246 



FUNDAMENTALS OF CYTOLOGY 



uniform chromosomes with more or less median kinetochores to shorter, 

 distinctly different types of chi-omosomes with nonmedian kinetochores. 

 Since morphologically similar species usually have similar karyotypes, 

 the genus is especiallj^ well suited to cytotaxonomic study. A change 

 in the number of chromosomes from 10 to 8, the most prevalent diploid 

 number in the genus, and from this to still lower numbers has occurred. 

 This was probably accomplished in a series of reciprocal translocations 

 by which all the essential portions of certain chromosomes were trans- 

 ferred to other chromosomes, the chromosomes losing these portions then 

 being eliminated. Following this was the formation of amphidiploids by 

 hybridization and chromosomal doubling, giving a series of species having 

 more than twice as many chromosomes as the primitive ancestral stock. 



Fig. 175. — Table showing the concept of species and the terms employed by certain experi- 

 mental taxonomists. {After J. Clausen, D. D. Keck, a?id W. M. Hiesey.) 



Many interspecific crosses in the breeding plot have aided in determining 

 the degree of relationship between the various species. 



Five processes of genetic change are regarded as significant in the 

 evolution of Crepis. Of primary importance has been (1) the structural 

 transformations of the chromosomes revealed in their number and 

 morphology, for these produce initial intraspecific sterility which makes 

 possible the accumulation of further intersterilitj^ along with morpho- 

 logical and physiological divergence. This and (2) gene mutation prob- 

 ably account for most of the progressive specialization within the genus. 

 To these causes of change, (3) interspecific hybridization is secondary 

 in importance, while (4) polyploidy and (5) apomixis have played 

 definite though relatively unimportant roles. 



A second series of investigations illustrating the methods and results 

 of experimental taxonomy is that being carried out in the Pacific Coast 

 region of the United States. Following the lead of Turesson in Europe, 

 American botanists are investigating the species of numerous genera with 



