Wood Anatomy of Roridulaceae: Ecological and Phylogenetic Implications

Description of quantitative and qualitative features of root and stem wood of the two species of Roridula , endemics of Cape Province, Republic of South Africa, are presented because no data on secondary xylem have hitherto been published for these shrubs. Of the two species, R. gorgonias has wood appreciably more primitive in such respects as longer vessel elements with more numerous bars on the scalariform perforation plates. This is interpreted as correlated with the fact that R. gorgonias occupies maximally mesic habitats (Sphagnum covered covered montane seeps). Roridula dentata shows more specialized wood features, hypothesized to be related to greater fluctuations in water availability in its characteristic habitats. These patterns parallel ecological correlations obtained for dicotyledons as a whole. Lack of procumbent ray cells may relate to limited stem diameter in the genus if procumbent cells tend to function in radial translocation of photosynthates. Wood anatomy of Roridula is congruent with relationships alleged by various authors to the genus Byblis , and very similar secondary xylem features can be found in such “rosoid” families as Saxifragaceae (sensu lato), Hamamelidaceae, Bruniaceae, and Grubbiaceae. Roridula should be excluded from Droseraceae, and should probably not be placed next to Droseraceae in a phylogenetic system.