Dispersal of heteromorphic seeds in Amphicarpaea bracteata (Fabaceae)

The dispersal of the chasmogamous, aerial cleistogamous, and subterranean cleistogamous seeds of Amphicarpaea bracteata was examined. The chasmogamous and aerial cleistogamous seeds are ballistically dispersed. Chasmogamous seeds were dispersed farther than the aerial cleistogamous seeds due to the height advantage of the chasmogamous pods. There was no difference in the firing angle or the initial velocity of the seeds discharged from the two aerial pod types. The subterranean cleistogamous seeds are "dispersed" the shortest distance by the elongation of runners. Differential dispersal of the three types of seeds may be a factor involved in the evolution of cleistogamy in A. bracteata. AUTOGAMY and self-fertilization have long been associated with long distance dispersal and colonizing ability (Stebbins, 1957; Allard, 1965; Jain, 1976). Recently, a number ofauthors have modeled the role of seed dispersal in the evolution of self-fertilization and cleistogamy (Schoen and Lloyd, 1984; Holsinger, 1986). Cleistogamy is the production of cleistogamous flowers, which have reduced corollas and androecia and always self-fertilize, and chasmogamous flowers, which have unreduced floral parts and are adapted for cross-pollination by a pollen vector (Lord, 1981; Uphof, 1938). Holsinger's (1986) model suggests that intermediate rates of self-fertilization can be evolutionarily stable ifinbred and outbred progeny are differentially dispersed to, and/or established in different habitats. Schoen and Lloyd (1984) proposed that seed dispersal plays a prominent role in the evolution of environmental chasmogamy (i.e., the increase in allocation to chasmogamous reproduction relative to cleistogamous reproduction as plant size increases) (Waller, 1980). One intriguing result of Schoen and Lloyd's (1984) model is that the differential dispersal ofchasmogamous and cleistogamous seeds may be sufficient to select for environmental chasmogamy even if the chasmogamous progeny I Received for publication 22 June 1987; revision accepted 1 February 1988. 2Current address: 2437 Overlook Rd., Apt. 28, Cleveland Heights, OH 44106. I thank Stephen Hendrix, Robert Cruden, Henry Howe, Eugene Schupp, Donald Waller, and Johanna Schmidt for helpful suggestions on earlier drafts of this manuscript. I also thank the Shaker Lakes Regional Nature Center for technical assistance. are not genetically more variable than the cleistogamous progeny. Their model predicts that environmental chasmogamy will be selectively advantageous if chasmogamous seeds are dispersed farther than cleistogamous seeds and if the increase in plant size enlarges the size of the chasmogamous seed shadow more than the size of the cleistogamous seed shadow. In this paper, I examine the dispersal of the chasmogamous, aerial cleistogamous, and subterranean cleistogamous seeds of Amphicarpaea bracteata L. (Fabaceae, hog peanut), and experimentally test the hypothesis that chasmogamous seeds are dispersed greater distances than either type of cleistogamous seeds. Species biology-Amphicarpaea bracteata is a slender, twining vine common in moist woods throughout the eastern and midwestern United States (Turner and Fearing, 1964). The chasmogamous and aerial cleistogamous fruits are dry explosively dehiscent legumes containing one to three small indurate seeds (Schnee and Waller, 1986; Schively, 1897; Trapp and Hendrix, 1988). The two types of aerial fruits are morphologically identical, but can be distinguished by the presence of a bract subtending the chasmogamous fruits (Trapp, 1986). The two types of aerial seeds are identical in size, shape, and weight (Trapp and Hendrix, 1988). The subterranean fruits are modified indehiscent legumes usually containing one large seed enclosed in a thin pericarp. The subterranean seeds are sixteen times heavier (dry weight) than either type of the aerial seed (Trapp and Hendrix, 1988). The subterranean cleistogamous seeds are dispersed in an unusual manner. The subter-