Sphenostemonaceae

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Taxonomy

The taxonomic position and rank of the only genus Sphenostemon has a chequered history. In the course of time it has, under various names, been attributed to the Aquifoliaceae (by BAILLON, as Sphenostemon, 1875), to the Icacinaceae (as a species of Phlebocalymna, by F. VON MUELLER, 1875), to the Guttiferae (as Nouhuysia, by LAUTERBACH, 1912), and to the Trimeniaceae (by GIBBS, as Idenburgia, 1917).
BAILEY & SWAMY (1953) and BAILEY (1956) examined the anatomy and concluded that the genus could not belong to either Guttiferae or Trimeniaceae cq. Monimiaceae, but they gave no clear alternative. When I summarised the complete generic synonymy (1955), I found it likely to retain Sphenostemon in Aquifoliaceae.
An other opinion approached that of F. VON MUELLER, viz. that by INGLE & DADSWELL (1961) who suggested, on the strength of the wood anatomy, a likeness with Platea in the Icacinaceae, and possibly also an affinity to Polyosma (Saxifragaceae).
HUTCHINSON (1959) and AIRY SHAW (1972) stuck to the Monimiaceous affinity, and I must admit that there is a distinct resemblance, in androecium in particular, with Trimenia, but this is overruled by the anatomical and other differences. They felt possibly also strengthened by the fact that LOESENER (1942) had expelled Sphenostemon from the Aquifoliaceae, and had suggested affinity with Theaceae or Ochnaceae, or as representative of a separate family.
In a good overview BERNARDI (1964) concluded that Sphenostemon should remain m Aquifoliaceae. In this he is followed by CRONQUIST (1981).
A very thorough anatomical research of Aquifoliaceae led BAAS (1975) to the conclusion that Sphenostemon is anatomically allied to both Aquifoliaceae and Icacinaceae, probably more to the latter. In fact, in comparing the macromorphological characters it appears that all of them occur in Icacinaceae. He proposed that the genus should be accommodated in a family of its own, an idea already advanced by AIRY SHAW (1972), allied to both Aquifoliaceae and Icacinaceae', this view is also held by THORNE (1983). The removal of Sphenostemon from Aquifoliaceae is well sustained by the seed structure. The fruit was mostly defined as a drupe containing a pyrene. BAILEY (1956) showed that the sclerified tissue is, however, not derived from the endocarp and that the fleshy envelope of it is really the whole of the pericarp. He accepted the sclerified tissue as derived from the testa. I expressed my doubt about this interpretation to Dr. W.A. VAN HEEL (Leyden), who found that the sclerified tissue surrounding the seed is of chalazal nature and that the seed belongs to a type characterized by CORNER (1976) as pachychalazal, a peculiar feature occurring in a limited number of families, amongst them Icacinaceae, but not Aquifoliaceae. Although not too fond of split families, I feel this new observation gives additional strength to recognize Sphenostemon representing a family of its own.
DICKISON& BAAS (1977) noted a remarkable similarity in vegetative anatomy and some gross morphological features between Sphenostemon and Paraeryphiaceae, a monotypic family from New Caledonia. This is compatible with the gradually accepted transfer of Aquifoliaceae, Icacinaceae, and Sphenostemonaceae from the heterogeneous order of the Celastrales to the Theales.

Synonymy

Sphenostemonaceae