1925] Mann: Chromosome Number and Individuality in the Genus Crepis 303 



TABLE 4 



The important point is that one can identify the chromosomes of 

 dioscoridis and of tectorum by measurement when they are in combina- 

 tion with those of setosa in an Fi hybrid, so that it is evident that the 

 specific differences in length noted are not the product of interaction 

 between a certain cytoplasm and its chromosomes. 



Since abundant material was available for capillaris (fig. 6), the first 

 measurements, which were made on ten figures about as good as the 

 average for all species, were checked by the use, first, of a mixture of 

 slightly different metaphase stages (beginning to almost complete divi- 

 sion) from a very short region of a single root tip, and, second, of a 

 mixture from undivided figures from two different roots. These 

 measurements show that averages for one chromosome in three different 

 sets of ten from the same species may differ by as much as 3.55 mm., 

 but that the averages give, in each case, very nearly the same differ- 

 ences between the lengths of the different pairs. 



COMPARISON OF SPECIES 



Crepis neglecta (fig. 7) has a very characteristic individuality, two 

 of the pairs being very similar and distinctly shorter than any of the 

 chromosomes of capillaris. Its total length is very similar to that of 

 capillaris, so much so that one is inclined to test the cross-division 

 hypothesis for this species. If the two shortest averages are added, 

 their sum is practically the same as the average for the intermediate 

 chromosome of capillaris and the other average lengths are very similar. 



capillaris.. 

 neglecta.... 



26.2 

 24.5 



11.2+9.8 = 



20.4 

 = 21.0 



14.8 

 16.2 



■1.7 



+0.6 +1.4 



Attempts to cross the two species have as yet been unsuccessful. 



