Xylem is nonuniform in its structure and function throughout the plant stem. Xylem structure varies from pith to bark, from root to apical meristem, from stem to branch, at nodes vs internodes, and at junctions of branches, stems or roots compared to the internodal regions nearby. At smaller scales, anatomy varies systematically within one growth ring and it varies among the layers of the cell wall. Xylem properties vary by the plane in which they are examined, owing to cell shape, cell orientation, and the orientation of microfibrils in the cell walls. As concluded by Larson (1967, p. 145), "more variability in wood characteristics exists within a single tree than among [average values for] trees growing on the same site or between [average values for] trees growing on different sites."
This structural heterogeneity results in spatial variation in hydraulic and mechanical performance of the xylem. Whereas wood technologists have long acknowledged the importance of wood variability (e.g., Northcott, 1957; Dadswell, 1958; Larson, 1962; Cown and McConchie, 1980; Beery et al., 1983; Megraw, 1986; Schniewind and Berndt, 1991), this heterogeneity often has been overlooked by botanists, who have tended to view stems a homogeneous organs ("biomass") with only a passive role in the biology of the plant. This chapter details the patterns of variation in xylem structure found within a wood plant, and emphasizes what is knpwn and what is not known about the functional consequences of this variation for shoot water movement and mechanics
