300 



University of California Publications in Agricultural Sciences [Vol. 2 



TABLE 1— (Continued) 



Table 1 shows that, while four is the most common haploid number 

 for the twenty species studied, five is also fairly frequent. The other 

 numbers (3, 6, 8, 9, and 20) are each represented by a single species. It 

 is obvious that chromosome measurement should show whether cross- 

 division, union into multiples, addition by non-disjunction, or combina- 

 tions of these methods are sufficient to account for the differences in 

 number found in the genus. It is also possible that hybridization 

 between species with different chromosome numbers might account for 

 the origin of certain cytological peculiarities. 



For some species the cytological material is far more abundant than 

 it is for others, so that it is possible to measure only somatic metaphases 

 in which all the chromosomes are fairly straight. The tendency of the 

 long chromosomes of Crepis to twist is a source of considerable error 

 where relatively poor material is available. The finest metaphase 

 figures are to be found in the upper portion of the rapidly growing 

 region of the root in seedlings, and in roots from adult plants. The 

 region containing fine figures is greater in roots from the latter than 



