ENVIRONMENTAL DETERMINANTS OF OUTCROSSING IN IMPATIENS CAPENSIS (BALSAMINACEAE).

Abstract

Many plant species produce both chasmogamous flowers, adapted to attracting pollinators, and tiny cleistogamous flowers, adapted to self-fertilization. Darwin (1877) interpreted these cases of floral dimorphism as adaptations to the two, frequently antagonistic, functions served by flowers: successful cross-fertilization with other plants, and the efficient setting of many seeds. He noted how sparing of pollen and nectar the cleistogamous flowers were, and realized how this economy would permit expanded seed set. He then asked why the more efficient mode of reproduction would not replace the other, and concluded that adaptations for crossfertilization implied the existence of some advantage for outcrossed progeny. This advantage is now taken to be heterosis, or the avoidance of inbreeding depression. From the time of Linnaeus, the environment has been known to influence the production of cleistogamous and chasmogamous flowers in many species (Uphof, 1938). Many researchers have searched for and found relationships between a species' life history or a population's environment and a particular breeding system (Darwin, 1877; Stebbins, 1950; Baker, 1955; Darlington, 1958; Grant, 1971; Levin, 1972; Solbrig and Rollins, 1976). This correlative approach is useful for suggesting relevant selective forces that might favor one breeding system over another. Although incapable of proving which selective forces are the most important, it may serve to discredit hypotheses whose predictions are violated. Jain (1976) and Sol-