1928] Wexclsen: Chromosome Numbeis and Morphology in Trifolium 373 



1 pair of satellited chromosomes ; but considering the fact that species 

 from very different sections have the satellites and that, on the other 

 hand, species with and without satellites occur in the same section, 

 this feature does not help much in establishing any relationship 

 between the species. It does not seem safe, either, to take chromosome 

 size in general as an evidence of relationship, when we remember 

 that the one species, T. repens, includes in itself almost the total 

 variability in chromosome size in the genus. It is apparently only in 

 the narrowest taxonomic groups that there is a similarity in chromo- 

 some morphology which points to common descent, and it is probably 

 here that the study of the chromosomes may be of help to the 

 taxonomist. Some facts pointing to this conclusion may be mentioned. 

 T. variegatum in the section Variegatae has 16 very small chromo- 

 somes. In the same section is T. ivormskjoldii with 48 equally small 

 chromosomes. This suggests that these two species, in regard to their 

 chromosomes, are more nearly related than the Californian clovers of 

 other sections. The two nearly related forms, T. fucatum and T. 

 virescens, have almost identical chromosome complexes. The chromo- 

 some sizes of the two related species, T. hybridum (16 diploid) and 

 T. repens (32 diploid), indicate that the latter may have a complex 

 which is twice that of the former. 



The situation in Trifolium is interesting because it seems to demon- 

 strate another type of differentiation of the chromosome complexes 

 than is found in many other genera studied. The genera which have 

 been most intensively investigated cytologically are those in which 

 interspecific hybridization has been carried out. There has been, then, 

 a preference for genera in which interspecific hybrids are fairly easily 

 obtained, and in which such hybrids are common in nature. This has 

 led some investigators to emphasize hybridization as the only factor 

 in species differentiation, and it may perhaps not be out of the way 

 to hold forth that there may be other ways of evolution of species. 

 It seems only fair to do so in connection with this study in Trifolium, 

 because all evidence suggests that hybridization has not played a 

 dominant role in the differentiation of this genus. Hybrids are very 

 rare in nature, if, indeed, ever observed, and no hybrids have been 

 obtained in experiments. Taking into account only the external 

 morphology of the chromosomes, in Trifolium no certain instance of 

 "homologous" chromosomes in different species is known, whether 

 in the form of one single chromosome, a group of chromosomes, or a 

 complete haploid set. The existence of a satellited chromosome pair 



