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The plant body of a typical angiosperm consists of two major organ systems: the root and the shoot. The shoot is composed of stems, leaves, and flowers, and each major organ in turn is composed of tissues that represent collections of one or more specialized cell types. All growth and organ formation of the plant body are initiated and controlled by regions of active cell division termed “meristems.” Meristems are composed of two categories of cells: the dividing initials and the immediate undifferentiated derivatives of cell division. The derivatives subsequently become differentiated and change into different types of mature cells and tissues. Following fertilization, the plant body begins development as a single-celled zygote that undergoes embryogenesis. After a series of early cell divisions, a globose proembryo is formed. During the later stages of embryo development, the embryonic axis of the plant body is formed along with the root and shoot apical meristems from which the rest of the plant body develops following seed germination. One of the most distinctive structural features of plant cells is the surrounding cell wall, typically composed of carbohydrate cellulose. Cell walls play important roles in plant development, cell-to-cell communication, physiology and environmental adaptation, and stress response.

Plants carry out numerous complex processes constituting their physiology to maintain themselves as living systems. Many of these physiological processes are closely related to plant structure. The basic activities are photosynthesis, respiration, metabolism, nutrition, and translocation and mobilization of water and photosynthetic solutes. The principal functional units of the phloem tissue of flowering plants include the major conducting conduit called the sieve tube element, along with its associated companion cells and a variety of integrating phloem parenchyma cells. The structure of the minor veins plays an important role in the process of loading sugars in the leaves. A symplastic mode of loading is indicated by high numbers of plasmodesmata. In some families, the sheath cells around the minor leaf veins are modified as phloem and xylem transfer cells. These cells are characterized by unlignified wall ingrowths. The leaves of some monocotyledons and dicotyledons possess enlarged vascular bundle sheath cells around the minor veins. The long-distance transport of water from roots to leaves occurs within the xylem in a series of highly specialized cells called tracheary elements.

Gene mutations provide the primary means of understanding gene structure and function in plants. A large number of mutants have been identified that affect a wide range of anatomical traits, ranging from embryogenesis to the adult plant. Numerous mutants have been isolated that directly control the root and shoot development, meristem behavior, epidermal cell formation, root hair and trichome development, and vascular tissue differentiation. Hybridization occurs when two different, naturally occurring plant populations show a breakdown in reproductive isolation and an exchange of genes between diverse individuals takes place. In several cases, the hybrids are more or less intermediate in quantitative features. The Aloineae are a well-defined tribe of the monocotyledonous family Liliaceae that provides an insight into the inheritance of leaf anatomical characters. Wood quality is determined by the combined effects of several interrelated and variable wood properties. Heritable wood properties include the amount of juvenile and mature wood, grain pattern, individual cell size and wall characteristics, and chemical properties of the cell wall. Some wood properties can be manipulated during processing, but others are stable features that can only be altered by a focused tree-breeding program.

Systematics is a broad field of inquiry that uses the characteristics and data from many disciplines for describing, naming, classifying, identifying, and determining relationships among plants. Anatomy broadens the base of systematic investigations by providing another set of characters that indicate relationships in harmony with external features. As with morphological characters, some anatomical features are useful in determining phylogenetic trends, whereas others delimit families, genera, or species. The secondary xylem has proved to be highly significant in elucidating higher-level plant phylogenies. One of the greatest achievements of plant anatomy has been the elucidation of the origin and subsequent trends of specialization of the angiosperm vessel element. The angiosperm vessel elements have been phylogenetically derived from scalariform-pitted tracheids by the progressive loss of pit membranes in the regions of tracheid overlap. The methodology of cladistic analysis has become the most widespread approach to modern phylogenetic reconstruction. A hypothesis of the relationship between two or more taxa is based on their shared possession of a derived character state. Anatomy provides important derived character states for the recognition of closely related taxa.

This chapter describes the various anatomical responses of higher plants to wounds or infections, and includes an analysis of how plants have evolved growth forms to cope with the changing stress patterns in natural environments. Multiple defenses occur in plants in response to stress and invasion of pathogens. Some of these defenses are structural and are often accompanied by chemical alterations in cells and tissues. Plant defenses can be pre-infectional or post-infectional responses. Defenses can be passive in the sense that they are preformed barriers to infection or active mechanisms of resistance that are formed in direct response to an injury or the activities of harmful invaders. The cuticle provides a very effective passive defense. The principal active anatomical response of plants to wounds and infections is to wall off the injured or diseased region. Lignification of cell walls and callose deposition are other common responses of plant tissues to injury. Several structural features that can potentially increase plant resistance include thickening and lignification of cell walls, and increasing the amount of sclerenchymatous tissue, in addition to foliar pubescence, surface waxes, and mineral accumulation. Laticifers can also serve a defensive function by clogging the mouthparts of insects attempting to feed on a latex-producing plant.

Many species of plants yield natural fibers that are interwoven to manufacture a variety of valuable products including fabrics, cordage, paper, and brushes. Plant fibers are also incorporated into various composite materials in which a matrix phase is reinforced with fibers. At maturity, fibers typically lose their protoplasts, and thus play no role in metabolism. Fibers lend strength and resilience to the materials made with them. Fibers positioned in the phloem or cortical regions of the stem and leaf fibers of monocotyledons include a number of commercially important cells that are very strong and flexible and which are used in the manufacture of various products. In response to being naturally or experimentally displaced from their normal growth orientation, the main axis or lateral branches of actively growing stems are stimulated to form wood with an eccentric growth pattern that is composed of cells having both an atypical structure and a chemical composition. This secondary xylem is known as reaction wood. The reaction wood of gymnosperms forms on the lower side of the woody cylinder and is known as compression wood. Tracheids of compression wood possess walls with visible discontinuities and abnormal microfibril angles.

Many plants produce secretions that serve as animal attractants or food rewards in the form of nectar, oils, and food bodies. Some chemicals provide plants with their only means of defense by discouraging the visitation of harmful insects and herbivores. Human beings use these products as condiments, medicines, fragrances, dyes, and ornaments. Secretory products are composed of a variety of organic molecules and are secreted by plants in specialized secretory structures and secretory cells. Different types of secretory structures often produce chemically identical products. Many plants possess elongated secretory canals, ducts, or cavities that become lined with specialized secreting epithelial cells. External secretory structures include glandular trichomes, colleters, glands, nectaries, and hydathodes. Glandular trichomes vary widely in structure but typically possess a basal cell, stalk, and unicellular or multicellular secretory head. At maturity, the head contains an enlarged subcuticular space that becomes filled with essential oil or resin as secretion proceeds. Stinging hairs represent another trichome type and produce skin-irritating substances. Internal secretory structures encompass oil cells, secretory cavities and canals, and laticifers.

One form of evidence of past human activity is the recovery of plant materials such as seeds, fruits, stems, and wood that have been routinely utilized as food, in construction, as clothing, in religious or other ceremonies, or as fuel. A careful study of the anatomical record of these materials can be used to help reconstruct an accurate picture of the life of prehistoric people, and to extend the knowledge of the undocumented climatic conditions under which they lived. Preserved wood and charcoal samples are especially valuable in dating historical buildings and archaeological sites and for interpreting past climates by studying the successive growth rings within a tree trunk. Tree-ring widths vary with climate. Abundant precipitation and soil moisture result in wide rings, whereas under drought conditions or less-than-average rainfall the rings are narrow or absent. Fire, pests, and disease also leave their marks within the ring sequence. The basis for all tree ring studies is the construction of a regional master chronology that extends back in time without interruption. The requirement of identifying and dating annual growth rings is achieved by utilizing a technique known as cross dating.

Forensic botany—the use of botanical evidence in courts of law—has proven to be useful in both civil and criminal cases. Evidence obtained from plants—including structural information—has gained wide acceptance as either direct or indirect evidence. The careful microanalysis of plant cells, tissues, or organ fragments at both the light-microscope and scanning-electron-microscope levels can provide compelling evidentiary arguments for resolving a variety of legal questions. Food plants also provide a source of forensic evidence. A record of what the victim ate just before death can be obtained by identifying the plant cells that are present in stomach contents. In some cases, the exact restaurant that served the last meal can also be located. The histological features of wood or wood fragments enable wood to be readily identified by family and genus. Obtaining diet and nutritional information on the domestic and wild herbivores represents a significant major application of plant anatomical research. Detailed food habit data and preferences are important because they relate to animal population size and fluctuation, range, and other complex interrelationships. Histological analysis of the herbivore diet has the advantage that materials can be microscopically examined at any stage of digestion, both recently consumed and as fine particles obtained from stomach contents and fecal droppings.

Wood technology deals with the physical appearance, anatomical structure, and mechanical and chemical properties of wood. All these features affect wood figure, strength, shrinkage patterns, and deformation. The physical properties of wood are derived from its particular molecular constitution, its cell wall organization, and the interaction of its structural units. External influence such as water content, shrinkage, and swelling, which may cause warping and checking, also affect the physical properties of wood. Weight is controlled by two major variables: the amount of solid cell wall material present and the quantity of water in the sample. Water is located in wood as bound water in the cell wall and as free water in the numerous spaces of the cell lumina and intercellular cavities. As the walls of individual cells lose water, they become thinner, and shrinkage of the wood occurs. Due to the stresses that are incurred in different directions by individual wood cells during the drying process and the complicated patterns of shrinkage that result, dried lumber tends to warp. The mechanical properties of wood are the multitude of measurable expressions that result from applied forces. These include strength properties, stiffness, elasticity, hardness, and toughness.

This chapter deals with details of the internal organization and the arrangement of cells and tissues within the three major organs of the primary plant body: the stem, the leaf, and the root. All stems are characterized by repetitive sequences of external nodes and internodes. The leaves and associated axillary buds are positioned at the nodal regions. Leaves are initiated as lateral protuberances on the domelike stem apex, and they occur in a precise arrangement—or phyllotaxis—on the stem. A mature leaf is composed of a leaf base, a stalk-like petiole, and an expanded, flattened blade called lamina. Some taxa have small, leaf-like structures called stipules, positioned between the petiole and the stem. The outermost layers on both surfaces of the blade constitute the upper and lower epidermis. The roots of a plant are collectively called the root system. Roots are composed of three primary tissue regions: the outermost epidermal layer, a wide middle region called the cortex, and an inner core or stele. The innermost layer of the cortex is the endodermis, and it is composed of cells with lignin-like wall material deposited in bands called the Casparian strip.

This chapter describes the origin and structure of the primary plant body. The postembryonic primary plant body arises from small, organized cell populations termed “meristems” that are located at the tips of the stems and the roots. Meristems are regions of active cell division, enlargement, and differentiation. The vegetative shoot apical meristem of angiosperms has a stratified organization, with two or more outer tunica layers in which cell divisions are primarily all anticlinal or oriented at right angles to the surface. Below the tunica layers is a region of corpus cells in which divisions occur in all planes. Active shoot apical meristems exhibit cytohistological zonation, in which the cells in the central zone are relatively large and divide infrequently as compared to the surrounding peripheral zone cells that divide more rapidly. A rib meristem underlies the central zone. Leaves are determinate organs initiated as groups of cells on the flanks of the shoot apex in precise and stable patterns. Leaf organogenesis commences with coordinated changes in the rate and pattern of cell division and expansion in different regions of the developing primordium.

Art objects, antiques, and musical instruments owe their unique qualities directly to their botanical origins. Therefore, it is possible to apply the special techniques of plant anatomy to help solve a number of fundamental questions relating to the history, preservation, and conservation of such materials. Plant anatomical evidence can be used to document attribution, date, and degree of deterioration, and to identify the materials used in object construction. Wooden objects often possess some cellular deterioration, and an anatomical examination at the light and electron-micrograph levels is essential to repair the damage. Deterioration occurs as a result of four general categories of decay processes: physical, chemical, mechanical, and biological. The recognition of decay provides valuable information that enables a museum curator to develop appropriate conservation and restoration procedures. Dendrochronological analysis is especially valuable for dating oil paintings on wooden panels, such as those produced on oak boards from the Middle Ages until the 17th century. Dendrochronological research has proven valuable in judging the authenticity of paintings given that a painting could not have been executed prior to the year of felling of the tree from which the panel board was constructed.

This chapter focuses on the origin and structure of the secondary plant body. All gymnosperms and most dicotyledons develop lateral meristems, which result in radial growth and are responsible for an increase in the width of the stems and the roots. Growth derived from the lateral meristems is called secondary growth and is composed of secondary tissues. Lateral meristems are termed “cambia” and function as biracial zones composed of initials and derivatives. The vascular cambium adds to the girth of the axis by the production of secondary xylem internally and secondary phloem externally. The outer protective layers of the axis are called periderm and are formed by the cork cambium, or phellogen. The vascular cambium develops a short distance behind the apical meristem. The cambium that forms within the vascular bundle is called fascicular cambium. The interfascicular cambium is initiated somewhat later from the renewed division of the parenchyma cells located between the original vascular bundles. A cambium in which the fusiform initials are overlapping and are of variable length is termed “unstratified.” A specialized stratified or stored cambium occurs when the fusiform initials are of nearly equal length, and are grouped in parallel and horizontal rows.

Within the phylogenetically unrelated genera Hibbertia and Erythroxylum multiple correlations among woody anatomy, plant stature, and details of leaf morphology and anatomy are clearly environmentally related. Xeric radiants typically have undergone plant and leaf size reductions, along with the development of foliar xeromorphy, deciduousness, and xeromorphic xylem specializations. Mesic radiants have evolved an increased plant stature and leaf size as well as retaining a primitively mesomorphic xylem structure. Clinal variation patterns are much more extreme and apparent in Hibbertia whereas Erythroxylum species possess more subtle degrees of habitat related variability. In species of both genera, modifications in leaf morphology and anatomy and the existence of deciduousness, appear to buffer the xylem and permit rather primitive and mesomorphic wood anatomy to exist within seasonally or permanently dry locations. The extent of the buffering influence of leaves upon the xylem tissues is presumably determined by the complex interrelationships between various climatic variables and such features as leaf size, duration, type and degree of scleromorphy, as well as non-foliar characteristics such as architectual forms, degree of stem succulence, and the nature of the root system. In this regard the foliage constitutes perhaps the most significant component of the plant hydrovascular system. Biological and evolutionary interpretations of wood anatomy within an ecological context, therefore, are ill-advised in the absence of correlative leaf structural data.