THE REPRODUCTIVE BIOLOGY OF CUPANIA GUATEMALENSIS RADLK. (SAPINDACEAE).

Abstract

Intraspecific floral polymorphism is of widespread occurrence in flowering plants. Perfect, staminate, and pistillate flowers may be found on the same or different plants in diverse combinations as exemplified by monoecy, dioecy, androdioecy, gynodioecy, polygamy and the like; in addition, within perfect-flowered species, polymorphism may exist in the form of distyly and tristyly. It is generally presumed that floral polymorphisms increase the probability of outcrossing. The level of outcrossing, however, has not been measured in any zoophilous species with polymorphic flowers except for some heterostylous species (Ornduff, 1971, 1975). Floral polymorphism, by introducing spatial variation in floral rewards, also influences the foraging behavior of pollinators. The foraging behavior of pollinators, in turn, is likely to influence not only gene flow but also the evolution of floral polymorphism itself. Yet it is somewhat surprising that the effect of floral polymorphism on the foraging behavior of pollinators has not so far been examined. This paper describes the reproductive biology of a monoecious species, Cupania guatemalensis Radlk. (Sapindaceae), a small tree found in lowland tropical forests of Central America. It examines the pattern of spatial and temporal variation in the relative frequencies of staminate and pistillate flowers at the individual and population levels; differences in floral reward of staminate and pistillate flowers; differences in the foraging behavior and visitation rates of pollinators on staminate and pistillate flowers; and the effect of flowering behavior and floral polymorphism on the foraging behavior of pollinators and the degree of outcrossing. In addition, the floral phenology of some other members of the Sapindaceae is interpreted in the light of findings reported here. The investigations are part of a wider study of the breeding systems of lowland tropical forest trees (Bawa, 1974; Bawa and Opler, 1975).