Structure and Function of Plants E-Book

Jennifer W. MacAdam

• Herausgeber: John Wiley & Sons
• Kategorie: Wissenschaft und neue Technologien
• Sprache: Englisch

Seitenzahl: 341

Veröffentlichungsjahr: 2009

Beschreibung

Plant anatomy and physiology and a broad understanding of basic plant processes are of primary importance to a basic understanding of plant science. These areas serve as the first important building blocks in a variety of fields of study, including botany, plant biology, and horticulture. Structure and Function of Plants will serve as a text aimed at undergraduates in the plant sciences that will provide an accurate overview of complex plant processes as well as details essential to a basic understanding of plant anatomy and physiology. Presented in an engaging style with full-color illustrations, Structure and Function of Plants will appeal to undergraduates, faculty, extension faculty, and members of Master Gardener programs.

Leseprobe

Contents

Preface

Acknowledgments

Chapter 1 The plant cell

Protoplast

Cell walls

Plasma membrane/cytoplasmic membrane/plasmalemma

Plasmodesmata

Cellular organelles

Variation in cellular structure

Chapter 2 Plant meristems and tissues

Meristems

Tissues formed during primary growth

Ground tissues

Epidermal tissue

Vascular tissues

Chapter 3 Plant roots

The functions of roots

Types of root systems

The organization of root growth zones

Root tissues

Symbiotic nitrogen fixation

Chapter 4 Plant stems

The function of stems

The structure of stems

Internal tissues of the stem

Secondary or woody growth of stems

Chapter 5 Plant leaves and translocation

The function of leaves

The structure of leaves

The development of leaves

Environmental effects on leaf development

Translocation

Chapter 6 Reproduction in flowering plants

Flower structure

Inflorescences

Vegetative reproduction

Chapter 7 Plant nutrition

Soil components

Acidity and alkalinity

Soil nutrients

Macronutrients

Micronutrients

Chapter 8 Plant–water relations

Uptake of water

Movement of water in plants

Transpiration

Drought

Water movement in stems

Chapter 9 Macromolecules and enzyme activity

Chemical bonds

Macromolecules

Chapter 10 Photosynthesis

Light and photosynthesis

Chloroplasts

Photosynthetic pigments

The “light” reactions of photosynthesis

The “dark” reactions of photosynthesis

Adaptations to make photosynthesis more efficient

Chapter 11 Respiration

When and where does respiration occur?

Sources of energy for respiration

Cellular respiration

Chapter 12 Environmental regulation of plant development

Photoperiodism

Other phytochrome-mediated responses

Plant environmental responses requiring cold temperature

Chapter 13 Hormonal regulation of plant development

Auxins

Gibberellins

Cytokinins

Ethylene

Abscisic acid

Chapter 14 Secondary plant products

Types of secondary plant products

Glossary

References

Index

Edition first published 2009

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Library of Congress Cataloging-in-Publication Data

MacAdam, Jennifer W.

Structure and function of plants/Jennifer W. MacAdam. – 1st ed.

p. cm.

Includes bibliographical references and index.

ISBN 978-0-8138-2718-6 (pbk.: alk. paper) 1. Plant anatomy–Textbooks. 2. Plant physiology–Textbooks. I. Title.

QK641.M33 2009

571.2–dc22

2008035480

Dedication

This book is dedicated to scientists in the fields of plant physiology and plant anatomy who continue to uncover how plants work, particularly my colleagues and mentors who communicate their unflagging enthusiasm for these subjects through their own excellence in teaching and research.

Preface

This book is written for anyone who is curious about plants and wants to better understand the plants we use in our fields and gardens for food, and the plants we love for the beauty they add to our lives. It addresses not only what plants do, but why, to provide insight for our interactions with the plants we cultivate, struggle with, and depend on in the natural environments that are our heritage and our future.

This book is intended for casual readers who want to know how plants actually work, not just how to care for a particular plant in a particular climate, and for students of plant science seeking an understanding of plant structure and function. It is hoped that both groups will be well-served by a simple but accurate overview of the subjects of plant anatomy and plant physiology.

The early chapters of the book are descriptive of roots, stems, leaves, and flowers, and the cells and tissues from which these structures are built. Such organs have much in common from plant to plant, but observing their differences helps us understand plants’ adaptations to the often challenging environments in which they developed. The balance of the book describes how plants work: how they extract water and nutrients from the soil, how they use sunlight to create the carbohydrates that are the first link in the food chain, and how they manage to survive and reproduce literally rooted in one spot.

The emphasis of this book is on inclusive explanations, and concepts are illustrated with examples from agricultural and horticultural plants. When used as a text, this material should be supplemented with a hands-on lab where students can observe the anatomical features and physiological processes introduced here. The book is arranged in 14 chapters, a useful length for a one-semester introductory course in plant anatomy and physiology.

Acknowledgments

I thank the many colleagues who generously allowed me to use their drawings and photographs, and the publishers who facilitated my sharing of research figures from the work of numerous other scientists with readers of this book. Adam Black turned sketches of concepts and mechanisms into the illustrations used in this book, and I thank him for his talent and patience.

Chapter 1

The plant cell

We appreciate plants for their beauty and usefulness, and on a different level, for the ability of plant species to adapt to an amazing diversity of climates and soils (two of many abiotic influences) as well as their ability to interact with microbes, animals, and other plants (biotic influences). The differences in characteristics such as stem, leaf, and flower structure that result from these and other adaptations were the original basis for classification of plants into different taxonomic groups. However, for all their differences in overall appearance (morphology), plants have the same basic structures at the cellular level, so we begin by looking at the cellular structures of plants. Figure 1.1 is a simplified illustration of a plant cell, and the structures labeled in Figure 1.1 are discussed in more detail in this and other chapters.

Protoplast

The protoplast is a collective term that includes the plasma membrane and the cellular objects it contains. It is filled with liquid, the cytosol, that bathes the cellular organelles including the nucleus. The protoplast includes all the “living” parts of the cell, so the cell wall to its outside is not included. The protoplast is composed of 60–75% proteins by dry weight.

Cytoplasm

The cytoplasm is the protoplast minus the nucleus. The nucleus directs the work that goes on in the cytoplasm.

Cytosol

The cytosol is the liquid portion (matrix) of the cytoplasm, which surrounds organelles and in which a number of proteins, salts (including nutrient ions), and sugars are dissolved. The cytosol has the thickened consistency of a gel. The cytosol of adjacent cells is continuous, by way of plasmodesmata.

Figure 1.1 Components of a plant cell.

Cell walls

The plant cell protoplast is enclosed by a fibrous wall that grows as the cell expands to its mature size, but which becomes cross-linked and eventually limits the growth of the cell, defining and supporting the cell and collectively providing support for stems and leaves. Some cells, like photosynthetic and storage cells, only have a thin primary cell wall, and other cells have both a primary wall and a thick, lignified and therefore rigid secondary cell wall, either to retain the cell’s shape against the tension of water movement through the plant, as in xylem cells, or to provide concentrated regions of support or protection as in fiber cells or sclerids. The trunk of a tree is made up of concentric layers of water-transporting (xylem) cells with secondary walls that serve both water-carrying and support functions.

Components of the cell wall

Cellulose

The fundamental component of cell walls is cellulose, which in turn is made up of long chains of glucose molecules, from thousands to tens of thousands of glucose units per molecule of cellulose. The chemical structure of glucose is illustrated in Figure 1.2, with each of the six carbon atoms (C) numbered. α- and β-Glucose differ in the orientation of the bonds at C-1. Starch and cellulose are both long chains of glucose, but starch is easily digested by monogastrics, like humans, while the bonds between glucose molecules in cellulose are most commonly broken by enzymes produced by microbes inhabiting the guts of ruminants, such as cattle and sheep (and termites). The difference in these chains of glucose is illustrated in Figure 1.3. Bonds in both starch and cellulose are between the 1- and 4-carbons of successive glucose molecules, but while in starch the orientation of each α-glucose molecule in the chain is the same, in cellulose every other β-glucose molecule is flipped on its horizontal axis.

Cellulose is the “fiber” in paper. Cellulose molecules are grouped together into microfibrils consisting of 50–60 cellulose molecules held together by hydrogen bonds, which are relatively loose bonds but effective in large numbers, as in cellulose (Figure 1.4). Cellulose is such a big molecule that it is synthesized at the plasma membrane rather than inside the protoplasm. Microfibrils are extruded into the extracellular matrix, like toothpaste from a tube (Figure 1.5). Other cell wall components are secreted into the cell wall by way of Golgi vesicles, and assemble around the cellulose microfibrils.

Figure 1.2 The structures of α- and β-glucose, demonstrating the difference in orientation of the –OH at Carbon 1.

Figure 1.3 The structure of starch and cellulose molecules, demonstrating the difference in bonds between the 1- and 4-carbons of α-glucose molecules in starch and the 1- and 4-carbons of β-glucose molecules in cellulose.

Hemicellulose

Hemicellulose also consists of chains of sugars, but the sugars are much more diverse than in cellulose, which contains only glucose. Hemicelluloses are highly branched because of the bonds that form among the sugars that make them up, and they form a network that coats the much larger cellulose microfibrils. Hemicelluloses adhere to cellulose by way of hydrogen bonds. Hemicellulose molecules coating individual cellulose microfibrils become cross-linked or bound together by covalent bonds, which limits cell wall expansion because the cellulose microfibrils can no longer slide past each other and allow the cell wall to grow. In Figure 1.6, the components of the cell wall are illustrated to show hemicelluloses forming cross-linkages between cellulose microfibrils.

Pectin

The middle lamella is the outermost layer of a plant cell and has a high concentration of pectins, which consist of uronic acids, the acidic (and therefore charged) forms of glucose and galactose, and other sugars. The middle lamella is the first boundary formed between what will become adjacent cells during cell division. In cell division, the genetic material of the cell is duplicated and the two groups of chromosomes move to opposite ends of the cell. In Figure 1.7, the middle lamella (yellow) is beginning to form as the boundary between the two new cells. The phragmoplast, a remnant of the organizing structure needed to divide the genetic material, which is shown as groups of white cylinders, is oriented between the daughter nuclei and the developing middle lamella.

Figure 1.4 Hydrogen bonds between cellulose molecules that result in cellulose microfibrils.

Figure 1.5 Cellulose is formed from glucose by a protein complex, cellulose synthase, at the plasma membrane.

After cell division, the primary cell wall forms to the inside of the middle lamella, and also has a relatively high content of pectin (up to 35%). The secondary cell wall, when present, then forms to the inside of the primary cell wall. The components of both walls are formed in the protoplast and secreted via the Golgi apparatus across the plasma membrane.

Extensin

A structural protein (in contrast to enzymes, which are soluble in the cytoplasm or the matrices of the cellular organelles), extensin, forms a network within the cell wall that can become cross-linked, like the hemicellulose network. Extensins make up, at most, about 10% of the cell wall, and were first identified in broadleaf plants (dicots), but proteins with similar functions are found in the grasses (monocots).

Figure 1.6 Components of the primary cell wall include cellulose microfibrils (gray), hemicelluloses (blue and green), and pectins (orange). In cells such as fibers with secondary cell walls, the space between cellulose and other molecules (black) becomes filled with lignin in both the primary and secondary walls.

Secondary cell walls

In structural cells like fibers and in the water-carrying xylem cells, additional cell wall layers are laid down inside the primary cell wall after cell growth stops. These secondary cell walls have a higher cellulose content than primary walls, and may be distinctly layered. In Figure 1.8, which is an electron micrograph of fiber cells, the middle lamella (ML), primary cell wall (CW1), and distinct layers of the secondary cell wall (S1, S2, and S3) can be seen. The walls of these cells also become lignified, a process in which small lignin precursor molecules are secreted into the cell wall and assemble into large, unorganized molecules that displace water (see Chapter 14). The function of lignin is to waterproof xylem vessels and to make cell walls resistant to degradation by invading pathogens. Lignin also greatly increases the rigidity of the cell wall, and is therefore an important component of wood. However, lignin must be extracted for the production of paper, and greatly reduces the digestibility of the fiber cells in plants such as the grasses used as animal feed. Xylem cells, which are the water-carrying cells in roots and shoots, and fiber cells do not contain a protoplasm at maturity and therefore are nonliving cells.

Figure 1.7 Cell division following the duplication of genetic material begins with the formation of a cell plate that develops into the new middle lamella, plasma membranes, and cell walls.

Plasma membrane/cytoplasmic membrane/plasmalemma

The plasma membrane, cytoplasmic membrane, and plasmalemma are all accepted names for the selectively permeable membrane that encloses the living contents of the cell and controls the movement of materials into and out of the cell.

Figure 1.8 Transmission electron micrograph of fiber cells from the stem of Canada yew (Taxus canadensis). Fibers provide structure and protection in leaves, stems, and roots. These cells develop secondary wall layers (S1, S2, S3) inside the primary cell wall (CW1). The middle lamella (ML) can be seen as a dark line between cells (plate 6.1, p. 98, Ledbetter and Porter 1970, used with kind permission of Springer Science and Business Media).

Membranes

Plant cell membranes are primarily made of lipids (fats and oils) and proteins. Membranes are usually described as consisting of a lipid bilayer because of the way the lipid molecules are arranged, but many proteins are embedded in this bilayer (Figure 1.9). In many cases, these proteins act as gateways for regulation of the contents of the cell.

Figure 1.9 The lipid bilayer of the plasma membrane self-assembles from phospholipids, which have hydrophilic, glycerol-containing “heads” oriented outward, and hydrophobic fatty acid “tails” oriented inward. Membrane proteins may extend through the lipid bilayer to act as channels or receptors (green), or they may be bonded to or embedded in the inner or outer surface.

Membrane lipids

The dominant lipids in the plasma membrane are phospholipids, which have a central glycerol molecule with a phosphate molecule attached at one end (the “head”) which is water-loving (hydrophilic) and a water-fearing (hydrophobic) “tail” composed of two fatty acids (making these phospholipids diglycerides). The fatty acids are partly unsaturated, making the lipid bilayer fluid, like oils (see Chapter 9).

Phospholipids spontaneously self-assemble into a bilayer in aqueous solutions like a plant cell. They turn their hydrophilic heads outward, some toward the cell wall that encloses the plasma membrane, and some toward the aqueous cell protoplasm, and turn their hydrophobic tails inward to form a double layer. Membranes are fluid—the molecules they contain can easily move past each other in the membrane—but they are also very stable. Membranes can exclude most charged molecules, like nutrient ions, which allows them to control movement of these nutrients into and out of the cell. Water and the gases oxygen and carbon dioxide, however, can cross the lipid bilayer relatively easily.

Other membranes, especially the internal membranes of the chloroplast, contain a large amount of glycolipids, where the head group contains one or two molecules of the sugar galactose instead of a phosphate, and sulfolipids, with a sulfate instead of a phosphate as part of the head group. In these cases, as for phospholipids, the heads are hydrophilic.

Membrane proteins

Proteins make up as much as 50% of the mass of cell membranes. The amino acid composition of proteins determines how the protein is incorporated into the lipid bilayer. If the protein spans the membrane from inside to out, it is an integral protein. If it is bound only to the inside or outside of the bilayer, then it is a peripheral protein. Proteins must have a region that is hydrophobic to be incorporated into the membrane. These membrane proteins can function in the selective transport of solutes across the membrane if they fully span the lipid bilayer (Figure 1.9), or they can act as enzymes like cellulose synthase, or they may form part of an electron transport chain, which are groupings of many different enzymes that are used in photosynthesis and respiration.

Plasmodesmata

The plasmodesmata are narrow channels between cells through which dissolved substances but not organelles can pass. Plasmodesmata form during cell division, and allow cell-to-cell communication and transport. One is termed a plasmodesma (Figure 1.10). Plasmodesmata are lined with extensions of the plasma membrane and have an inner structure, the desmotubule, which is continuous with endoplasmic reticulum of the two adjacent cells. Dissolved substances can pass between the plasma membrane and the desmotubule to move between cells. The cytoplasm of adjacent cells connected by plasmodesmata forms a continuous living network among cells called the symplast.

In contrast, the apoplast is the nonliving space outside the protoplast and includes the cell wall, the intercellular space, and xylem tissue through which water is transported. The larger spaces between cells in leaves and stems is usually filled with air, although cells are coated with a film of water; in roots, these spaces between cells can contain water being taken up by the plant.

Figure 1.10 Plasmodesmata provide channels between adjacent cells through which dissolved substances can pass.

Cellular organelles

Organelles are membrane-defined compartments inside the cell, each with specific functions. The following are major plant cellular organelles.

Nucleus

The nucleus is the location of the genetic material (DNA, deoxyribonucleic acid) contained in almost all cells; the sieve tubes of the phloem are one exception. The nucleus directs the synthesis of the majority of enzyme production and is therefore considered the control center of the cell, since enzymes perform the work (or metabolism) of cells. DNA is organized into chromosomes in plants, and genes are discrete regions of DNA within chromosomes. DNA is the template used to synthesize RNA (ribonucleic acid), which is termed “transcription.” RNA is exported from the nucleus to the cytoplasm, where it directs the synthesis of proteins, which is termed “translation.” These processes are discussed further in Chapter 9.

Vacuole

Defined by a membrane called the tonoplast, the vacuole is filled with water, and may comprise 80 or 90% of the volume of a mature plant cell. The vacuole enlarges during growth, and this enlargement occurs by water uptake. The vacuole contains dissolved salts, sugars, organic acids, enzymes, and may contain pigments. Vacuolar transport processes are illustrated in Figure 1.11. The energy of a phosphate bond from ATP (see Chapter 11) is used to pump hydrogen ions (H+ or protons) into the vacuole; the higher concentration of H+ in the vacuole reduces the pH of the vacuole compared to the cytosol (for a discussion of pH, see Chapter 7). These H+ can be exchanged for other positively charged ions such as calcium (Ca2+) and balanced by the uptake of negatively charged ions such as chloride (Cl−), nitrate (NO3−), or the organic acid malate.

Figure 1.11 The vacuole functions as storage for nutrients, salts, and organic acids, and is active in other transport processes of the cell.

Endoplasmic reticulum

The endoplasmic reticulum (ER) is a tubular network that is formed from and continuous with the nuclear envelope, and which fills much of the volume of the cytosol. In Figure 1.1, slices through the ER are indicated as ovals covered with red dots that represent ribosomes. Ribosomes may also occur free in the cytosol. The function of the ER is the synthesis of lipids and proteins that are either used to make cellular membranes or exported from the cell. The space enclosed by the membrane layers is called the lumen of the ER. The smooth ER, without ribosomes, is involved in lipid synthesis. The rough ER, with ribosomes, is the site of protein synthesis. Proteins that will leave the cell are made on the rough ER and passed into the lumen of the ER after synthesis. In the ER lumen, they are altered by posttranslational modifications such as the addition of sugars to form glycoproteins, which help determine the specific function and location of the protein. The modified proteins move through the lumen to the smooth ER and small, enclosed pieces of the smooth ER bud off to form transport vesicles, with the proteins inside. The transport vesicles move to the Golgi apparatus.

Golgi apparatus (formerly dictyosomes)

A Golgi apparatus consists of a stack of separate flattened sacs (cisternae) where products of the ER are processed further. Transport vesicles from the ER deliver their contents by fusing with a membrane of the Golgi apparatus. After processing, products of the Golgi are packaged in secretory vesicles that bud off for transport within the cell, or fuse with the plasma membrane to secrete their contents outside the cell (Figure 1.12). Some of the carbohydrates that make up the cell wall are also formed in the Golgi.

Mitochondria

Mitochondria are the site of respiration (Chapter 11), which releases the chemical energy stored in food (most commonly carbohydrates and fats) and transfers this energy to form ATP (adenosine triphosphate), a chemical compound that can be transported to other locations in the cell. The outer membrane of a mitochondrion is readily permeable, and the inner membrane, which has many invaginations called cristae, is very high in protein (70%), and is much more selective. Inside the inner membrane is a matrix that has a high concentration (40–50%) of dissolved protein (enzymes) where key steps in respiration occur (Figure 1.13).

Figure 1.12 The Golgi apparatus functions in the processing and export from the cell of glycoproteins (proteins with sugars attached) synthesized in the endoplasmic reticulum (ER), and in the synthesis and export of some cell wall carbohydrates (hemicelluloses and pectins).

Figure 1.13 Mitochondria are the cellular organelles in which most steps of respiration occur.

Figure 1.14 Chloroplasts are the cellular organelles in which photosynthesis occurs.

Chloroplasts and other plastids

Chloroplasts are football-shaped organelles found primarily in mesophyll cells of leaves and stems and guard cells of the epidermis. These organelles are the site of photosynthesis, the metabolic process that synthesizes sugars in plants. Chloroplasts consist of three membranes: a readily permeable outer membrane, a selective inner membrane, and the thylakoid membrane system. The matrix that fills the space around the outer surface of the thylakoid membranes is called the stroma, and is protein-rich (Figure 1.14). The reactions of photosynthesis that produce sugar occur in the stroma. Chlorophyll is a pigment that is embedded in the thylakoid membranes, which enclose a space called (as with the ER) the lumen. The lumen is where water is oxidized for the generation of the O2 that is a by-product of photosynthesis, and where protons (hydrogen ions; H+) accumulate, subsequently driving ATP synthesis. Some thylakoid membranes occur in stacks called grana. Other plastids are the storage sites for starch (amyloplasts; common in roots; Figure 2.4) and for pigments (chromoplasts), such as those in carrot roots, tomato fruits, and the petals of yellow, orange, or red flowers.

Endosymbiosis

Both chloroplasts and mitochondria are thought to have originated as bacteria (prokaryotic cells that have no nucleus; Figure 1.15a) that invaded or were consumed by eukaryotic cells, which do have nuclei (Figure 1.15b), forming a symbiotic (mutually beneficial) relationship (Figure 1.15c). The invasion by the bacteria that became chloroplasts (Figure 1.15d) followed the invasion by the bacteria that became mitochondria (Figure 1.15e), creating plant cells (Figure 1.15f). Our understanding of endosymbiosis was developed over many years by Dr Lynn Margulis. A unique feature of mitochondria and chloroplasts among plant cell organelles is the presence of inner and outer membranes in both organelles. The outer membrane is thought to have formed from an invagination and eventual budding off of the plasma membrane as the cell tried to contain an invasion by the bacterium. In addition, both organelles contain plasmid (bacterial-type) DNA, and still use it to carry out their own protein synthesis. These invaders benefited from their new surroundings, and were thought to have been tolerated because their presence confers clear advantages to the cells that contain them. Therefore, the relationship is mutually beneficial or symbiotic.

Figure 1.15 These figures show the stepwise evolution of plant cells with mitochondria and chloroplasts via endosymbiosis. (a) Cells exist that have a nucleus but which cannot carry out aerobic respiration or photosynthesis. (b) The cell with a nucleus is invaded by a cell that is capable of aerobic respiration. (c) Rather than digesting the invading bacterium, the cell and the bacterium provide benefits to each other. (d) Other bacteria are capable of photosynthesis. (e) The eukaryotic cell (with a nucleus) may have consumed the bacterium capable of photosynthesis. (f) The further benefits of a resident “organelle” carrying out photosynthesis causes a further symbiosis. This was the basis for the organisms that developed into plants. The bacterial precursor of chloroplasts allowed the new cells to produce sugars, and the bacterial precursor of mitochondria allowed the new cells to produce energy from these sugars much more efficiently.

Figure 1.16 A vascular bundle from a pineapple (Ananas comosus) leaf surrounded by cells for photosynthesis (mesophyll). The vascular bundle contains cells for the transport of water (xylem) sugars and amino acids (phloem). The two fiber caps support the leaf and protect the transport tissues.

Variation in cellular structure

Plant cells have many basic structures in common, but as can be seen in the vascular bundle in Figure 1.16, variation in these structures can result in enormous differences in cellular appearance and function. Most remarkably, these very different cells are connected to one another through plasmodesmata; they divide and expand together as plants develop and work together through the life of the plant.

Chapter 2

Plant meristems and tissues

Plants are usually described in terms of their stem height, their leaf shape, and their flower shape and color. These plant organs are in turn composed of tissues, and grow into their final form through cell division, expansion, and differentiation. In this chapter, the regions of cell division (meristems) that develop into plant organs are described. Tissues are also described in the general terms used by botanists, which apply to similar cells wherever they occur in the plant. In the chapters that follow, we explore how these different types of tissues—specialized for protection, support, or transport—are organized into roots, stems, leaves, and flowers.

Meristems

In contrast to animals that attain a mature size and stop growing, plant roots and stems continue to grow throughout the life of the plant. Perennial plants produce new branches, leaves, and flowers each year. Meristems are regions of cell division that produce new cells for primary and secondary growth in all tissues of the plant. Primary growth results in an increase in the length of shoots and roots, including branches, while secondary growth results in an increase in the girth of woody stems and roots.

Plant meristems

Apical meristems

Apical meristems are located at the tips (apex) of stems and roots, and leave cylindrical roots or stems behind them as they produce new cells. The apical meristem of a grass plant is located at the top of the stem, but if the stem is not elongated, the apical meristem is enclosed in leaves (Figure 2.1). In meristems, cells divide, increase in size, and divide again. Apical meristems are usually protected by the plant; they are covered by a root cap in the soil or enclosed by leaves, bracts, or scales on the shoot (Figure 2.2). There are apical meristems at the tips of all living roots; just a few are indicated by red arrows in Figure 2.3.

Figure 2.1 Diagram of a grass plant with four mature leaves comprising horizontal blades and vertical sheaths, and a fifth elongating leaf emerging through the whorl of older leaves. Leaves are attached to an elongating stem. The apical meristem is green, the intercalary meristems are yellow, and the axillary buds are blue (Dodds 1980).

Figure 2.2 Diagram of the shoot of a broadleaf plant showing an axillary bud at the junction of a leaf petiole and the stem, an intercalary meristem located at the base of a stem internode, and the apical meristem enclosed in young leaves.

Figure 2.3 Diagram of a young onion (Allium) plant (Weaver and Bruner 1927) with the location of some of the many root apical meristems noted with red arrows.

In both roots and shoots, there are slowly dividing groups of initials, the cells that constitute a meristem for the apical meristems. These groups of initials are called the quiescent center in roots and the central mother cells in shoots. Infrequent divisions by initial cells produce the cells that divide more actively to produce the primary plant body. Mutations are more likely to occur the more often a cell divides, so if a frequently dividing cell mutates, it can be replaced from the initials “storehouse” where genetic information is still correct.

Axillary buds

Axillary buds are located in leaf axils, the junctions of the leaf, and the stem (Figures 2.1 and 2.2). The activity of these meristems is similar to that of apical meristems, but they produce branches or flowers. The pattern of branching of a stem is established by the axillary buds produced by apical meristems, and regulated by the hormonal activity of the apical meristem.

Intercalary meristems

Intercalary meristems are located between mature tissues, most commonly in stems (Figure 2.2), and leaves of grasses. These meristems are located at the base of leaves and stem internodes; the activity of intercalary meristems results in the regrowth of grasses following mowing or grazing. In Figure 2.1, the intercalary meristems of the grass stem are colored yellow.

Lateral meristems

Lateral meristems form a cylinder around the stems and roots of woody plants and produce secondary growth. One of these is the vascular cambium, which produces the secondary xylem and phloem; another is the cork cambium, which produces the cork layer that replaces the epidermis of woody perennial shoots and roots.

Tissues formed during primary growth

1. Ground tissue is the group of simple tissues that comprise the bulk of the plant.

2. Epidermal tissue is the “skin” or outer covering of shoots and roots.

3. Vascular tissues are the “plumbing” system, carrying water, amino acids, and sugars throughout the plant.

Ground tissues

Parenchyma

Parenchyma cells have a relatively thin primary wall and form the bulk of many plant parts, like the cortex and pith of the stems of herbaceous plants, and the cortex of roots. In the taproots of starch-storing plants, starch is stored in cortical cells in membrane-bound organelles called amyloplasts. In Figure 2.4, the walls of starch-storing cells are transparent, and amyloplasts can be seen filling the cells. The winter-hardy legume cicer milkvetch (Astragalus cicer) stores starch under ground in rhizomes, and amyloplasts can be seen in the iodine-stained rhizome section in Figure 2.5. The mesophyll (photosynthetic) tissue of leaves is also parenchyma tissue. Mesophyll cells are an example of ground tissue that has an important function in plant metabolism.

Figure 2.4 Cells from a Dutch iris (Iris hollandica) bulb scale filled with amyloplasts and stained for starch with iodine.

Figure 2.5 Cross section of a cicer milkvetch (Astragalus cicer) rhizome stained with iodine. Many of the cells near the vascular bundles contain dark-stained amyloplasts.

Figure 2.6 Cross section of a begonia stem. Collenchyma cells, located just inside the epidermis, have primary walls thickened at triangular cell junctions. Walls are stained dark red in this photograph.

Collenchyma

Collenchyma cells have primary cell walls that are thickened, often at cell junctions, appearing as darkly stained triangles in Figure 2.6. These cells resemble fibers because they are very elongated (up to 2 mm in length) and they provide support, but their walls do not become lignified. Collenchyma cells are found in growing tissue of the shoot, and they are alive at maturity. The stalks of celery (Apium) and rhubarb (Rheum rhabarbarum) are actually petioles (the stalk of a leaf) that contain strands of collenchyma tissue (the strings in celery) for support. Because these cell walls are not lignified, they are not rigid, and depend therefore on turgidity (water under pressure) to assist in their support function. This lack of lignification is why celery wilts as it dries while a woody tissue does not.

Sclerenchyma

Sclerenchyma cells also provide support, but have thick secondary cell walls inside their primary cell walls that become lignified at maturity, adding considerable rigidity. Sclerenchyma cells are often dead at maturity. There are two types of sclerenchyma cells: fibers and sclerids. Fibers are long, slender sclerenchyma cells that can sometimes be used to produce cloth like linen, or rope like hemp. Cotton (Gossypium hirsutum) fibers, however, are trichomes or extensions of epidermal cells. Fibers often occur alongside vascular tissue in plants, providing protection and support. In Figure 2.7, the walls of groups of fibers of a young flax (Linum usitatissimum) stem appear as white rings. The cellulose in fiber cells of grasses and other forages can be utilized by rumen microbes in the digestive system of grazing animals, but the greater the lignification of these cell walls, the less digestible the feed is. Sclerids are short and of variable shapes; sclerids can be encountered as the grit in pear (Pyrus communis) fruit and they form the shell of nuts such as walnut (Juglans). Figure 2.8 shows a still-living pear sclerid with a primary (CW1) and secondary (CW2) wall. This cell still retains its nucleus (N), mitochondria (M), and plastids (P). There are also islands of the cytoplasm seen in the CW2 with endoplasmic reticulum (ER). The inset shows the middle lamella and primary wall with plasmodesmata (Pd) coinciding with an area of cytoplasm.

Figure 2.7 Cross section of a young flax (Linum usitatissimum) stem. Fiber cells appear as white rings and are grouped in semicircles to the outside of the phloem.

Figure 2.8 Transmission electron micrograph of a living sclerid from pear (Pyrus communis) fruit. A primary cell wall (CW1) encloses a still-forming secondary cell wall (CW2). The protoplast contains a nucleus (N), mitochondria (M), plastids (P), and endoplasmic reticulum (ER). The inset shows the middle lamella and walls of two sclerids crossed by plasmodesmata (Pd) connecting the cytoplasm of the two cells (Plate 6.2, p. 102, Ledbetter and Porter 1970, used with kind permission of Springer Science and Business Media).

Epidermal tissue

The epidermis is a continuous protective layer of cells on the outer surface of both roots and shoots. In the shoot, epidermal cells are coated with a layer of waxy cutin (the cuticle) over their outer walls to limit water loss from the surface. In Figure 2.9, the cuticle of yucca appears as a gray layer continuing from the epidermis across guard cells of the stoma. The waxy cuticle causes water to bead on the surface of leaves, necessitating the use of a surfactant when applying compounds to leaf surfaces. The epidermis is usually just one cell thick except in shoots of succulents such as jade (Crassulaovata) or rubber plant (Ficus elastica), where the extra epidermal layers are used for water storage. Epidermal cells typically do not have chlorophyll and are transparent, but their vacuoles may contain other pigments such as anthocyanins which absorb ultraviolet light and therefore act as a sunscreen.

Figure 2.9 Epidermis (red) and stoma of pineapple (Ananas comosus) with a thick cuticle (gray) covering the outer surface of the epidermis to prevent evaporation.

Pores in the epidermis are termed stomata (one is a stoma) and are composed of two guard cells that are specialized epidermal cells that control the movement of CO2 and, in drought conditions, of water into or out of the leaf. The guard cells are flanked by two or four subsidiary cells that assist in stomatal function, and the guard cells, inner air space, and subsidiary cells constitute the stomatal complex (Figure 2.9). The leaves of most plants have several thousand stomata per square centimeter. In dicots, stomata tend to be on the lower sides of leaves, while in grasses, they are more evenly distributed on both the upper and lower leaf surfaces. A replica of the lower epidermis of English ivy (Hedera helix; Figure 2.10) shows that the frequency of stomata (arrows) is high relative to the frequency of epidermal cells in dicots.

The swelling of guard cells that leads to stomatal opening is caused by the uptake of potassium (K+) and other charged substances into guard cells. The guard cells of dicots are “kidney”-shaped, and the stoma opens as the guard cells swell because of the pattern of reinforcement of the cell walls. Monocot guard cells are “dumbbell”-shaped when they are swollen, and open because the ends of the cells enlarge and force the centers of the cells apart. The transparent lower epidermis peeled from a leaf of the grass tall fescue (Lolium arundinaceum; Figure 2.11) shows rows of stomata (arrows) that occur on either side of each vein. Also notice the barbs along the margin of the leaf, which act as a defense against grazing. Ledges on some guard cells reduce evaporation by helping to maintain still air immediately above the stomatal opening, and also keep liquid water from entering stomata.

Figure 2.10 Replica made by coating the lower epidermis of an English ivy (Hedera helix) leaf with clear nail polish and lifting it from the leaf with cellophane tape. Some of the many stomata are indicated by red arrows.

Leaf rolling in grasses is caused by specialized epidermal cells called bulliform cells which are located in groups between the ridges formed by veins. In Figure 2.12, a section of an unstained tall fescue leaf, the groupings of transparent bulliform cells (under the red arrow) can be seen between mesophyll cells filled with green-pigmented chloroplasts. Drought stress causes these large, water-filled cells to lose water and shrink, which causes the leaf to roll, minimizing the exposed surface area and reducing further water losses.

Trichomes are specialized epidermal cells in the form of hairs. Cotton fibers are trichomes of cottonseed epidermal cells, which can grow up to 6 cm (2.5 in.) in length. Root hairs such as the maize (Zea mays) root hairs in Figure 2.13 are also trichomes. In this case, root hairs increase the surface area for uptake of water in young roots. Trichomes can deter insects from attacking leaves by providing a physical barrier or in some cases by impaling insects that crawl across the leaf. Other trichomes contain irritating chemicals or sticky substances that entrap or scare off attackers (e. g., stinging nettle) and still others send distress signals to attract predators of the plant pest.

Figure 2.11 Lower epidermis peeled from a leaf of tall fescue (Lolium arundinaceum). The stomata in one file of epidermal cells are indicated with red arrows.

Vascular tissues