Acerca del autor

David F Cutler is currently an Honorary Research Fellow at the Royal Botanic Gardens, Kew, a Visiting Professor in Botany, Reading University, and an Honorary Lecturer, Imperial College, London University. He was formerly Deputy Keeper and Head of the Plant Anatomy Section, Jodrell Laboratory, RBG Kew. He is currently President of the Linnean Society of London. His research is in pure and applied plant anatomy, including systematic anatomy of angiosperms, functional aspects of plant structure and identification of fragmentary plant material.

Ted Botha is Professor of Botany and was Head of Department at Rhodes University in Grahamstown, South Africa. His research into plant structure and function is recognized internationally as well as locally, by the National Research Foundation. He serves on the organizing committees for several international plant structure–function groups, and is a former President of The South African Association of Botany. His contribution to teaching at Rhodes University was recognized through the award of the Vice Chancellor’s Distinguished Teaching Award in 2005.

Dennis Wm Stevenson is currently Rupert Barneby Curator and Vice President for Botanical Science at the New York Botanical Garden. He also serves as editor of the journal Botanical Review, has published over 200 peer-reviewed papers, and edited 11 books ranging from horticulture to paleobotany to genomics. He holds adjunct faculty positions at City University of New York, Columbia University, Cornell University, New York University, and Yale University.

De la contraportada

This indispensable textbook provides a comprehensive overview of all aspects of plant anatomy and emphasizes the application of plant anatomy and its relevance to modern botanical research.

The companion website, ‘The Virtual Plant’, offers a collection of high quality photographs and scanning electron microscope images giving students access to the microscopic detail of plant structures essential to gaining a real understanding of the subject. Exercises for the laboratory are also included, making this work an indispensable resource for lectures and laboratory classes. Vist: http://virtualplant.ru.ac.za/Main/virtual_Cover.htm to access these resources.

Plant Anatomy is an essential reference for undergraduates taking courses in plant anatomy, applied plant anatomy and plant biology courses; and for researchers and postgraduates in plant sciences.

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Plant Anatomy

John Wiley & Sons

Chapter One

General background

Because each organ of the plant will be discussed in detail in later chapters, this section is intended only to be a reminder of basic plant structure and arrangements of tissue systems. It is not intended to be comprehensive, and by its very nature it oversimplifies the complex and wide range of form and organization existing in the higher plants. When a specialized term is first used, it is normally defined. The glossary forms an essential part of the book, and should be consulted if the meaning of a term is not clear.

This book concentrates on the vegetative anatomy of land plants, and in particular on monocotyledons and dicotyledons (flowering plants, angiosperms, with the seeds enclosed in carpels). Some anatomical features of conifers (gymnosperms - plants with seeds but without carpels fruits, enclosing the seed) are also described. Monocotyledons (Fig. 1.1) are flowering plants that when the seed germinates start life with one seed leaf, and lack the tissues that form new (secondary) growth in thickness, the vascular cambium, and a long-lived primary root. Examples include the grasses, orchids, palms and lilies. Dicotyledons (Fig. 1.2) are also flowering plants but have two seed leaves, and like the conifers have stems that generally have the ability to grow in thickness through a formal vascular cambium, and have a long-lived primary root. Examples of dicotyledons include the bean, rose and potato families, and the conifers include such plants as pines, larches and araucarias. There are, of course, other features that distinguish the angiosperms from gymnosperms (e.g. reproductive structures and reproductive cycle).

The plant organs are shown in Figs 1.1 and 1.2. Most land plants have roots, which anchor them in the ground, or attach them to other plants (as in epiphytes). Roots also absorb water and minerals. Roots first arise in the embryo and are there attached to the stem through a specialized region called the hypocotyl. Later in development if growth in thickness occurs, the hypocotyl becomes obscured. Many species grow additional roots, called adventitious roots, because they arise from other parts of the plant (although some roots themselves canal so give rise to adventitious roots, but these do not develop from the normal sites for secondary roots). When leaves are present, they arise from the stem, either from the apical meristem (see next chapter), or from axillary bud meristems. Their particular arrangement (phyllotaxy) is usually recognizable, for example opposite one another, alternate or in an obvious spiral. Buds may be present in the axils of leaves, that is, between the leaf and the stem, close to where they join. Sometimes buds develop from other parts of the plant; these are called adventitious buds.

Adaptation to aerial growth

To understand the structure-morphology and anatomy-of land plants we have to remember that plant life started from single-celled organisms in an aquatic environment. There are still many thousands of different species of unicellular algae both in water and exposed-on tree trunks, leaves, soil and rock faces for example, in suitably moist places. Evolution of algae in the water has produced some very large, multicellularforms, for example Laminaria species, kelps. These large plants are fine in water, but lack the adaptations necessary for terrestrial life. They need to be bathed in water, which is a source of dissolved nutrients. Because they can absorb nutrients over most of their surface area, there is no need for a complex internal plumbing system, like the xylem (woody tissue) and phloem (cells adapted to conduct synthesized materials in the plant) in vascular bundles of land plants. They lack roots, but have holdfasts, structures adapted to anchor them to a firm substrate, but which are not absorbing organs for minerals and water, such as roots usually are. They lack a waterproof covering, a modified outer layer of epidermal cells of land plants, and rapidly desiccate if exposed to the air. Their mechanical support comes from the surrounding water, so they do not need the woody tissue (xylem) or fibres (elongate, thick-walled cells with tapered ends whose cell walls become strengthened with lignin, a hard material, at maturity; form part of the sclerenchyma) of land plants. True, they are tough and very flexible, and most can survive violent wave action. Even their reproduction depends on the release of male and female gametes into the water around them.

Some types of land plants still rely on a film of water for their male gametes to swim in to reach the female gamete and effect fertilization, for example mosses and ferns, but the higher plants like gymnosperms and angiosperms have their male gametes delivered in a protective package, the pollen grain, to a receptive female part of the cone or flower.

There is a very wide range of land habitats, and land plants show a remarkable range of shapes and sizes. This book is mostly about the anatomy of flowering plants (angiosperms), and the vast majority of these share distinct vegetative organs that are readily recognized. They are leaf, root and stem (Figs 1.1, 1.2). These organs cope with the need to obtain, transport and retain enough water to help prevent wilting, carry dissolves minerals and keep the plants cool when necessary. Most land plants contain specialized cells and tissues for mechanical support and others for movement within the plant of materials they synthesize. The tough skin (epidermis, together with a cuticle and sometimes waxy materials) prevents water loss but permits gas exchange. Small pores in the epidermis of most leaves and young stems can be opened and closed and regulated in size (see Chapter 6 for details). These are called stomata and they regulate the rate of movement of water and dissolved minerals through and out of the plant. Sometimes the epidermis is the main part of the mechanical system as well, and holds the main leaf or stem material inside under hydraulic pressure.

In many plants, the strength of the 'skin' is supplemented by tough mechanical cells arranged in mechanically appropriate areas. These are forms of sclerenchyma cells with lignified walls: fibres which are elongated cells and sclereids, which are usually relatively short; a range of types exists (see the Glossary). Collenchyma is also a supporting or mechanical tissue which occurs in young organs and in certain leaves; the walls are mainly cellulosic. Here walls are thickest in the angles between the cell walls, or in lamellar collenchyma wall thickening is found mainly on anticlinal cell walls; see below for details.

Plants submerged in water are afforded some protection from damaging ultraviolet (UV) light. Land plants need other mechanisms to prevent UV damage. The green pigment, chlorophyll, is readily damaged by UV. Since this pigment and its cohort of specialized enzymes is responsible for transforming the energy of sunlight through its action on C[O.sub.2] and [H.sub.2]O into sugars, the starting point for nearly all stored organic energy on earth, it is vitally important that the UV screening methods developed are effective.

All green plants need light for photosynthesis. Plants have evolved different strategies which bring leaves into a good position for obtaining the sunlight. Some (annuals, ephemerals) put out their leaves before others neighbouring plants, complete their annual or shorter cycle and form seed for the next generation. Others retire to a dormant form (some perennials and biennials) at a time when they may be shaded by taller vegetation. Many species develop long stems or trunks and expose their leaves above the competition (some are annuals or biennials and but most are perennials). Some species do not have mechanically strong stems, but use the support provided by those which do, climbing or scrambling over them (they can be either annuals or perennials). Biennials are plants with a two-year life cycle. They build up a plant body and food reserves in the first year, and then flower and fruit in the second.

In summary, the main factors which all terrestrial plants with aerial (above-ground) stems and their associated leaves have to overcome are:

1 Mechanical, i.e. support must be provided in one way or another so that a suitable surface area with cells containing chloroplasts can be exposed to the sunlight to intercept and fix solar energy. These chlorenchyma cells may be on the surface, or just beneath translucent layers of cells. See below for more detail of the cell types that give mechanical strength. Secondary growth in thickness is another strategy that provides mechanical strength to parts both above and below ground. The growth in thickness may be relatively small in annuals, but in perennial plants it may be extensive, and requiring the use of large quantities of energy in its production. When present, the way secondary growth occurs differs between monocots and dicots.

2 Risk of excess water loss, i.e. they must be provided with protection against too much water loss from the exposed surfaces. This is generally done by a combination of a waxy outer layer and a fatty cuticle above an epidermis (the outer skin). Because water has to evaporate from some exposed surfaces so that movement of water and dissolved minerals can take place through the plant (transpiration), most leaves, and stems which retain the epidermis, have regulated pores, stomata, which can be opened and closed in response to prevailing conditions.

3 The ability to move water and minerals from the soil (transpiration) through the roots to regions where they can be combined with other materials to build the plant body, and the movement of synthesized food material from the site of synthesis to places of growth or storage and from the stores to growing cells (translocation). Of particular interest is the level of structural and physiological control of the phloem loading process. Epiphytes are attached by their roots to other plants, and obtain their water and minerals in different ways.

4 Reproduction, placement of reproductive organs enabling the pollen or gamete receptor mechanism to operate successfully, and after fertilization and spore/seed production, ensuring dispersal of the propagules.

The first three issues outlined above are dealt with by well-organized (if complex) systems in the higher plants, and will be summarized here. The fourth, reproduction, is outside the scope of this book. Secondary growth is discussed in Chapter 2, under lateral meristems.

Mechanical support systems

1 Using inflated or turgid, thin-walled cells (parenchyma): these are present in growing points, and the cortex and parenchymatous pith of many plants. They constitute the bulk of many succulent plants, for example, Aloe, Gasteria leaves, Salicornia from salt marshes and Lithops from desert regions. The cell wall acts as a slightly elastic container; internal liquid pressure inflates the cell so that it becomes supporting, like the air in an inflated car tyre. Its support properties depend on water pressure, so a water shortage can lead to a loss of support and wilting. Some fairly large organs can be supported by this system, but they usually rely on the additional help of devices that reduce water loss, such as a thick cuticle, and perhaps also thick outer walls to the epidermal cells, and specially modified stomata. A strong epidermis is particularly important, because it acts as the outermost boundary between the plant cells and the air. A split in the skin of a tomato, for example, rapidly leads to deformation of the fruit, or a cut in the succulent leaf of a Crassula or Senecio rapidly opens up. Not many plants rely on the turgid cell and strong epidermis principle alone.

2 Both monocotyledons and dicotyledons and have specially developed, elongated, thick-walled fibres, in suitable places, which assist in mechanical support. Alternatively, they have especially thick-walled, generally elongated parenchyma cells (also sometimes called prosenchyma); or, in those primary parts of the stem where growth in length is continuing, collenchyma cells may be present. Although there are only a few common ways in which specialized mechanical supporting cells are arranged in the stem, leaf or root, it is the variations on these themes which are of particular interest to those who have to identify small fragments of plants, or make comparative, taxonomic studies. The variations will be dealt with in detail in the chapters dealing with each organ. Obviously, to be effective the mechanical system must be economical in materials, and the cells must not be arranged in such a way as to hinder or impede the essential physiological functions of the organs.

The mechanical systems develop with the early growth of the seedling. Whilst turgid cells are the only means of support at first, collenchyma may rapidly become established, particularly in dicotyledonous plants. This tissue is concentrated in the outer part of the cortex, and is frequently associated with the midrib of the leaf blade, and the petiole.

Collenchyma is essentially the strengthening tissue of primary organs, or those undergoing their phase of growth in length. The cells making up this tissue have thickened cellulosic walls at their angles, are rich in pectin and are often found with chloroplasts in their living protoplasts.

Sometimes the only other mechanical support is provided by the wood (xylem) composed of tracheids (imperforate tracheary elements, i.e. cells with intact pit membrane q.v., between them and adjacent elements of the vascular system), as in most gymnosperms, or by the tracheids, vessels (tube-like series of vessel elements or members with perforate common end walls; vessel elements are the individual cell components of a vessel, with perforated end walls) and xylem fibres of the angiosperms. However, far more commonly there are also fibres outside the xylem (extraxylary fibres) which are arranged in strands or as a complete cylinder, such as in Pelargonium which can give considerable strength to herbaceous plants, and particularly in herbaceous monocotyledons in their stems and leaves. The much elongated fibres, with their cellulose and lignin walls, are not so flexible and do not stretch as readily as does collenchyma; consequently they are often found most fully developed in those parts of organs that have ceased growth in length.

Figure 1.1 shows some fibre arrangements in monocotyledon stems and leaves. In the leaf, fibres commonly strengthen the margins (e.g. Agave) and are found as girders or caps associated with the vascular bundles. In the stem, strands next to the epidermis can act rather like the iron or steel reinforcing rods in reinforced concrete. Together with a ribbed outline that they often confer on the stem section, they produce a rigid yet flexible system with economy of use of strengthening material.

Tubes are known to resist bending more effectively than solid rods of similar diameter; they also use much less material than the solid rod. It is not surprising then, that tubes or cylinders of fibres commonly occur in plant stems. They may be next to the surface, further into the cortex, or may occur as a few layers of cells uniting an outer ring of vascular bundles (Fig. 1.1).

The various arrangements within leaves, stems and roots will be discussed in more detail in Chapters 4-6. Mention must be made here that in some monocotyledon stems individual vascular bundles scattered throughout the stem can each be enclosed in a strong cylinder of fibres, which form a bundle sheath. Each bundle plus its sheath then acts as a reinforcing rod set in a matrix of parenchymatous cells and with a sieve cell centre so the whole unit acts as a hollow cylinder with maximum efficiency of both transport and strength.

(Continues...)

Excerpted from Plant Anatomyby David F. Cutler Ted Botha Dennis Wm. Stevenson Copyright © 2007 by David F. Cutler. Excerpted by permission.
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