1929] Bah cock-Clausen: Meiosls in Crepis 421 



These results are in fairly good agreement with the taxonomic 

 relationship as determined by comparative morphology. It will be 

 noted, however, that meiotic irregularities are just about as high and 

 fertility nearly as low in aspera X hur si folia (both in Subgenus A) as 

 in taraxacifolia X tectorum (Subgenus A X -S). This must indicate 

 profound physiological diversity between Barkhausia and Nemau- 

 chenes. It provides additional reason for maintaining these species 

 of Nemauchenes as a separate group from their close relatives in Bark- 

 hausia. It also shows, however, that when the relationship between 

 two species is below a certain threshold value, the meiotic irregulari- 

 ties and degree of sterility exhibited by hybrids between them are of 

 little value in determining the degree of relationship betAveen the 

 species. These criteria, in other words, are useful only within certain 

 limits in the study of taxonomy, and should be considered onlj^ in 

 relation to other criteria such as number and morphology of the 

 chromosomes, and the distribution, ecology, and comparative morphol- 

 ogy of the plants themselves. 



SUMMARY 



1. This paper deals with the two pure species, Crepis aspera L., 

 and C. bursifoJia L., together with the following three hybrids: Crepi§ 

 aspera X C. hursifoHa, C. taraxacif olia Thuill. X C- tectorum L., and 

 C. aspera X G- aculeata (DC.) Boiss. 



2. All five species have w = 4 chromosomes. After a discussion 

 of methods of fixation and staining, meiosis in the two species, C. 

 aspera and C. hursifolia, is described. In C. hursifolia one chromo- 

 some pair, which is shorter than the other three pairs, can be recog- 

 nized through all phases. The chromonema thread in the early zygo- 

 phase shows chromomeres, and parasyndesis has been found. In the 

 diplophase the chromonema coils up into a spiral filament which in 

 the diaphase or diakinesis appears to be doubled by splitting. In this 

 phase chiasmas between the partners of the bivalent chromosomes are 

 often seen. These chiasmas may be regarded as the result of inter- 

 change accomplished at an earlier stage, before the chromosomes have 

 become shortened by the spiral coiling of the chromonemata. Extrac- 

 tion of the crossed-over chromatids would occur in the heterotypic 

 anaphase. 



