A study in the open field in the Netherlands of the genus Erodium leads us, so far as the segregation of indigenous forms is concerned, to a division between those of the fields and those of the dunes. In the first we find one main form, Erodium cicutarium L’Her. subsp. arvale nov. subsp., in the second we meet with two main forms, viz. the intermediate and the extreme dune-form, resp. Erodium cicutarium subsp. dunense nov. subsp. and Erodium glutinosum Dum. How these three groups are to be discerned, has been described in the chapters III, IX. This subdivision into three main types, belonging to two species only, means a decrease in the number of our native Erodia, four of which have been described in the most recent flora (Heukels & Wachter, 1942), viz. Erodium cicutarium, E. neglectum, E. glutinosum and E. dentatum. This statement is based on field-, herbarium- and literature-studies, on cytological investigation, and on experiments in cultivation and hybridization. To Jansen and Wachter, to whom the polymorphy of the species was well-known, it was obvious that such investigations would be worth while and that, in the Netherlands at least, there was more to be seen than mere Erodium cicutarium, as appears from an article in the ,,Nederlandsch Kruidkundig Archief” 1923. In that paper the difference was pointed out between the fields and the dunes as far as this genus is concerned, and also the polymorphy of Erodium in the Netherlands. The said authors realised that there was a good deal of confusion in the different systems, but they inclined to a belief in a relation between our field-Erodium and the form pimpinellifolium (though appreciating the vagueness of the latter) and they insisted on a study of the literature beginning with Cavanilles. Plants both, with spotted and with unspotted flowers were already known to the above-mentioned authors and they also pointed out the possibility of particular areas in which these might be found. During our investigations we also arrived at the conclusion that Erodium in the Netherlands is quite polymorphous, that in the fields the form with maculate flowers is generally found, but that there is also a form with unspotted petals and that, in addition to these, there are two white-flowered varieties and one with many glandular hairs. In the Netherlands a definite area for each of them could not be determined, except perhaps in southern Limburg, where immaculate Erodia on calcareous and maculate plants on silicated soils are said to be found. The field-form mentioned belongs to the species Erodium cicutarium L’Her. This is subdivided into two subspecies, named after the habitats of the plants, viz. Erodium cicutarium subsp. arvale and Erodium cicutarium subsp. dunense nov. sspp. The latter is identical with our intermediate dune-form. It was not only on the strength of their morphological properties (character of indumentum, number of flowers in the inflorescences, features of flowers and carpels, length of beaks) that we came to the conclusion that the form of the fields and the intermediate dune-form (the so-called Erodium neglectum of the floras) should be comprised into one species, viz. Erodium cicutarium; the results of cytological investigation and of hybridization experiments too pointed to the same conclusion; both forms have the same chromosome number and in both the number 2n = 40 was determined from root tips. In the literature concerning Erodium cicutarium we meet with numbers varying from 36 to 40 (Warburg, Gauger, Heitz). In our hybridization experiments we found that the field-form and the intermediate dune-form could be hybridised in both directions, the hybrids being of complete or barely diminished fertility. Part of the decreased fertility seemed, moreover, to be due to weather conditions since the same phenomenon was observed in the pure parents when growing under identical conditions. A greater or lesser morphological similarity, the possibility of hybridization of different forms, the degree of fertility or sterility of the hybrids, all of these are used as criteria for determining the affinity of the individuals, hybrids between species in general being sterile, hybrids between units of a lower rank having a different degree of fertility. Consequently, on account of their morphological characters as well as of the results of our hybridization experiments we may decide that the very polymorphous Erodium of the fields and the intermediate dune-plant belong to one and the same species. In the Netherlands these two groups represent the species Erodium cicutarium. They are ecological vicariants; the one is especially found in the interior in sandy fields and along roads (sometimes in fields in the dunes), the other is restricted to the dune-region and is, at most, occasionally met with in those places where dune-sand has been deposited. A study of the literature elucidated the relation between Erodium cicutarium subsp. arvale and Erodium pimpinellifolium. We found the latter to be a synonym of part of the first, viz. of the maculate form of the fields, Erodium cicutarium L’Her. subsp. arvale a typicum Andreas. As to Erodium pimpinellifolium, this name is mentioned for the first time in Sibthorp’s Flora Oxoniensis (1794); here the species is distinguished not, as might be expected, on account of the form of the leaves but of the spots on the petals. Though the name Erodium pimpinellifolium is often attributed to a pre-Sibthorpian and post-Linnean author, viz. Cava-NILLES (Diss. bot. tySy-’SS), under the name of Geranium pimpinellifolium, we failed to find it in that publication. That this quotation is erroneous, was already known to Jansen and Wachter (Nederl. Kruidk. Archief 1931). The name Erodium pimpinellifolium, however, was given by Sibthorp to a plant, which L’Heritier in 1789 had already described as Erodium cicutarium. In their first paper Jansen and Wachter supposed the form arenarium Jordan to be one occurring in our dune-region. This, however, seems incorrect, since Jordan’s description of his Erodium arenarium in Pug. plant, p. 44 differs from the Dutch duneplants by the colour of the flowers (intense purpureis), by having both 2-4-fIowered inflorescences and rather long beaks (26-30 mm), as well as by the lack of glandular hairs except on the calyx; moreover, the two upper petals are pale-spotted at base. It is, therefore, not to be wondered at, that in their second paper Jansen and Wachter do not mention the form arenarium Jord. any longer and in the case of the Dutch dune-plants they only speak about the following foreign Erodia: Erodium bipinnatum, Erodium dentatum, Erodium glutinosum and Erodium neglectum. It seems to us that the first, with its finely cut leaves, hairless stalks, leaves and beaks, based by Willdenow on Cavanilles’ Geranium bipinnatum from North-Africa, is not to be considered as a synonym of the plants occurring in the dune-region of the Netherlands. A study of the literature and oral information has led us to the conclusion that the mysterious Erodium dentatum should be dropped; the reason is given in chapter VIII. Specimens from DU Mortier’s collection do not, in the critical feature, agree with his description. In no Dutch herbarium is this plant extant and no Dutch botanist has ever come across it in this country. In this way only Erodium neglectum Baker & Salmon and Erodium glutinosum Dumortier are left as the material identical to the two main groups of Erodium in our dune-region, the intermediate and the extreme dune-form; as a matter of fact, the latter is identical with Erodium glutinosum Dum. Erodium neglectum was supposed by Jansen and Wachter to occur abundantly in our dunes; it was believed to be the same as our intermediate dune-form. Others, however, had their doubts about this identity. Unpublished notes of J. L. v. Soest to his herbarium of Erodium supply important data on this subject. In consequence of his floristical observations the said investigator arrived at the conclusion that the genus Erodium presents many difficulties owing to its great variability as well as by its confused nomenclature. When studying the intermediate dune-plant, hitherto supposed to be the same as Erodium neglectum, described by Baker and Salmon in 1920, van Soest stated that this view is probably incorrect, as the former possesses a more or less distinct furrow around the pits of the carpels, whilst in the case of Erodium neglectum the lack of these furrows is emphatically pointed out. In the opinion of van Soest two forms could be distinguished, on considering the degree of development of these furrows, one with carpels > 5 mm and one with carpels < 5 mm; for the rest, however, the difference was very small. We found no grounds for this subdivision into two groups and we provisionally would conceive the intermediate dune-form as a unit. We agree, however, with van Soest in not identifying this unit with Erodium neglectum B. & S. In our opinion, van Soest was right in including the intermediate dune-form in Erodium cicutarium; in accordance with Hegi’s system he« provisionally placed it as var. dunense (nom. nud.) in the subsp. eucicutarium, next to var. pimpinellifolium. Thusfar we have had no opportunity to investigate the English Erodia, nor could we examine Baker and Salmon’s material in order tc ascertain that the furrow around the pit of the carpel is actually lacking. The phenomenon of the furrows generally needs accurate observation for it is not nearly so clear as might be expected on consulting the literature. Anyhow, it is an error to make a systematic classification chiefly based on the development of these furrows and it is undoubtedly necessary to consider a combination of features. We have already pointed out that too much centralisation in the system of Erodium may be the cause of errors and this appeared to be the case in our extreme dune-form, Erodium glutinosum Dum., the second species of Dutch indigenous Erodia. To what extent this species may be linked up with other species elsewhere could not be ascertained, but in the Netherlands it is a species that can be very well distinguished morphologically, a species which, it is true, can be hybridised with Erodium cicutarium, but which always gives entirely or almost sterile hybrids. Meanwhile it is easier to hybridise it with subsp. arvale than with subsp. dunense, the hybridization being, however, only possible in case Erodium glutinosum is the mother. In the course of our cytological investigations it was found that Erodium cicutarium is tetraploid with regard to Erodium glutinosum; in root tips the latter has somatically 2n = 20 chromosomes. The same number was arrived at by Warburg in the case of various English maritime Erodia, not more fully identified. The sterile hybrid Erodium glutinosum X cicutarium was met with wild, but it was never before artificially obtained, at any rate not in the flowering stage. It was called Erodium anaristatum because of the total or nearly total lack of the characteristic beaks; its chromosome number is 2n = 30. Its features are paternal and maternal as well as intermediate; the paternal characteristic of the spotted petals appears to be a dominant one. Erodium glutinosum reaches the northern limit of its area in the Netherlands, on the W. Frisian islands. .It has an atlantic distribution and is found in the coastal regions of Central England, Belgium, France, Spain and probably also of Portugal and on the island of Corsica; it does not occur in Denmark. Erodium cicutarium subsp. arvale is found nearly all over the world, in some places as an adventitious weed. The distribution of Erodium cicutarium subsp. dunense outside the Netherlands could not yet be determined. The number of so-called indigenous Erodia in the Netherlands, recently a growing one, has now been decreased; only Erodium cicutarium and Erodium glutinosum are accepted as good species in this country.