1928] Wexelsen: Chromosome Numbers and Morphology in Trifolium 375 



then, to mutational changes in species isolated by interspecific sterility. 

 It is not contended that crosses have not taken place in this genus, 

 but it is held that the species have been thus isolated for a long time 

 and that many mutations have occurred. 



SUMMARY 



1. Somatic chromosome numbers have been obtained in eighteen 

 species of Trifolium, twelve of which had not been counted before; 

 the reduction division was studied in two species. 



2. The ten American species studied have the diploid numbers 

 16, 32, 48. No representative is found of the 7-series which is found 

 in European species of Trifolium. 



3. The chromosomes of Trifolium are in general small, but they 

 exhibit a great variation in size, both as to single chromosomes and 

 to total amount of chromatin. 



4. In T. repens L. the two varieties gig ant sum, and sylvestre proved 

 to have chromosomes of different size. Giganteum is a giant variety 

 and has large chromosomes ; sylvestre is a small variety and has small 

 chromosomes. F^ plants between these two varieties showed chromo- 

 somes of intermediate size. 



5. Ten species from several sections of the genus have been shown 

 to have 1 pair of satellited chromosomes and one species probably 

 has 3 such pairs. 



6. The satellites are in some plates without visible connection with 

 any chromosome and appear like an extra pair of small chromosomes. 

 In a fcAv diakinesis plates of T. pratense were observed bodies which 

 must be interpreted as 1 pair of satellites attached to a bivalent 

 chromosome. 



7. On the basis of satellites, constrictions, and chromosome size, 

 a scheme of chromosome morphology has been given for some of the 

 species. 



8. Species crosses were attempted between nine species in eighteen 

 different combinations, but with completely negative results. 



9. The suggestion is made that the diversity of chromosome com- 

 plexes in Trifolium is a result of mutational changes in species which 

 have become isolated by intersterility rather than the result of 

 hybridization. 



