Search

The comparative vegetative and reproductive morphology and anatomy of the endangered, monotypic, dicotyledonous genusMedusagyne was studied, and detailed descriptions of leaf, axis, nodal, wood, floral, pollen, fruit, and seed structure are presented. Overall, the genus has many specialized features, including the possession of extreme, habitat-related specializations. Flowers are either bisexual or staminate, and are interpreted as retaining some primitive aspects, such as many free parts spirally arranged on an elongate floral axis. One of the most salient structural features of the plant is the massive development of ensheathing fibrous elements around the vascular system of both vegetative and reproductive tissues. Diffuse foliar sclereids are absent. Particular attention is paid to the unusual multicarpellate, synovarial gynoecium and comparisons are made with theCaryocaraceae. The totality of morphological and anatomical evidence confirms the view thatMedusagyne is a very distinct and evolutionarily isolated genus, best treated as forming the monotypic familyMedusagynaceae. Observations are presented, including the occurrence of stamen fascicle traces, that link the family to the dillenialean and thealean assemblage. Like other isolated thealean taxa,Medusagyne shows affinities to several different families, without having especially close relationships with any particular extant taxon.

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.

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.

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.

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.

This report concerns itself with the morphology and anatomy of the stem of flowering plants. Research results dealing with both the primary body and secondary xylem are discussed. For the most part, literature dealing with fossil plants, tissue culture, the phloem, the periderm, and regional, purely descriptive, or applied wood anatomy is not treated. I have made no attempt to summarize systematic conclusions, some of which are reviewed in a previous volume (Kadereit 1989). With few exceptions, this discussion is restricted to literature published in the years 1986–1990. During this interval much new factual information has been obtained which has been responsible for the formation of new morphological concepts as well as the reinforcement, substantial modification, or even abandonment of existing theories on the nature, structure, origin, and evolution of angiosperm stems and tissues.

Five genera and 12 species of Staphyleaceae were examined with respect to systematic floral morphology and anatomy. Detailed descriptions of floral structure and vascularization are provided. Features of systematic and evolutionary importance in the family include the number and type of fusion of floral parts, placentation type, occurrence of a vascularized disk, and the degree of independence of floral vascular traces. Flowers of Tapiscia sinensis lack a disk, and the bicarpellate gynoecium, containing no internal ovarian septation, culminates a trend toward carpel fusion and reduction in ovular number that has resulted in a unilocular structure with a solitary ovule attached to a more derived basal or subbasal placenta. The progressive fusion of floral parts in the Staphyleaceae has been accompanied by the characteristic union of the androecial vascular supply and sepal-plane bundles, which subsequently divide to form an externally situated stamen vasculature and internal sepal strands. Huertea and Tapi...

The secondary xylem of five species representing all three genera of Alseuosmiaceae was studied. Salient anatomical features that circumscribe the family include narrow vessel ele- ments with many-barred, scalariform perforation plates, pores distributed as a combination of both solitary elements and radial multiples, living septate fibers with stored starch at maturity, and the scarcity or absence of axial parenchyma. An absence of rays characterizes Alseuosmia and Wittsteinia vacciniacea. The insular species, W. balansae from New Caledonia, contains exclusively multiseriate rays that are very wide and tall and composed completely of erect and square cells. The large rays and the occasional scalariform lateral wall pitting of vessel elements of this species are probably best interpreted as paedomorphosis related to the increased size of this plant. The ancestral habit of the family remains unclear since Crispiloba disperma has retained a more primitive xylem structure as evidenced by the occurrence of axial parenchyma and shorter and narrower heterocellular rays. Xylem anatomy can be used to support a position for the Alseuosmiaceae in or near the woody Saxifragaceae.