ABSTRACT

Facultatively xenogamous species are self-compatible, adapted for cross-pollination, have delayed autogamy, occur primarily in climax or other stable communities, and flower when pollinator activity may be low or unreliable. In most species both fruit and seed set are high. With the exception of species that produce both chasmogamous and cleistogamous flowers, species with mixed mating systems such as facultative xenogamy have received minimal attention. The chapter examines facultative xenogamy, a mixed mating system of strictly chasmogamous, perfect-flowered plants. It discusses three general objectives: to demonstrate that the flowers of facultatively xenogamous species can function as Cruden predicted, shows that facultative xenogamy is different from both xenogamy and facultative autogamy, as well as other mixed mating systems, and indicate when and/or where facultative xenogamy is adaptive. The chapter concludes that facultative xenogamy is a unique and evolutionarily stable mating system rather than an intermediate stage in the evolution of autogamy.