Aust. J. Bot., 1994,42,441-448 Amphicarpy in the Cyperaceae, with Novel Variation in the Wetland Sedge Eleocharis caespitosissima Baker Jeremy J. Bruhl Department of Botany, University of New England, Armidale, NSW 2351, Australia. Abstract Amphicarpy, the heterocarpic production of aerial and basal fruits, occurs in the Cyperaceae in species of the primarily mesophytic genera Bulbostylis and Trianoptiles and in species of the helophytic genera Eleocharis and Schoenoplectus. There is a variety of breeding systems and growth forms across these amphicarpic species. Eleocharis caespitosissima is amphicarpic, with basal fruits buried by positively geotropic culms. The seeds of basal fruits in this species may arise asexually. The selective pressures likely to be important in the development of amphicarpy in the Cyperaceae are discussed. Introduction Amphicarpy is the production of two kinds of fruit, differing in character (Jackson 1928). Here, amphicarpy is considered to include the production of aerial and basal flowers where the basal seeds or fruits are larger than the aerial (see Haines 1971; Raynal 1976). The term 'basal' is used to include both ground-level and subterranean inflorescences. Stricter definitions of amphicarpy involve the production of subterranean fruits (van der Pijl 1969; Cheplick 1987). The occurrence of amphicarpy was reviewed by Cheplick (1987) who listed 26 amphicarpic species from nine families (Asteraceae, Brassicaceae, Commelinaceae, Fabaceae, Poaceae, Polygalaceae, Polygonaceae, Scrophulariaceae, and Urticaceae). In these families the basal cleistogamous flowers produce fruits that are larger than the aerial fruits produced from chasmogamous flowers. No Cyperaceae were listed, despite literature references to amphicarpy in the sedge family extending back as far as 1876 (Table 1). Four hypotheses have been proposed to account for the selection processes leading to the evolution of arnphicarpy (Cheplick 1987): (1) placement of the basal fruits in a suitable site, as defined by the occurrence of the parent plant; (2) protection of the basal fruits from extremes of microclimate, with higher maintenance of viability over time of the basal seeds relative to the aerial seeds; (3) protection of the basal fruits from predation; and (4) protection of the basal fruits from perturbations, such as fire. The first hypothesis generated considerable support (see Koller and Roth 1964; van der Pijl 1969; Gopinathan and Babu 1986) though there is evidence that does not fit this model (Ellner and Shmida 1981; Cheplick 1987). Weiss (1980) has provided evidence supporting the second hypothesis for Emex spinosa from a xeric habitat, though there is no supporting evidence for amphicarpic species in mesic environments. Hypothesis three requires testing. A relationship between amphicarpy and perturbation by fire has been shown for Vigna minima (Gopinathan and Babu 1987) and Amphicarpum purshii (Cheplick and Quinn 1988). In these cases basal fruits are protected from the heat damage of fires that destroy the aerial fruits. Here, the occurrence of amphicarpy in the Cyperaceae is reviewed and a novel form of amphicarpy is described in the wetland sedge Eleocharis caespitosissima. The above hypotheses are explored for the Cyperaceae.