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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.

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.

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.

The structure and development of roots and haustoria in 37 species of parasitic Scrophulariaceae was studied using light microscopy. The mature haustorium consists of two regions: the swollen “body” and the parent root, which resembles non-haustorial roots in structure. The body arises from the parent root and is composed of an epidermis, cortex, central region of xylem (the vascular core), a region of parenchyma (the central parenchymatous core), and the portion of the haustorium contained in the host tissue (the endophyte). The xylem of the vascular core is composed predominately of vessel elements. The central parenchymatous core is composed of parenchyma and col-lenchyma. Vessels extend from the vascular core through the central parenchymatous core to the endophyte. The endophyte is composed of parenchyma cells and vessel elements. No phloem is present in the body of the haustorium. Early stages in the development of the haustorium are exogenous. Initial periclinal divisions in the epidermis or outer cortex are followed by hypertrophy of cortical parenchyma. These events are followed by development of the vascular core from the pericycle, attachment of haustorium to the host by a specialized layer of cementing cells or root hairs, and penetration of the host by dissolution of host cells.

Vegetative and reproductive anatomy and morphology are described for the first time for Tetracarpaea tasmannica Hook., a small shrub endemic to Tasmania. Tetracarpaea, a monotypic genus, has many characteristics of other woody Saxifragaceae, such as wood with solitary pores, scalariform perforation plates, sparse axial xylem parenchyma, tracheids, spiral thickenings in tracheary elements, and perforated ray cells. The tracheary elements of Tetracarpaea are much smaller than those characteristic of the Escallonioideae, a feature probably related to its montane forest habitat. Other features of Tetracarpaea inconsistent with most Escallonioideae include dark-staining deposits in the ray cells; a unilacunar, one-trace nodal pattern; lack of unicellular foliar trichomes; simple craspedodromous venation; areole development that is lacking or incomplete; straight and tapered veinlets; abaxial fibers associated with the foliar vascular bundles; and lack of bundle sheaths. The genus is further characterized by complete, hypogynous, tetramerous flowers. The essentially apocarpous gynoecium has multiovulate carpels, each supplied by three veins that reach the stigma. Ovules are anatropous, bitegmic, and crassinucellate. Lateral sepal bundles are derived either from the sepal midrib or from the petalplane bundles; stamens are supplied by independent traces or by bundles originating from compound traces in both sepal- and petal-planes. The follicular fruits possess a sclerenchymatous endocarp and contain winged seeds that have a membranous testa, a ridged surface, and a cellular endosperm. Reproductive morphological and anatomical features are more consistent with features of the Saxifragoideae than with the Escallonioideae or the Cunoniaceae, although the essentially apocarpous gynoecium with multiovulate carpels is not found in these groups. Vegetative and reproductive characteristics indicate that Tetracarpaea is more closely related to Saxifragaceae than to Cunoniaceae. It is possible this isolated genus should have separate familial status.

The first description of the wood of the monotypic Madagascan genus Diegodendron is provided. The xylem of D. humbertii is characterised by short , solitary vessel elements with alternate lateral wall pitting and simple perforation plates, imperforate tracheary elements of the libriform fibre type, nearly all biseriate, imperfectly storied, homogeneous rays composed of procumbent cells only, and diffuse and diffuse-in-aggregates axial parenchyma. The specialised wood anatomy of Diegodendron supports a close alliance with both Sphaerosepalaceae and Malvales.

Woods of over 40 species representing nine genera of Styracaceae were studied. Features present in most taxa include growth rings, diffuse porosity, combinations of both solitaries and pore multiples, exclusively scalariform perforation plates, opposite to alternate intervessel pitting, imperforate tracheary elements with indistinctly bordered pits, both uniseriate and multiseriate heterocellular rays, and axial parenchyma distributed as a combination of diffuse, diffuse-in-aggregates, and scanty. Prismatic crystals occur in species of the genera Bruinsmia, Halesia, and Styrax, and silica is present in a few Neotropical species of Styrax. The characteristic solitary pore distribution and high scalariform perforation plate bar number of Huodendron are of potential evolutionary significance. The xylem of Lissocarpa differs from the Styracaceae in possessing more highly evolved vessel elements with both simple and scalariform perforations and prominently banded axial parenchyma. The occurrence of simple perforation plates in the wider, earlywood vessel elements, along with an increased pore frequency and decreased vessel element length, in Styrax platanifolius and S. texanus is documented. Both species inhabit seasonally dry habitats of the southwestern United States, thus supporting similar specialisations observed in other plants growing in xerophytic conditions. The apparent variation in perforation plate condition within different geographic varieties of S. officinalis is discussed. Significant correlations of wood anatomical characters and latitude of provenance are present among species of Styracaceae. Increasing latitude is strongly correlated with increased pore and multiseriate ray frequency, and decreased vessel element length and wall thickness. Increasing latitude is less strongly correlated with an occurrence of decreased pore diameter, increased bar number per perforation plate, increased fibre-tracheid and intervessel pit diameter, and increased frequency of uniseriate rays. Weak correlations are also evident between increasing latitude and shorter ray height and narrower, shorter, and thinner-walled fibre-tracheids.

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.

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.