MODERN BOTANY - P. RAVEN - 1990

PART V. STRUCTURE AND DEVELOPMENT OF THE ANGIOSPERM BODY

CHAPTER 20. PLANT CELLS AND TISSUES

As the embryo develops, The formation of new Cells, Tissues, and Organs is almost entirely restricted to Meristems—tissues that retain physiological juvenility and are fundamental to growth. As noted in Chapter 1, There are two primary types of meristems: apical and lateral. Apical meristems are primarily responsible for lengthening growth. They are located at the tips (apices) of shoots and roots. Growth that gives rise to primary tissues is termed primary growth, and the part of the plant composed of these tissues is referred to as the primary plant body.

Class="center">Fig. 20-1. Transverse section of the phloem of the fern Platycerium bifurcatum, showing portions of two transfer cells whose walls exhibit numerous invaginations (see arrows). These cells lie adjacent to the lighter-staining sieve element, a nutrient-conducting Cell

Lateral meristems — vascular¹ and cork cambiums — produce secondary tissues, which make up the secondary plant body. The cambium produces secondary xylem and phloem, whereas the cork cambium primarily produces cork.

¹ The vascular cambium is commonly referred to simply as the cambium. — Editor's note.

In both apical and lateral meristems, certain cells divide continuously. Following each division, one of the daughter cells remains in the meristem, while the other is incorporated into non-dividing tissues. The cells that continue to divide are called initials, and the others are called derivatives. Notably, derivatives typically divide one or more times before beginning to differentiate into specialized cell types. Consequently, a meristem comprises the initials and their immediate derivatives.

Cell divisions occur outside of apical and lateral meristems as well. For example, the protoderm, procambium, and ground meristem, which represent partially differentiated tissues, are termed primary meristems because (1) they give rise to primary tissues and (2) many of their cells remain meristematic for some time before differentiating. Primary meristems originate from apical meristems, with the exception of those arising during Embryogenesis within the embryo proper.

Plant growth is the result of Cell Division and cell elongation. As meristematic tissues age, the overall size of their cells increases; toward the end of differentiation, cell elongation becomes the primary mode of growth in young regions of roots, stems, or leaves.

Differentiation — the process by which cells with the identical genotype acquire individual differences, including differences from the meristematic cell that gave rise to them. This process often begins even while the cells are elongating. Upon maturation, i.e., upon completing differentiation, some cells remain alive while others die. A multitude of types exists within both categories. How cells of identical origin become so structurally and functionally diverse remains one of the most pressing questions in modern biology. Certain factors involved in The regulation of Cell Differentiation are discussed at the end of Chapter 8 and in Chapter 24 (p. 101).

The basic arrangement of tissues is established during the Cytology/cytology/16.html">Early stages of meristematic activity. Their Organization and the overall form of the plant are determined by both cell division and cell elongation. The acquisition of a species-specific form is termed morphogenesis (from morphé, meaning form, and genere, meaning to create).

Tissue Systems

Botanists have long recognized that the major tissues of a vascular plant are organized into larger complexes known as tissue systems. Their presence in roots, stems, and leaves reflects the fundamental similarity of these organs and The Unity of the plant body. There are at least three such systems: (1) the ground tissue system, (2) the vascular tissue system, and (3) the dermal tissue system. As noted in Chapter 19, these systems are laid down during embryogenesis and are represented at that stage by the primary meristems—the ground meristem, procambium, and protoderm, respectively.

The ground tissue system consists of parenchyma (the most abundant tissue), collenchyma, and sclerenchyma. The vascular tissue system is represented by xylem and phloem. The epidermis, the outer protective layer of the primary plant body, and the periderm, the secondary protective tissue that eventually replaces it, together constitute the dermal tissue system.

Tissues and Their Cells

Tissues can be defined as structurally and/or functionally specialized groups of cells. If they consist of only a single cell type, they are termed simple, whereas if they comprise two or more cell types, they are termed complex. Parenchyma, collenchyma, and sclerenchyma are simple tissues, whereas xylem, phloem, and epidermis are complex.

Parenchyma

Parenchyma, the precursor of all other tissues, is composed of parenchyma cells. In the primary plant body, they typically occur as continuous masses in the cortex of stems (Fig. 20-2) and roots, the pith of stems, the mesophyll of leaves (see Fig. 20-23), and the flesh of fruits. In addition, these cells form vertical strands in both Primary and secondary Vascular Tissues, as well as horizontal strands (rays) in secondary vascular tissues (see Chapter 23).

Fig. 20-2. Transverse section of the stem cortex of Canada elderberry (Sambucus canadensis). Collenchyma cells with unevenly thickened walls are visible at the top, and parenchyma cells are visible at the bottom. The light spaces between them are intercellular spaces. In some parenchyma cells, a reticulate Structure is visible on the primary cell walls; its light (thin) areas represent primary pit fields

Typically living at maturity, parenchyma cells are capable of division. While they generally retain primary walls, they sometimes develop secondary walls as well. Because of their capacity for division, parenchyma cells with primary walls play a vital role in regeneration and wound healing. They are the cells that give rise to adventitious roots on stem cuttings. These cells participate in Photosynthesis, storage, and secretion—processes involving living protoplasm. Furthermore, they may play a specific role in Water movement and nutrient transport throughout the plant.

Transfer Cells

In recent years, considerable attention has been drawn to a specialized type of parenchyma cell characterized by distinct Cell wall invaginations that greatly increase the surface area of The Plasma Membrane (see Fig. 20-1). These cells, termed transfer cells, are hypothesized to play a crucial role in short-distance solute transport. Although their presence in various organs has long been known, it has only recently become clear that they are widely distributed throughout the plant and likely perform similar Functions everywhere. Transfer cells are associated with the xylem and phloem of the minor Veins of cotyledons and leaves in many herbaceous dicotyledons, as well as with the phloem and xylem of leaf traces at the nodes of dicotyledons and monocotyledons. Additionally, they are found in various reproductive structures (Placenta, embryo sac, endosperm) and glandular structures (nectaries, salt glands, carnivorous plant glands). Each of these zones represents a potential site for intensive short-distance solute transport.

Collenchyma

Collenchyma is composed of collenchymatous cells, which, much like parenchyma cells, remain alive at maturity (Figs. 20-2 to 20-5). It typically occurs as discrete strands or a continuous cylinder beneath the epidermis of stems and petioles, and also frames the veins in dicot leaves (forming almost entirely the ridges on the outer surface of celery petioles and leaf veins). Typically, elongated collenchyma cells (Fig. 20-3) feature unevenly thickened, non-lignified primary walls that make them exceptionally well-suited for strengthening young, growing organs (see the Description of the primary cell wall in Chapter 2). The very name collenchyma derives from the Greek word colla, meaning "glue," originating from these characteristically thickened walls that appear glistening in fresh tissues (Fig. 20-5). Being primary, these walls are highly extensible and practically do not hinder the elongation of the plant part in which they reside. Furthermore, because mature collenchymatous cells remain alive, they can continue to deposit thick, flexible walls concurrently with the lengthwise growth of the organ.

Fig. 20-3. Longitudinal section showing elongated collenchyma cells in a squash stem (Cucurbita maxima). A trichome, which is an epidermal appendage, is also visible (several trichome types are shown in Fig. 20-22)

Fig. 20-4. Transmission electron micrograph of a cross-section of a mature collenchyma cell from a wheat stamen filament (Triticum aestivum). The protoplasmic contents and the unevenly thickened wall are clearly visible

Fig. 20-5. Cross-section of collenchyma from a rhubarb leaf petiole (Rheum rhaponticum). In fresh tissue (as seen in this micrograph), the unevenly thickened walls of the collenchymatous cells appear glossy

Sclerenchyma

Sclerenchyma consists of sclerenchymatous cells, which can develop in any or even all PARTS OF THE primary and secondary plant body; their protoplasts are frequently absent at maturity. The term comes from the Greek skleros, meaning "hard," reflecting the defining feature of sclerenchyma cells—their thick, often lignified secondary walls. These walls make these cells vital strengthening and supporting elements in plant parts that have completed their longitudinal growth (see the Discussion of secondary cell walls in Chapter 2).

Sclerenchymatous cells are divided into two main types: fibers and sclereids. Fibers are typically long, slender cells usually grouped into strands or bundles (Fig. 20-6). Economically important bast fibers, such as hemp, jute, and flax, are obtained from the stems of dicots, whereas others (such as Manila hemp) come from monocot leaves. Sclereids vary greatly in shape and are often branched (Fig. 20-7), but they are relatively short compared to most fibers. They may occur individually or in clusters throughout the ground tissue. Sclereids form seed coats, nut shells, and the endocarp (stone) of stone fruits; they also give the flesh of pears its characteristic gritty texture (Fig. 20-8).

Fig. 20-6. Primary phloem fibers from a basswood stem (Tilia americana) in cross (A) and longitudinal (B) sections. The secondary walls of these long, thick-walled fibers feature relatively inconspicuous pits. Only a portion of their length is visible in part B

Fig. 20-7. A branched sclereid from a water lily leaf (Nymphaea odorata) under normal (A) and polarized (B) light. Its cell wall contains numerous minute, angular crystals

Fig. 20-8. Sclereids (stone cells) from the flesh of a pear (Pyrus communis). The secondary walls exhibit distinct simple pits with numerous branchings, known as branched (ramified) pits. As stone cell clusters form within the pear fruit flesh, successive divisions occur concentrically around earlier-formed sclereids, as new cells differentiate into stone cells and enlarge these clusters

Xylem

Xylem is the principal water-conducting tissue in vascular plants. It is also involved in The transport of minerals and the storage of nutrients, as well as providing mechanical support. Together with the phloem, xylem forms a continuous vascular tissue system permeating the entire plant body (Fig. 20-9). Based on its origin, xylem can be either primary or secondary. Primary xylem is produced by the procambium, whereas secondary xylem originates from the cambium (see Chapter 23).

Fig. 20-9. Cross-section of a vascular bundle from a squash stem (Cucurbita maxima). The phloem is located on both sides of the xylem, with the cambium developing between the outer phloem and the xylem

The primary conducting Cells of the xylem are tracheary elements, which comprise two types: tracheids and vessel elements. Both cell types are elongated, devoid of protoplasts at maturity, and possess pits on their secondary walls (Figs. 20-10A–D). In addition, vessel elements feature perforations, which are areas lacking both primary and secondary walls, effectively forming holes in The Cell wall. These occur primarily at the end walls of the elements, though they may also be found on lateral walls. The perforated region of the wall is called a perforation plate (Fig. 20-11).

Fig. 20-10. Secondary xylem (wood) cell types from an oak (Quercus) tree, isolated from unpainted macerated (dissociated) tissue. A, B — wide vessel elements. C — narrow vessel element. D — tracheid. E — fiber-tracheid. F — libriform fiber (the longest fiber type in oak wood). The speckled appearance of these cells is due to pits in their walls (except for F)

Fig. 20-11. Scanning electron micrograph of perforated end walls of secondary xylem vessel elements. A. A simple perforation plate with a single large opening between two vessel elements in American basswood (Tilia americana). B. A scalariform perforation plate between vessel elements in red alder (Alnus rubra). Pits can be seen on the walls adjacent to the perforation plates

Vessel elements are arranged end-to-end to form long, continuous tubes known as vessels (Fig. 20-12). The pits of the long, tapered tracheids are concentrated on their adjoining end walls. Tracheids are the sole water-conducting cell type in most seedless vascular plants and gymnosperms; the xylem of angiosperms typically contains vessel elements as well.

Fig. 20-12. Scanning electron micrograph of three vessel elements from the secondary xylem of red oak (Quercus rubra). External view, showing the rims between them.

Intensive comparative studies of tracheary elements across a broad range of vascular plants have established that tracheids are a more primitive (less specialized) cell type than vessel elements, which are the main water-conducting elements of flowering plant xylem. It is likely that vessel elements arose independently from tracheids in several vascular plant groups, including dicots and monocots (angiosperms), Gnetophytes (gymnosperms with vessels), certain unrelated fern species, several species of spikemosses (lycophytes), and horsetails. The Evolution of the vessel element is a prime example of convergent evolution—The Development of similar structures in unrelated or distantly related organisms (see the Appendix on Convergent Evolution in Ch. 22).

The term tracheary element was introduced in the seventeenth century by the Italian physician Marcello Malpighi, one of the founders of plant anatomy, who sought to discover similarities between plants and animals. While examining the xylem, he observed air bubbles emerging from a vessel with spiral wall thickenings, leading him to compare this structure to the Trachea—the air-conducting tube of insects—and later to apply the same term to all xylem vessels. Since then, the term has been used to designate the water-conducting cells of the xylem as a whole.

Vessel elements are generally considered to conduct water more efficiently than tracheids because water can flow relatively unimpeded from one element to the next through perforations, whereas in tracheids it must traverse pit membranes (see Ch. 2). However, these membranes likely offer relatively little resistance to water flow, as they become partially hydrolyzed during the final stages of differentiation, leaving only a highly permeable framework of Cellulose microfibrils.

In secondary and late primary xylem (metaxylem), the secondary walls of tracheids and vessel elements coat their entire primary wall except in the region of pits and perforations (Fig. 20-10, A–D). Subsequently, these walls become rigid and cease to stretch. During the elongation of roots, stems, and leaves in many early-formed tracheary elements of the early primary xylem (protoxylem), the secondary walls form rings or spirals (Fig. 20-13). Such annular and helical thickenings allow tracheary elements to stretch, although they are frequently disrupted during overall organ elongation. In the primary xylem, the pattern of wall thickening is largely influenced by the intensity of stretching. When stretching is slight, primarily pitted elements develop, whereas strong stretching yields numerous elements with annular and helical thickenings. Fig. 20-14 illustrates several stages of differentiation in a vessel element with spiral thickenings.

Fig. 20-13. Portions of tracheary elements from the early primary xylem (protoxylem) of castor bean (Ricinus communis). A. Annular (left) and helical cell wall thickenings in somewhat stretched elements. B. Double-helical thickenings in stretched elements. The element on the left is stretched so severely that the turns of the helix are far apart.

Fig. 20-14. Schematic diagram of vessel element development. A. Young, highly vacuolated element lacking a secondary wall. B. The cell has enlarged and secondary thickenings are appearing; the primary wall thickens at the site of the future perforation. C. Formation of secondary thickenings is complete, and the cell is in the lysis stage; The Nucleus is deformed, the tonoplast has ruptured, and the cell wall at the perforation site is disintegrating. D. Mature cell lacking a protoplast, open at both ends.

In addition to tracheids and vessel elements, the xylem includes parenchyma cells that store various substances. These typically form vertical strands and, in secondary xylem, are also found in rays. Fibers (see Fig. 20-10, D, E) and sclereids are also present in the xylem. Many of these fibers remain alive at maturity and perform a dual storage and mechanical support function.

Phloem

Phloem is the principal food-conducting tissue of vascular plants (see Fig. 20-9). It can be primary or secondary in origin. Like primary xylem, early primary phloem (protophloem) is frequently stretched and destroyed during organ elongation.

The primary conducting cells of the phloem are sieve elements of two types: sieve cells (Fig. 20-15) and sieve-tube elements (Figs. 20-16 and 20-17). The term "sieve" refers to clusters of pores (sieve areas) through which the protoplasts of adjacent sieve elements interconnect. In sieve cells, the pores are narrow, and The structure of the sieve areas is fairly uniform across all walls. Most of the areas are concentrated on the overlapping ends of these long, narrow cells (Fig. 20-15, A). In sieve-tube elements, the tubules of the sieve areas are wider on some walls than on others. The region of the wall bearing sieve areas with larger tubules is called a sieve plate (Fig. 20-16, B and 20-17, A, B). Although sieve plates may occur on all walls, they are located predominantly at the cell ends. Sieve-tube elements are arranged end-to-end in longitudinal series known as sieve tubes. Thus, the fundamental difference between the Two Types of sieve elements is the presence of sieve plates in sieve-tube elements and their absence in sieve cells.

Fig. 20-15. A. Longitudinal (radial) section of the secondary xylem of yew (Taxus canadensis) showing vertically oriented sieve cells, strands of parenchyma cells, and fibers. Portions of two horizontal rays intersecting the vertical cells are visible. B. Portion of the secondary xylem of yew, showing sieve areas with callose (stained blue) on the walls of the sieve cells, and albuminous cells (see p. 25) forming the uppermost row of ray cells here.

Fig. 20-16. A. Longitudinal (radial) section of the secondary phloem of basswood (Tilia americana) with sieve-tube elements and prominent groups of thick-walled fibers. B. Compound sieve plates (consisting of two or more sieve areas) of basswood sieve-tube elements. Each sieve area is traversed by tubules bordered by cylinders of callose, which is stained blue in this section.

Sieve cells are more primitive than sieve-tube elements. In most seedless vascular plants and gymnosperms, they are the only food-conducting cell type, whereas angiosperms possess exclusively sieve-tube elements.

The walls of sieve elements are generally considered primary. In sections of phloem tissue, the pores of sieve areas and the tubules of sieve plates are typically sealed with callose, a polysaccharide composed of helically wound chains of glucose residues. Its presence here has long puzzled botanists; it seemed counterintuitive that regions designed for intercellular transport should contain a compound that apparently hinders this transport. It is now understood that most, if not all, of this callose is deposited in response to wounding associated with microtechnique preparation.

Unlike tracheary elements, sieve elements retain living protoplasts at maturity, which differ from the protoplasts of all other living plant cells in being completely devoid of a nucleus or containing only its remnants. Furthermore, most mature sieve elements lack a distinct boundary between the Cytoplasm and vacuoles. A young sieve element contains several vacuoles, each separated from the cytoplasm by a tonoplast, or vacuolar membrane. During The final stage of differentiation, the tonoplasts disappear, and the demarcation between cytoplasmic and vacuolar contents ceases to exist. In a mature sieve element, all remaining protoplast components are distributed along the walls; these consist of the plasma membrane, smooth Endoplasmic reticulum adjacent to it, and a small number of Plastids and Mitochondria. Ribosomes, dictyosomes, microtubules, and nuclei are absent.

The protoplasts of dicot (and some monocot) sieve-tube elements are characterized by the presence of a proteinaceous substance known as slime, or P-protein ("P" stands for phloem). P-protein appears in the young element as discrete slime bodies, or P-protein bodies (Fig. 20-17, C, E), which elongate and disperse during the final stage of differentiation. In sections of phloem tissue, "slime plugs" of P-protein are commonly observed near sieve plates (Fig. 20-17, D). Like callose, they are absent in intact cells and are therefore considered an artifact resulting from injury to the Contents of the elements during slide preparation. In normal, mature sieve tubes, P-protein appears to be distributed along the walls, passing from Cell to Cell through the tubules of sieve areas and sieve plates—that is, the tubules are lined rather than plugged by it (Fig. 20-17, A, B). The function of P-protein remains unclear, though some botanists suggest that, together with wound callose, it participates in blocking the tubules upon tissue damage, thereby preventing the leakage of contents from the sieve tubes.

Fig. 20-17. Phloem of pumpkin stem (Cucurbita maxima) shown in electron (A, B) and light (C–E) micrographs. A. Longitudinal section of portions of two mature sieve-tube elements and a sieve plate, showing the parietal distribution of P-protein (see arrows). B. Simple sieve plate (with a single sieve area) between two mature sieve-tube elements. The tubules are open. C. Cross section showing two immature sieve-tube elements. Slime bodies, or P-protein bodies, can be seen in the element on the left, and an immature sieve plate in the element to the upper right. Small, densely stained companion cells are visible. D. Cross section showing mature sieve-tube elements. A slime plug can be seen in the element on the left, and a mature sieve plate in the element on the right. Small, densely stained companion cells are visible. E. Longitudinal section showing mature and immature sieve-tube elements. Arrows indicate P-protein bodies in immature cells.

Sieve-tube members are typically accompanied by specialized parenchymatous companion cells (Fig. 20-17, C–E) containing all the typical components of plant cells, including a nucleus. A sieve-tube member and its associated companion cells share a common origin (arising from the same mother cell) and are interconnected by numerous plasmodesmata. Figure 20-18 illustrates various stages in the differentiation of a sieve-tube member containing P-protein. The functions of companion cells are crucial, as they are largely responsible for the active secretion of substances into the sieve-tube members and their retrieval. This topic will be discussed in detail in Chapter 27, which examines the Mechanism of phloem transport in angiosperms.

Fig. 20-18. Diagram of sieve-tube member differentiation. A. The mother cell divides. B. This results in the formation of a young sieve-tube member and a companion cell. Following division, one or more P-protein bodies appear in the cytoplasm, which is separated from the vacuole by the tonoplast. The walls of the member are thickened at this stage, and the sites of the future sieve-plate pores are represented by plasmodesmata. C. The nucleus degenerates, the tonoplast breaks down, and the P-protein bodies become evenly distributed throughout the cytoplasm, lining the cell walls; simultaneously, the plasmodesmata of the developing sieve plates begin to widen into pores. D. In the mature sieve-tube member, the nucleus and vacuole are absent. All other protoplast components, including the P-protein, are located along the walls; the pores of the sieve plates are open

The sieve cells of gymnosperms are generally accompanied by specialized parenchymatous albuminous cells (Fig. 20-15, B). Although they typically do not share a common origin with the sieve cells from the same mother cell, their functions are believed to be analogous to those performed by companion cells. Like the latter, albuminous cells contain a nucleus and other typical cytoplasmic components of living cells.

The sieve elements of most species are probably short-lived, dying in less than a year after their formation. However, this is not universally true. In the secondary phloem of the American basswood (Tilia americana), some sieve elements remain alive and apparently function in conduction for 5 to 10 years. They are also known to live for many years in perennial monocots, surviving for over a century at the Base of the main stem in certain palms. When sieve elements die, their accompanying companion cells or albuminous cells also die, which provides further Evidence of the close functional relationship between these structures.

Other parenchymatous cells (Figs. 20-15 to 20-17), serving primarily for storage, as well as fibers (Figs. 20-15 and 20-16) and sclereids, are also found in the primary and secondary phloem.

Epidermis

The epidermis is the outermost layer of cells of the primary plant body, forming the protective tissue system of leaves, floral parts, fruits, and seeds, as well as stems and roots that have not yet undergone significant secondary thickening. Epidermal cells are highly diverse in Structure and function. In addition to ordinary epidermal cells, which make up the bulk of its surface area, the epidermis may include Stomata (Figs. 20-19, 20-21), various appendages or trichomes (Fig. 20-22), and other specialized cell types adapted for specific functions.

Fig. 20-19. Scanning electron micrograph of the leaf epidermis surface of the eucalyptus (Eucalyptus globulus). A stoma and numerous fibrous deposits of epicuticular wax are visible

Fig. 20-20. Electron micrograph of a stoma of corn (Zea mays). Cross-section through mature thick-walled guard cells, each adjacent to a subsidiary cell

Fig. 20-21. Developing stoma in surface view (A) and cross-section (B). C. Diagram of a mature stoma, showing its relationship with the epidermis and subepidermal cells. Guard cells arise from an unequal division of a protodermal cell. The smaller of the two resulting cells is called the stomatal mother cell or guard-cell mother cell; its division directly produces the two guard cells (A, B). Following the Formation of the guard cells, the intercellular substance of their common wall swells and dissolves, leaving a pore. This process is accompanied by uneven wall thickening in the guard cells: the walls bordering the pore are much thicker than those adjacent to neighboring epidermal cells. Mature stomata may be raised above the epidermal surface or sunken within it. Often, a large air chamber, or substomatal cavity, is located directly beneath them. Unlike ordinary epidermal cells, guard cells contain METABOLISM/14.html">Chloroplasts

Fig. 20-22. Trichomes. A. Surface view (top) and sectional view (bottom) of a peltate Hair, or scale, from an olive leaf (Olea europaea). B. Dendritic hair of a plane tree (Platanus orientalis). C. Water vesicle of the ice plant (Mesembryanthemum crystallinum). D. Short unbranched hair from a tomato stem (Lycopersicon esculentum). E. Glandular hair from a tomato stem. F. Stinging hair of a nettle (Urtica). It consists of a long needle-like portion and a broad base surrounded by other epidermal cells. Upon contact with a body, the tip breaks off, and the toxic cell contents (histamine and acetylcholine) are injected into the Skin. G–I. Stages in the development of a ROOT hair, a simple tubular outgowth of an epidermal cell

In most plants, the epidermis is single-layered. However, in the protoderm of leaves of certain plants, surface-parallel (periclinal) divisions occur, resulting in a multi-layered (multiple) epidermis (see Fig. 20-23). It is found, for example, in the leaves of such popular houseplants as the rubber plant (Ficus elastica) and Peperomia. Such an epidermis is believed to serve as a water-storage tissue.

Fig. 20-23. Cross-section of the upper portion of a leaf blade of the rubber plant Ficus elastica. A thick cuticle is visible, covering a multi-layered epidermis composed predominantly of large cells. The shovel-shaped structure in the largest epidermal cell consists mainly of calcium carbonate deposited on a cellulosic stalk. Mesophyll cells lie beneath the large, translucent epidermal cells

For the most part, epidermal cells fit tightly together, providing effective mechanical protection for plant parts. In aerial plant parts, their walls are covered with a cuticle that minimizes water loss and consists mainly of cutin and wax (see Chapter 3). In many plants, wax is secreted onto the cuticle surface either as smooth layers or in the form of rods or threads projecting above the surface (so-called epicuticular wax) (Fig. 20-19; see also Fig. 3-11). It is this wax that creates the whitish or bluish bloom on certain leaves and fruits.

Interspersed among the flat, tightly packed epidermal cells are specialized, chloroplast-containing guard cells of stomata (Figs. 20-20 and 20-21) that regulate the opening of stomatal pores on aerial plant parts, thereby controlling the entry and exit of gases, including water vapor (the stomatal opening and closing mechanism is discussed in Chapter 27). Although stomata are present on all aerial organs, they are particularly abundant on leaves. They are frequently associated with specially shaped epidermal cells known as subsidiary cells (Fig. 20-20).

Trichomes perform various functions. Root hairs facilitate the absorption of water and minerals from the soil. Recent studies of plants in arid habitats have shown that dense pubescence on their aerial parts increases the reflection of solar radiation, thereby lowering their Temperature and reducing water Transpiration. Many epiphytes, such as bromeliads, use leaf trichomes to absorb water and mineral nutrients. Conversely, in saltbushes (Atriplex), secretory trichomes remove toxic salts from leaf tissues, preventing their accumulation in the plant. Trichomes can provide insect defense. For instance, in many species, a positive correlation is observed between hair density and resistance to insect damage. The hooked hairs of certain plants impale insects and their larvae (Fig. 20-24). Glandular (secretory) hairs may provide chemical defense.

Fig. 20-24. Nymphs and adults of leafhoppers (Empoasca sp.) are frequently trapped by hooked trichomes on bean leaves (Phaseolus vulgaris). In this scanning electron micrograph, a trichome is seen embedded in the membranous tissue between the leg segments of an adult insect

Periderm

The periderm typically replaces the epidermis of stems and roots during their Secondary Growth. It consists primarily of a protective cork tissue (phellem) made of mature dead cells with heavily suberized walls, a cork cambium (phellogen), and a living parenchymatous tissue known as phelloderm (Fig. 20-25). The phellogen produces cork tissue on its outer surface and phelloderm on its inner surface. The Origin of the cork cambium varies depending on the plant species and plant part. The periderm will be discussed in detail in Chapter 23.

Fig. 20-25. Cross-section of a tree periderm in apple (Malus sylvestris). Here, it consists mainly of suberized layers laid down outwardly in radial rows by the phellogen. A single layer of phelloderm cells lies internal to it.

Conclusion

General Information on Plant Tissues and their constituent cell types is presented in Table 20-1, while the characteristics, localization, and functions of various cell types are summarized in Table 20-2.

Table 20-1. Plant tissues and their constituent cell types

Table 20-2. General characteristics of cell types