Evidence for a Primitively Epiphytic Habit in Orchidaceae

Abstract

Both terrestrial and epiphytic orchids share a specialized reproduc- tive biology that is particularly marked in contrast to the facultative mechanisms of a basically terrestrial family, Asteraceae. The specialized biology of the orchids is attributed to an epiphytic ancestry for the family, with evidence from pollination mechanism, seeds, and roots. The zygomorphic orchid flower form can be traced to a pendant form of primitive inflorescence. Repeated reversions of orchids to the terrestrial habit are indicated, and extinction of primitive epiphytic forms is believed to follow a general pattern of extinction of primitive forms in other evolutionarily successful families such as Asteraceae. The studies of the senior author on members of both Orchidaceae and Asteraceae, each containing 20,000 to 25,000 species, has raised a serious question regarding the very different mechanisms of these two evolu- tionarily successful families of flowering plants (Wagenitz 1981). Aster- aceae, a family of unquestionable terrestrial ancestry, has very facul- tative reproductive mechanisms, while Orchidaceae, even in terrestrial members, has highly specialized reproductive biology. The contrast has been sufficient to call the ancestry of the terrestrial members of the orchids into question. Independently, the junior author, on the basis of her work in the primarily terrestrial orchid subfamilies Neottioideae, Orchidoideae, and Spiranthoideae (sensu Balogh and Robinson unpub- lished), had begun to suspect that such groups were primitively epiphytic. A joint effort is made here to explore the latter possibility using initially the example of Spiranthoideae and extending the conclusions to other terrestrial subfamilies. Probably ninety-five percent of species of Spiranthoideae are terres- trial (Balogh in prep.), including all of the north temperate genera such as Spiranthes Rich. and Goodyera R. Br. that are familiar to most taxon- omists. As such, the group is thoroughly adapted to the soil, and it is not surprising that the group is generally regarded as primitively terrestrial (Dressler 1981, pers. comm.). Problems arise in this assumption, how- ever, when one notes all the specializations that are shared by Spiran- thoideae and the predominantly epiphytic Epidendroideae. In seed type, floral details, and root structure, Spiranthoideae seem to show special- izations that are, as elaborated below, unlikely in plants lacking an epi- phytic ancestry. We recognize that specializations, in themselves, prove only that such specializations are able to be produced and to survive, but the numerous and sometimes extreme specializations of Spiranthoideae are believed to be impossible except in response to circumstances not