Wood Anatomy of Buxaceae: Correlations with Ecology and Phylogeny

Abstract

Summary The family Buxaceae consists of four genera, with the desert shrub Simmondsia dubiously included in the family as a fifth genus. Styloceras (Andean rain forest) has the most mesomorphic and primitive wood features, followed by Sarcococca and then by Buxus, which can be grouped closely with Notobuxus in terms of wood anatomy. Pachysandra was excluded from this study because of its essentially herbaceous habit. For a small family, Buxaceae have a wide span in expression of wood features. Characters in Buxaceae s.s. which show a range within the family probably explainable in terms of ecology include vessel element length, number of vessels per mm2, vessel diameter, number of bars per perforation plate, presence or absence of helical thickenings in vessels and tracheids, and presence or absence of growth rings. The woods in the family can be ranked as highly mesic (Styloceras, Sarcococca) to moderately mesic (Buxus, Notobuxus), with a few species in the latter pair of genera notably more mesomorphic in wood construction. Retention of scalariform perforation plates throughout Buxaceae s.s. may be due to compensatory low transpiration capabilities of the foliage, which may have very thick cuticles and tend toward microphylly. Features which are susceptible to evolutionary interpreation but which do not bear as direct a relationship to ecology include ratio of tracheid length to vessel element length, presence of tracheids with fully bordered pits, degree of aggregation of axial parenchyma cells (diffuse, diffuse-in-aggregates, abaxial) and degree of cellular heterogeneity in ray histology. The ray types of Sarcococca reflects a type of juvenilism, and scarcity of axial parenchyma in the genus also seems related to innovation of numerous shoots, each with limited duration. Simmondsia differs from Buxaceae s.s. in its desert habitat, its successive cambia, and its anemophily. Distinctive features of wood anatomy (lack of acial parenchyma, juvenilism in ray histology, and presence of very short vessel elements with helical thickenings) can be attributed to the first two factors. No features of wood anatomy clearly rule out relationship to Buxaceae. Presence of tracheids in wood of Simmondsia may be related to the high selective value for tracheids in a desert environment; this feature (and the encyclocytic stomata) suggest affinity to Buxaceae, even if the genus is segregated in Simmondsiaceae. Buxaceae have been claimed to be related to four major groups of dicotyledons: Celastrales, Euphorbiales, Hamamelidales, and Pittosporales. Of these, Euphorbiales shows the least degree of resemblance to Buxaceae in terms of wood anatomy. The placement of the isolated Madagascan genus Didymeles may be related to the problem of placement of Buxaceae, and is discussed in this context.