Secamone (Asclepiadaceae subfam. Secamonoideae) in Africa

Abstract

Secamone R. Br., as currently defined, is a genus of 80 mostly small-flowered woody twiners or scrambling shrubs native to tropical and subtropical regions of the Old World. The greatest diversity occurs in Madagascar where 62 species can be found (Klackenberg, in press). A further 16 species, treated in this paper, occur in continental Africa. Of the remaining two, one is found in southern India and Sri Lanka while the second occurs over parts of south-east Asia and as far as northern Australia (Forster & Harold 1989; Klackenberg 1992). The genus is notable for its minute pollinaria, each with four pollinia attached (Schill & Jakel 1978) by flattened caudicles to a porous corpusculum, and its terete or laterally compressed staminal corona lobes. In contrast, the related genus Toxocarpus Wight & Arn., which shares a similar pollinarium, has dorsally flattened corona lobes. Generic delimitation in this part of Asclepiadaceae is in urgent need of clarification, with this and other characters traditionally used to separate Secamone and Toxocarpus breaking down in Madagascar (Klackenberg, in press). Since his treatment of Secamone in Australia with Harold (Forster & Harold 1989), Forster (1991) has expanded his concept of the genus to include Toxocarpus. The traditional boundaries of Secamone have been retained for this study pending a revision of the subfamily at generic level. Secamone and the allied genera Genianthus Hook. f., Goniostemma Wight, Menabea Baillon, Pervillaea Decne., Rhynchostigma Benth., Toxocarpus and Secamonopsis Jumelle, many of which are of uncertain status, form a bridge between the Asclepiadoideae (with two pollinia per pollinarium) and the Periplocoideae (with granular pollen), sharing a number of so-called primitive characters with the Periplocoideae and, in many cases, also with Apocynaceae. These are listed by Safwat (1962) as 4-locular anthers (2-locular by phylogenetic suppression in the Asclepiadoideae); simultaneous meiotic divisions of the pollen mother cells (cf. successive division in Asclepiadoideae); rhomboid or T-shaped pollen tetrads (cf. linear tetrads in Asclepiadoideae) and the stigma head which is entirely glandular at maturity (cf. 5 glandular patches in Asclepiadoideae). Rosatti (1989) adds longitudinally introrse or latrorse anther dehiscence (apical in most Asclepiadoideae) to this list. Characters