Biosystematic studies on Spergularia media and S. marina in the Netherlands. III. The variability of S. media and S. marina in relation to the environment

In this third report of this series concerning biosystematic studies of the Dutch halophilous species of Spergularia, the relation between the variation and the environment is discussed. In the case of S. media, a correlation between the occurrence of populations with a high representation of plants with unwinged and subalate seeds and the degree of extremity of the abiotic factors prevailing in the habitat seems to be manifest. In some cases overgrazing by sheep or cattle also results in a relatively high representation of such plants in the population, even if the abiotic factors are favourable. The possible causes of a selective advantage of winged seeds over exalate seeds in certain types of habitats (and vice versa) are discussed. The predominantly autogamous S. marina occurs in populations consisting of plants with unwinged seeds. However, one population was found containing plants with winged seeds; the possible ecological causes of this phenomenon are mentioned.