Sporophyte Production and Sexuality of Mosses in Two Northern Michigan Habitats

Abstract

The hypothesis that dioicous mosses produce sporophytes less fre- quently than monoicous mosses was tested by sampling populations in two con- trasting habitats. In a dry aspen forest the mean percentages of populations with sporophytes were 75.9 for monoicous and 19.3 for dioicous species, while in a wet coniferous forest the means were 84.1 for monoicous and 12.3 for dioicous species. Dioicous species with antheridia borne in splash cups showed significantly higher sporophyte production than those without splash cups. However, dioicous species with male plants dwarfed and epiphytic on female plants did not show significantly greater sporophyte production. Differences in sporophyte production between hab- itats were minimal. Since there is no evidence that apogamy is frequent in mosses, it is reasonable to assume that sporophyte production is dependent upon fertilization. Sperms are released from the antheridia in the presence of free water and must travel to an egg within an archegonium to effect fertilization. Although the sperms are biflagellate, it is doubtful that they rely much upon swimming for dispersal. Showalter (1926) showed that sperms of Riccardia pinguis placed at one side of a 1 x 0.5 cm drop of water swam vigorously but never dispersed themselves throughout the drop. Rather, because of lipids in the sperm mass, sperms are carried along passively as surface films on splashing drops of water or by water flowing over the plants (Muggoch & Walton 1942; Paolillo 1981). Insects and other minute invertebrates have also been recorded as vectors of sperm transfer (Harvey- Gibson & Miller-Brown 1927). Both self-fertile and self-sterile monoicous species have been reported (Lazarenko & Lesnyak 1972; Lazarenko 1974; Ashton & Cove 1976). While sperms need only travel a short distance to fertilize an archegonium in self-fertile species, in dioicous and self-sterile monoicous species sperms must travel from one plant to another. Sperms are short-lived and can disperse only limited distances. Sperm transfer distances have been measured for several dioicous species (Bedford 1938, 1940; Longton & Greene 1967, 1969; Newton 1971; Riemann 1972; Clayton-Greene et al. 1977; Wyatt 1977; Reyn- olds 1980) and one monoicous species (Anderson & Lemmon 1974). Most measurements were minimum estimates, determined by measuring the distance from plants bearing spo- rophytes to the nearest male, and most of the studies indicated that sperms rarely have an effective range of fertilization over 10 cm. Thus, separation of the sexes on different plants would seem to decrease the probability of fertilization; and in general, dioicous species would be expected to produce sporophytes less frequently than monoicous species. Using data on fruiting derived from Dixon's Handbook of British Mosses (1924), Gem- mell (1950) compiled a contingency table demonstrating the relationship of sexuality to frequency of sporophyte production. Of the 239 monoicous species, 232 fruited frequently. Of the 252 dioicous species, 148 fruited frequently, 47 rarely, and 57 very rarely. Lefebvre (1969) presented data based on 350 specimens showing that dioicous species of Belgian