A detailed analysis was given in time (1940-1946) and place — horizontally and vertically — of the habitats of some characteristic species of plant communities on a shore (salt marsh) at the westpoint of the Dutch island Vlieland. The distribution of Carex extensa, Pholiurus filiformis and of Blysmus rufus was studied, more especially in connection with the chlorine contents of the ground water (soil liquid). In this respect new habitats of Carex extensa and of Pholiurus filiformis had the same oecological relations. On salt marshes, directly accessible for the springtides, Carex extensa occurred together with the var. subcurvatus of Pholiurus filiformis. On the shore behind coastal ridges, only indirectly accessible for the sea water at springtides, Carex extensa settled together with the var. strictus of Pholiurus filiformis. It was found that never Blysmus rufus and Carex extensa occurred in the same places. This is in accordance with the old floristic literature, mentioning the preference of Blysmus rufus for desaliniting stages on the shore, whereas Carex extensa tolerates 3-7 gr. per 1. in the soil liquid. In connection with these data the historical evolution of some plant communities (Junceto-Caricetum extensae, Artemisietum maritimae), as determined in their oecological relations by the oecology of their characteristic species Carex extensa, Pholiurus filiformis and Blysmus rufus, was shortly discussed. Finally caution was recommended in applying the results of descriptive, synthetic, regional sociology, without testing these by the standard of the local state of affairs.