1929] Navashin: Triploidy in Crepis 391 



penetrate into the barrier zone, thus continuing the process of in- 

 vasion. Some evidence confirming these suggestions may be seen in 

 C. biennis. This polyploid species (J. Collins and M. Mann, 1923), 

 owing perhaps to its biennial life-cycle, has acquired the ability to 

 inhabit an area which is unavailable to another closely related species, 

 C. capillaris. 



Furthermore, as was shown above, triploidy stimulates interspecific 

 crossability, probably by reason of the higher viability of the hybrid 

 zygote derived from a diploid egg, and the presence of a normal 

 balanced chromosomal complex of one of the species crossed. (3ther 

 features may also play a considerable part; among them the partial 

 male sterility of triploids, which may facilitate successful cross- 

 pollination. There is a possibility, moreover, that the structure of 

 the stigma and of the style facilitates hybridization between some 

 species, owing to enlarged dimensions of the cells, etc. 



Such polyploid hybrids, as has been shown, are not only more 

 viable, but, moreover, show a higher degree of fertility than normal 

 ones; and the peculiarities of their chromosomal constitution afford 

 favorable conditions for the production in subsequent generations of 

 various recombinations of chromosomes of the parental species. 

 Finally, crossing of two triploid individuals belonging to different 

 species may produce directly a balanced amphidiploid (M. Navashin, 

 1927) hybrid, as a result of the meeting of two diploid gametes. The 

 results of investigations on polyploid interspecific hybrids in Crepis 

 will be reported in another paper. 



The other combinatory alterations of the cell nucleus occurring 

 in the progeny of triploids, i.e., trisomies and tetrftsomics of various 

 kinds, could hardly play any part in species formation. For Datura 

 there is even direct evidence against such a suggestion, for balanced 

 tetrasomic types show greatly reduced viability, and it is unreasonable 

 to ascribe to them the role of originators of new forms. In Crepis, 

 as a matter of fact, such types do not even occur. 



For the purpose of genetic analysis the trisomies in Crepis are of 

 the highest interest because of the fact that the individual chromo- 

 somes may be easily identified. It should be pointed out that the 

 excellent morphological features of the chromosomes in Crepis allow 

 a visual verification of the hypothesis of the origin of secondaries, the 

 latter being supposed according to Belling (1924) to have duplicated 

 ends of certain chromosomes. A careful study of trisomies in Crepis 

 shows, however, that none of them possess even the slightest abnor- 



