For the study of the section Eu-callitriche in the Netherlands the cytological investigations formed the base for the outlines of the species. Owing to the chromosome sets, very different among themselves, the section might be split up into 5 fractions. This division proved to be consolidated as well by morphological, geographical and ecological characteristics and by the differences in the lifecycles. The combination of these characteristics at last led to a division into 5 species: C. hamulata Ktzg, C. obtusangula Legali, C. platycarpa Ktzg, C. stagnalis Scop, and C. palustris L. While cultivating under various conditions we saw that—C. stagnalis excepted, being little variable—all species are very variable, especially in the vegetative parts. Many varieties, described in literature, must therefore be considered as modifications. Also C. pedunculata DC., described as a species, is probably a modification, and in that case belongs to C. hamulata. Only a few varieties in ecological and morphological qualities are probably founded on hereditary factors (different leafshape and larger salt tolerance in C. obtusangula from the province of Zeeland, the brookform of C. platycarpa). For the time being no further systematic division was based on these characteristics. In the species C. obtusangula, C. stagnalis, C. palustris and C. platycarpa plants occur with and without SAT-chromosomes. Between the two types of one species no further differences were observed, only in the case of C. obtusangula the existence of SAT-chromosomes proved to coincide with a differing leafshape and larger salt tolerance. When studying the early stages in the mitosis, it became apparent that each satellite is composed of 2 parts, which unite before the metaphase into one (plate 4). Most species show a clear preference for a certain environment, which also appears from the distribution regions. In order to unravel the nomenclature, the knowledge of morphology, distribution etc. proved to be an aid. Thus e.g. C. intermedia Hoffm. must be rejected as a synonym for C. hamulata Ktzg, because of the quite different structure of flower. A short survey of the principal data follows below. C. hamulata Ktzg. In several respects this species occupies a place of its own in the section. It deviates in the number of chromosomes (2n = 38; see plate 1) from the other species which have 2n = 10 or 2n = 20. The chromosome set must be considered as the most asymmetrical; the lengths of the chromosomes are very different and as for the position of the centromeres, we find transitions from median to subterminal centromeres (fig. le). In contrast to the other species of the section no plants with SAT-chromosomes were observed. Also in the structure of the flowers and in the submerged pollination C. hamulata differs from the other species (plate 14a; fig. 5). Because of its submerged pollination it is the only species able to fructify in deep water and in swiftly flowing water. With regards to the distribution it is, for the Netherlands, typically a plant of sandy soils (pleistocene soils and dune soils, see map 1,2; p. 336) where it grows in slightly acid – neutral water (cf. app. II, table I). C. hamulata is the only winterannual species. By the fruit, with reflexed stigmata rests, laterally pressed close to it, it may easily be distinguished in any habit from other species. C. obtusangula Legall has 2n = 10 chromosomes (plate 3d). Some chromosomes belong to the longest of this section. The position of the centromeres is submedian – subterminal (fig. 1 a). This salt tolerant species occurs in Europe in the mediterranean-atlantic coastal areas (map 5, p. 342). In the Netherlands we especially find it in oligohalinic-mesohalinic water of the wadden-, dune-, half- and fluviatile districts (map 3, 4; p. 340). The water is neutral to slightly alcalic (cf. app. II, table II). Well developed waterforms may be recognized by the dense rosette with rhombic, thick leaves (plate 8a), the fruit with rounded edges (fig. 8a), the non-winged seeds (plate 16a) and the ellipsoidal pollengrains (fig. 10). The species is a perennial. A different form was found in the prov. of Zeeland. Those plants show an other leafshape (plate 8b), the seeds germinate in a higher salt concentration (cf. Ch. VI, table VI-X). The chromosome picture of these plants shows 2 SAT-chromosomes; the plants from Texel have no SAT-chromosomes. The satellite is connected to the small arm of the chromosome (plate 3e). C. stagnalis Scop, also has 2n = 10 chromosomes (plate 3a). But they are different in shape and size from those of C. obtusangula. All chromosomes have a subterminal centromere (fig. lb). In this species plants were found with 0, 1 or 2 SAT-chromosomes (plate 3a, b, c). The satellite is connected to the long arm of the chromosome. The chromosome picture, observed by us, differs from that of the Russian plants of Sokolovskaja (1932); (cf. fig. lb; fig. 3). Just like C. obtusangula C. stagnalis is salttolerant (cf. Ch. VI, table XIV, XV) and occurs in coastal regions of Europe. However it also appears inland (map 10, p. 347). In the Netherlands (map 9, p. 347) we find it in the brookvalley areas and here and there along the coast (Biesbos). It is the only species that varies but little, and never forms linear leaves. The round rosetteTleaves (plate 9a), the globular pollen (fig. 13), the characters of the female flower (fig. 11a, b) and the broadly winged seeds (plate 16e) are characteristic for this species. C. platycarpa Ktzg. The number of chromosomes is 2n = 20 (plate 2a). All centromeres are subterminal (fig. 1c). Also in this species plants occur with 0 or 2 SAT-chromosomes (plate 2c, d; fig. 2). The satellite is connected to the long arm of the chromosome. C. platycarpa is an extremely variable species. In the Netherlands it proves to be the most common. It is much less restricted to special conditions of environment than the other species and occurs everywhere, except in brackish water (map 7, p. 345). On account of its polymorphy and because it is often found in a sterile condition, this species often can be recognized as such after culture-experiments or cytological research only. That is probably the reason why it was not recognized thus far in our country. C. palustris L., just like C. platycarpa, has 2n = 20 chromosomes (plate 3f). They are smaller, however, and are nearly equal in size. (C. palustris has the smallest chromosomes of the 5 species of the section, fig. Id). Here we also find plants with 0 or 2 SAT-chromosomes (plate 3f, g). The satellite is connected to the small arm of the chromosome. For this species the connections between the metaphase-chromosomes in the mitosis are remarkable (plate 6a, b). In habit C. palustris is the smallest of the 5 native Eu-callitriche species, the plants are tiny and little branched; as landform, the form in which it is mostly found, it is apogamous: the stigmata are abortive in these plants, while the stamens often have vanished altogether (plate 14b, c). The embryosac probably develops from a somatic cell (plate 5). The area of distribution is—so far as western Europe and Greenland are concerned—principally arctic-alpine (map 12, p. 349). In the intermediate regions it appears only rarely, and then only under very special environmental conditions (“cowpuddles”). Also in the Netherlands only few finding-places are known (map 11, p. 348). The species is a summerannual and well discernable by the very small dark fruit. The seeds are winged at the top only (plate 16d).