SUMMARYThe considerable range of forms within the collective species Valeriana officinalis Linn. sensu lato is due to three main causes: (a) a high degree of phenotypic plasticity in natural habitats; (b) a polyploid differentiation with tetraploids (2n = 28), representing the presumably older types, and octoploids (2n = 56) representing the younger type; (c) a distinct genie differentiation within each chromosomic group, which brings about a considerable number of forms with more or less distinct morphological differences.A detailed morphological analysis of a number of characters in the various forms has shown that purely morphological criteria are inadequate for the delimitation of the two polyploids which are connected by a range of intergrading forms. The various characters are not regularly correlated in the strains occurring in nature; on the contrary, they appear in a variety of combinations. In view of this, any attempt to subdivide the polymorphic species into smaller units must fail.The various forms are produced by sexual reproduction. Repeated cross‐pollination is followed by Mendelian segregation and recombination, while vegetative propagation contributes to the fixation of the numerous heterozygous forms. The two polyploids are intersterile.Distinct differences occur in the distribution of the two polyploids. The area of the tetraploids is much smaller than that of the octoploids. The former is limited to the south of England and parts of the Midlands, the latter extends from the south of England to the north of Scotland. Within the common area, the tetraploids are confined to particular dry habitats in hilly regions and are ecologically separated from the octoploids which grow at lower altitudes in moist soil. In their distribution the tetraploids follow some definite geological formations, chiefly Chalk, Oolitic limestone, as well as some areas of Carboniferous limestone in the south‐west. The general area roughly corresponds to that of plants representing in the British flora the continental southern geographic element. The spreading northwards of the tetraploids seems to be prevented by a climatic barrier, and in contrast, the octoploids were able to advance beyond this area and to extend notably the geographical and ecological range of the species.The remarkably wider range of forms of the octoploids is possibly due to their crossing with the extreme sambucifolia plants followed by segregation and recombination. On the other hand, in the tetraploids the narrower range of forms is presumably due to the ecological separation and the barrier of incompatibility which in the tetraploids limits the exchange of genes to crosses within this group. Therefore, the process of partial fusion with the sambucifolia type has been achieved exclusively on the octoploid level.In the formation of the wide range of forms within the collective species V. officinalis Linn, sensu lato, presumably two different major mechanisms were at work: the doubling of chromosomes and the intercrossing of the newly produced higher polyploids with closely related plants belonging to the sambucifolia t