Microgonium (Hymenophyllaceae) in Malesia, with Special Reference to Peninsular Malaysia

Abstract

Summary. The six species of Microgonium occurring in south-east Asia and Malesia are described, illustrated and distinguished and their distributions reviewed. M. tahitense is reported for the first time from Malaya and Thailand and M. mindorense from Java and Borneo. In west Malaysia the latter is much commoner than M. bimarginatum, with which it has been confused hitherto and which is now known from Borneo only by one recent collection. Specimens of M. henzaianum (previously known by a single Burmese collection) from south Vietnam and southern Thailand suggest that it might occur in northern Malaya. Microgonium is one of the more easily recognised and circumscribed of the filmy-fern genera segregated from Trichomanes when the latter is subdivided by taxonomists wishing to recognise more than two (sometimes four) genera in the diverse and complex family Hymenophyllaceae. The genus Microgonium, established by Presl (1843) and maintained by van den Bosch (1861) and Copeland (1938), is characterised by small fronds, with entire, glabrous margins, often simple, sometimes lobed (rarely pinnatifid) apically in an irregular fashion. The veins are dichotomously, not pinnately, branched and in some species are radial in arrangement. All species have false veins (veinlets) between and parallel to the true veins and the two may connect proximally as well as distally at a submarginal false vein, which is possessed by some species. The sorus has a conical or tubular indusium, widening to a flared circular mouth, usually completely immersed in the lamina. Microgonium is closely related to Didymoglossum (whose frond margins have short, stout black-brown hairs) and especially to Crepidomanes, whose species are distinguished by having the lamina pinnatifid or more dissected, true veins pinnately divided and no connection between true and false veins (including the submarginal vein) and in many species the sori are free with bilabiate lips. Crepidomanes species also have chromosome numbers based on n = 36, rather than n = 34 as in Microgonium. There is a very small species of this alliance, T. parvifolium Baker, occurring in peninsular Thailand (and north to southern Burma), which has simple to quadrifurcate fronds and is thus intermediate in dissection between the two genera. It has usually been placed in Microgonium (Copeland 1938, Tagawa & Iwatsuki 1967, 1975) but on the basis of its venation (Iwatsuki 1978) and its possession of a clearly bilabiate and pointed lip to the indusium, it is better placed in Crepidomanes. Microgonium consists of about 19 species, four in the neotropics (Wessels Boer 1962), four in mainland Africa (Pichi Sermolli 1982) and about five in Mad- agascar and the Mascarene and Seychelle Islands (Christensen 1932, Tardieu-