and spongy. Palisade mesophyll, located beneath the past was intermittently very dry and very swampy has sto- mata only on its upper epidermis. Was the leaf from a des- upper epidermis, consists of one or more layers of elongated, ert plant or from a floating aquatic plant? Explain. chloroplast-rich cells that are specialized for light capture. 3. MAKE CONNECTIONS How are root hairs and microvilli Spongy mesophyll, located inward from the lower epidermis, analogous structures? (See Figure 6.8 and the discussion of consists of irregularly shaped cells that have fewer chloro- analogy in Concept 26.2.) plasts. These cells form a labyrinth of air spaces through which For suggested answers, see Appendix A. CHAPTER 35 Vascular Plant Structure, Growth, and Development 771  CONCEPT 35.4 Secondary growth consists of the tissues produced by the vascular cambium and cork cambium. The vascular cam- Secondary growth increases bium adds secondary xylem (wood) and secondary phloem, thereby increasing vascular flow and support for the shoots. the diameter of stems and roots The cork cambium produces a tough, thick covering of waxy in woody plants cells that protect the stem from water loss and from invasion by insects, bacteria, and fungi. Many land plants display secondary growth, the growth in In woody plants, primary growth and secondary growth thickness produced by lateral meristems. The advent of second- occur simultaneously. As primary growth adds leaves and ary growth during plant evolution allowed the production of lengthens stems and roots in the younger regions of a plant, novel plant forms ranging from massive forest trees to woody secondary growth increases the diameter of stems and roots vines. All gymnosperm species and many eudicot species in older regions where primary growth has ceased. The pro- undergo secondary growth, but it is unusual in monocots. It cess is similar in shoots and roots. Figure 35.19 provides an occurs in stems and roots of woody plants, but rarely in leaves. overview of growth in a woody stem.. Figure 35.19 Secondary growth 1 of a woody stem. 1 Primary growth from the activity of the apical meristem is complete here. The vascular cambium has formed, and its cell divisions will give rise to the bulk of secondary growth. Epidermis Pith Cortex Primary xylem 2 Although primary growth continues in Primary Vascular cambium Epidermis the apical bud, only secondary growth phloem Primary phloem Cortex occurs in this region. The stem thickens as the vascular cambium forms Vascular secondary xylem to the inside and cambium 2 secondary phloem to the outside. Primary th xylem Grow 3 Vascular 3 Some stem cells of the vascular cambium Pith ray give rise to vascular rays. 4 As the vascular cambium’s diameter increases, the secondary phloem and other tissues external to the cambium Primary can’t keep pace because their cells no xylem longer divide. As a result, these Secondary xylem tissues, including the epidermis, will eventually rupture. A second lateral Vascular cambium meristem, the cork cambium, develops Secondary phloem from parenchyma cells in the cortex. 4 Primary phloem The cork cambium produces cork cells, First cork cambium Cork which replace the epidermis. 5 In year 2 of secondary growth, the vascular Periderm