Seed heteromorphism in a population of Spergularia media in relation to the ambient vegetation density

This paper presents basic data regarding the seed heteromorphism in a population of the short-lived perennial halophyte Spergularia media. Essentially two seed types exist, unwinged and winged seeds. The variation in seed size and the existence and the extent of seed wings was investigated in a population from the western coast of Sweden. Seeds were collected both directly from plants growing under natural conditions, and from plants developed out of soil samples brought into a greenhouse after the seed set in the autumn. The seed size varied considerably and the width of the seed wing was positively related to the length of the seed body, but in contrast to the situation in the closely related annual S. marina (where the winged seeds have appreciably larger seed bodies than the unwinged seeds), the average seed body length differed only marginally (but in the same direction) between the seed types in S. media. Most of the plants produced either exclusively winged or unwinged seeds, but still 45%Field or 10%Greenhouse of the plants were found to produce capsules with heteromorphic seeds (strongly dominated by the winged seed type). The spatial distribution of the different seed types in an environmental gradient comprising surrounding vegetation of sparse, intermediate and dense covering was also investigated. Seed capsules containing only winged seeds were more common in the dense vegetation. Inversely, capsules with unwinged seed contents were more frequently encountered in open areas. No such relation was found between the vegetation density and the proportion of capsules with heteromorphic seeds. A hypothesis is presented saying that the production of two seed types in S. media is dictated by the pressure for greater efficacy in seed dispersal in combination with differential survival and reproduction of the two different seed morphs.