1. The interrelationships of five western European violets are determined by examining chromosome pairing in their F1 hybrids. Viola stagnina Kit. (n= 10), V. pumila Chaix. (n= 20), V. canina L. sspp. canina and montana (L.) Fries. (n= 20) and V. lactea Sm (n= 29) belong to the group Arosulatae and Viola riviniana Rchb. (n= 20) is in the group Rosulantes.
2. (a) Seven artificial interspecific hybrids were studied and the most frequent meiotic chromosomal arrangements can be summarized as follows: Viola canina × lactea (2n= 49)—2011+91; V. canina × stagnina (2n= 30)—101 + 101; V. canina (sspp. canina and montana) × riviniana (2n= 40)—1011+201; V. lactea × riviniana (2n= 49)—1011+291; V. pumila × canina (2n= 40)—1011 + 201; V. lactea × pumila (2n= 49)—1011+291.
(b) V. canina ssp. canina × riviniana and V. lactea × riviniana also had supernumerary chromosomes, derived from their riviniana parent. These varied in number, even between the p.m.c. of a single hybrid plant, which suggests somatic chromosomal irregularity, possibly during premeiotic mitosis.
(c) The two hybrids involving V. pumila frequently showed a high proportion of multivalents, as well as ‘clumping’ of the bivalents; the univalents were generally distinct but some tended to show secondary association at M1.
3. The genomic constitutions of the species were deduced from the chromosome pairing in the hybrids. The species are seen to be related in a reticulate pattern which is based on the combination of genomes by allopolyploidy. Within this pattern the groups, Arosulatae and Rosulantes, merge into one another, at least one genome being shared by species in each group.
4. The affinities and origin of Viola lactea are discussed. It is an allohexaploid deriving two genomes from V. canina. The species carrying the third genome cannot be found in western Europe and it is suggested that, if it still exists, it may be among the diploid Arosulatae in eastern Asia.