In five populations of Entodon cladorrhizans in central Pennsylvania during the 1981 growing season more sporophytes were initiated than developed to maturity; the greater the number of sporophytes initiated per stem, the lower the probability that a given sporophyte developed to maturity; vegetative growth of the gametophyte was reduced during sporophyte development; sporophytes tended to abort early in their development; and the amount of vegetative growth on a stem prior to the initiation of sporophytes was positively correlated with the size and number of sporophytes produced. These findings suggest that sporophyte production in E. cladorrhizans is resource-limited and that sporophyte abortion allows matching of reproductive output to the available resources. Many higher plants consistently produce more flowers than mature fruits. Recent studies reveal that these species usually produce more morphologically female flowers (flowers with a gynoecium) than can be developed into mature fruits with the resources available for fruit maturation. When resources rather than pollination limit fruit and seed production (and it frequently does in many species), the pollinated flowers and immature fruits compete for the limited resources. Flowers and immature fruits then abort until the resource demands of the reproductive structures match the resource supply (van Stevenick 1957; Janzen 1976; Willson and Price 1977; Lloyd 1980; Stephenson 1980, 1981; Udovic and Aker 1981). This contention is supported by the findings of many investigators who have employed a variety of experimental techniques (see review by Stephenson 1981). Firstly, the number of pollinated flowers under natural conditions often exceeds the number of fruits that initiate and mature. Furthermore, when all of the flowers on a plant or inflorescence are outcrossed by hand-pollinations, a sizeable portion of the flowers and juvenile fruits still abort. In many cases hand-pollinations do not increase the number of mature fruits compared to natural pollination. Secondly, thinning experiments, in which flowers or juvenile fruits are removed from a plant, reveal that the remaining fruits have a higher probability of developing to maturity. Thirdly, in many species it is possible to increase the proportion of flowers that initiate and mature fruits by irrigation, by the addition of fertilizer, or by the removal of competing species during the reproductive episode. Conversely, shading experiments, defoliation experiments, and natural herbivory decrease fruit initiation and maturation in many species. Two lines of evidence suggest that analogous phenomena may occur among mosses. First, just as fruits depend upon parent plants for many of the resources necessary for growth and development, moss sporophytes are partially dependent upon gametophytes for resources. Although moss sporophytes contain chlorophyll (Bold 1940), recent studies have established that the sporophytes are incapable of complete selfnutrition (Rastorfer 1962; Paolillo and Bazzaz 1968). Proctor (1977) demonstrated the one-way transfer of photosynthate from gametophyte to sporophyte in Mnium hornum. Browning and Gunning (1979a, b, c) investigated growth and development of Funaria hygrometrica sporophytes using 14CO2 and found that nearly onequarter of the CO2 fixed by the gametophytes was transferred to elongating sporophytes and that larger sporophytes are more powerful carbon sinks than smaller ones. Secondly, in most mosses, only one sporophyte usually develops per perichaetium (a cluster of archegonia); this condition is known as monosety. When polysety (development of two or more sporophytes from a single perichaetium) does occur, the monosetous capsules are often significantly larger than polysetous capsules (Lowry 1949; Dalby 1959; Longton