422 University of California Publications in Agricultural Sciences [Vol. 2 



3. In the hybrid, C. aspera X hursi folia, the structure of the 

 chromosomes can be recognized as similar to that of the parent species 

 and chiasmas can be found between the partners of the bivalent chro- 

 mosomes. In pollen mother cells where four pairs of chromosomes are 

 present, a pair consisting of two unequal partners is seen and the short 

 bursifolia chromosome can be followed through and recognized in all 

 phases. 



4. Of the three hybrids, C. iaraxacifoJia X tectorum and C. aspera 

 X bursifolia show comparatively many irregularities during meiosis, 

 while C. aspera X aculeata is fairly regular (table 1 and 2, p. 412). 



5. The data on meiotic irregularities in these hybrids are in close 

 agreement with the taxonomic relations between the parental species; 

 also the data on percentage of "good" pollen formed in the hybrids, 

 and their fertility, agree well, in the main, with the irregularities 

 observed in meiosis as well as with the taxonomic relations, with the 

 exception that, if their relationship is below a certain threshold value, 

 these criteria cannot be used for determining how remote two species 

 are from each other. 



