In the Netherlands the distinction of the species Spergularia media (L.) C. Presl (Syn.: S.marginata (DC.) Kittel) and Spergularia marina (L.) Grisebach (Syn.: S.salina J. et C. Presl) is not always easy. These taxonomical difficulties gave cause to investigate the variability of these two halophilous species. From a nomenclatorical study it appeared that and S.marina are the legitimate names (Chapter I). The variability of both Spergularia-species has been studied by means of population samples (Chapter II). The seeds of and S.marina vary from unwinged to broadly winged; however, the variation per plant is limited, the plants mostly producing in their proximal capsules either unwinged to rudimentary or narrowly winged, or only broadly winged seeds. These differences in the width of the seed wing are apparently genetically controlled. In general the winged seeds are larger than the unwinged seeds. The most important diagnostic characters of both Spergularia-species are (a) the number of stamens: in (0-)10, in S.marina (0-)2-5 (-10) and (b) the diameter of the flower: in (7-)10-12(-13) mm, in S.marina (4-)5-7(-8) mm Both species show a reduction of the androecium; the total number of stamens and staminodes of is always 10, but when staminodes are present the diameter of the flower is smaller; flowers 7-8 mm in diam. always have a considerable number of staminodes. S.marina rarely has staminodes: if present there are at most only three of them. Other diagnostic characters are the length of the bracts and the position of the stigmas. In Chapter II the variability is correlated with the geographic distribution and the environment. The distribution of on the outside of the dikes can be correlated with the salinity of M(ean) H(igh) W(ater). The limit of distribution upstream in the estuaries is found near the transition of Polyhalinicum to Mesohalinicum (see page 72). is a differential species of the order Glauceto-Puccinellietalia. In some respects this species shows an independent behaviour in its relation to the other species of this order. occurs in a zonation of vegetation in the Artemisietum maritimae and not in the Puccinellietum maritimae and vice versa. In sandy regions is found in the zonation below Puccinellia maritima. Populations of with a high frequency of plants producing unwinged seeds are found in open pioneer vegetation, on sandy soil in an extreme and unstable environment (see photo 2, page 81) or in intensively grazed vegetations (also on clayey soils). Populations of composed of plants that produce exclusively winged seeds, are found in closed vegetation on clayey soil in a less extreme and more stable environment (see photo 1, page 81). In the population complex of there is apparently an ecotypical differentiation. Populations composed of plants producing exclusively unwinged seeds do not occur in the Netherlands, the pressure of selection probably not being strong enough. On the isle of Römö (Denmark) and according to reports in the literature also elsewhere in Europe and North-Africa there are populations exclusively composed of plants with unwinged seeds. As the genetically controlled variability of the size of the seeds and the seed wing is largest in relative extreme and unstable environments, the variability decreases when the environment becomes less extreme and more stable. This also happens when the environment becomes very extreme. When population of extreme and less extreme environments are compared, the population density is higher, and the mean age of the individuals lower, in the first case. The distribution of S.marina on the outside of the dikes can be correlated with the salinity of M(ean) H(igh) W(ater). In the Mesohalinicum (see page 72) S. marina is found on the level of M.H.W., in the Polyhalinicum and Buhalinicum S.marina occurs at a higher level with regard to M.H.W. Upstream in the estuaries the limit of distribution is found mostly near the transition between Mesohalinicum and Oligohalinicum. S.marina is a differential species of the alliance Puccinellio-Spergularion. The species occurs in the subordinate associations with high presence. In one locality in the Netherlands, (see photo 3, page 110), namely in the eastern part of Terschelling, a population of S.marina is found that for the greater part is composed of plants producing exclusively broadly winged seeds in their proximal capsules. In general it can be concluded that incidental factors have a considerable influence on the variation pattern in the Netherlands as a result of the small size of the local populations, the scattered habitats and the obligatory autogamy. As regards dissemination, and S.marina are polychorous, namely anemochorous, hydrochorous, zoochorous and anthropochorous (Chapter IV). Both Spergularia-species show a number of differences in floral structure and floral ecology (see page 124 e.v.). is, dependent on the circumstances, allogamous or autogamous; S.marina is almost invariably autogamous, very rarely allogamous. Both species do not hybridise (Chapter IV). The diploid number of chromosomes of is 18 and of S.marina 36 (Chapter V). In Chapter VI a taxonomic description of and S.marina is given and a key of all the Spergularia-species native in the Netherlands. The final conclusions of the general discussion (Chapter VII) are that and S.marina are clearly distinguishable on morphological, cytological and also ecological grounds. The species are reproductively isolated. Both species are very variable. In the population complex of a great deal of ecotypical differentiation is found. The local populations of are not reproductively isolated, and the same applies to the local populations of S.marina. As the variation is clinal no infraspecific taxa are described.