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. THE FAMILY Roridulaceae consists of two species, Roridula dentata Linnaeus and R. gorgonias Planchon. These are shrubs endemic to Cape Province, Republic of South Africa. Once they were thought to be insectivorous because of their viscid leaves and tentacle-like trichomes (see Bruce, 1907), but there is no evidence that insect protein is absorbed by leaves (Lloyd, 1942). At most, one might speculate that by the catching of insects on the viscid leaves, and by the ultimate fall of these leaves laden with insects to the ground, the nitrate content of the soils in which Roridula grows might be enhanced. The soils in which Roridula grows are derived from Table Mountain Sandstone, and have long been known to be highly acidic and low in nitrates (Marloth, 1925). Both species can be said to grow in mesic pockets within the summer-dry mediterranean scrublands of Cape Province. Roridula gorgonias grows in seeps or marshy areas on montane slopes often cloudy in summertime. Colonies of this species were observed in the mountains above Hermanus (Carlquist 4960, RSA and other herbaria), where shrubs grow in thickets on Sphagnumcovered black soil. Occasional individuals of 1 Received for publication 22 August 1975. This research was supported by grants from the National Science Foundation (GB-38901) and the John Simon Guggenheim Foundation. Field work in South Africa was aided by Miss Elsie Esterhuysen, Dr. John Rourke, and Dr. Ion Williams. Dr. Gary Wallace helped in preparation of data. Grubbia rosmarinifolia, also an excellent indicator of wet seeps, grow with R. gorgonias. Roridula dentata can be found on "vlakte" (flats) of the Kold Bokkeveld mountains, as at Boboskloof (Carlquist 4644, RSA and other herbaria). These sandy flats are clearly drier than the R. gorgonias localities in summer, but underground water is undoubtedly available from nearby streams that flow for most of the year. Thus both species of Roridula can be termed mesophytes, but R. dentata is a plant with some xeromorphic adaptations. This is, for example, suggested in the leaf anatomy. Leaves of R. dentata are smaller, more condensed in form, and have compact chlorenchyma with small intercellular spaces; contrasting conditions characterize leaves of R. gorgonias (Bruce, 1907). Because ecology proves to be closely related to wood anatomy in dicotyledons (Carlquist, 1975), the two species of Roridula prove suitable material for testing potential correlations. Phylogenetically, various authors have hypothesized two placements for Roridula. The first of these includes Roridula in Droseraceae (Bentham and Hooker, 1865; Harvey, 1868; Harvey and Sonder, 1884; Phillips, 1951). This view is still current among South African botanists. The alleged insectivorous habit probably has been the overriding reason for inclusion of Roridula in Droseraceae by these authors. Because numerous features of floral anatomy, embryology, and wood anatomy differentiate Roridula from Droseraceae (sensu stricto), this placement should probably be abandoned. The second viewpoint on relation-