The flexibility of adaptation in plants to variable environmental conditions is greatly increased by the production of two or more different kinds of seeds. Such heteromorphic seeds, either with or without a winged margin, are characteristic of several species belonging to the genus Spergularia (Caryophyllaceae). By examining the seed type variation and the extent of seed heteromorphism in S. media, and by investigating the differential demographic and dispersal characteristics of the different seed morphs, an explanation was sought to the phenomenon itself, as well as to the distribution of this species in its natural habitat: salt marshes. A nine-year field study of the demography of this short-lived perennial herb revealed that, in addition to density dependent mortality of the seedlings, unpredictable climatical fluctuations profoundly affected the survival and the growth of the plants. The production of winged and unwinged seeds was differentiated along a gradient of increasing vegetation density, with the winged seeds most frequently occurring where the vegetation is dense. The winged seeds which have a slightly larger seed body germinated more readily, the survivorship of their resulting plants was higher, the growth was quicker and the reproduction was more profuse than among the unwinged ones. In the closely resemblant S. marina the experimentally determined dispersal distance was almost the same in the two seed types when dispersed by wind, but when dispersed by water in the presence of vegetation, the winged seeds were more easily trapped at a short distance. In want of detailed knowledge of the phylogenetic interrelations in the genus needed to settle the question of whether the seed heteromorphism represents independent evolutionary events, or if the presumed adaptation has a common origin, by referring to the connection between the occurrence of seed wings and increasing seed size at both intrageneric and intraspecific levels, and the restriction of the seed heteromorphism to species living under conditions where secondary dispersal by water is important, I conclude that the heteromorphic condition is adaptive in relation to seed dispersal.