Wood Anatomy of Stilbaceae and Retziaceae: Ecological and Systematic Implications

Wood anatomy of ten species in five genera of the Cape Province (South Africa) family Stilbaceae is reported in quantitative and qualitative terms. Wood anatomy for stem, root, and lignotuber is reported for the monotypic Cape genus Retzia. Stilbaceae and Retziaceae are alike in wood anatomy but differ from Verbenaceae by having scalariform perforation plates with few and wide-bordered bars (simple plates and modified scalariform plates are also present); vessel elements clearly fibriform in shape; very scarce axial parenchyma; and long uniseriate wings on multiseriate rays. When added to endosperm presence and ericoid habit, these features may serve to segregate Stilbaceae from Verbenaceae. The wood of Stilbaceae is xeromorphic in having very narrow vessels, numerous vessels per mm' , and pores grouped into radial multiples. The least xeromorphic wood within Stilbaceae is found in species from relatively moist montane localities; these species also have scalariform perforation plates in vessels. The most xeromorphic wood occurs in those Stilbaceae restricted to lowlands or lowlands plus dry montane sites. Scalariform perforation plates are interpreted as an indicator of primitiveness in Stilbaceae and Retziaceae, an interpretation reinforced by presence of borders on pits of imperforate tracheary elements in Eurylobium serrulatum (all other Stilbaceae have libriform fibers with simple pits). The rock-crevice species Stilbe rupestris has caudex wood specialized for storage, stem wood adapted for mechanical strength. Species characters appear to be related to ecology, but sampling is inadequate for establishment of many features as systematic indicators. Wood anatomy confirms close relationship between Stilbaceae and Retziaceae, and union of the families is endorsed.