308 Vniversity of California Publications in Agricultural Sciences [Vol. '2 



41.9+32.4 



sibinca • =37.1 27.6 23.2 18.5 



2 



dioscoridis 35.9 29.3 24.9 19.3 



Difference 1.2 1.7 1.7 0.8 



If we suppose that this group of species has been derived from a 

 type like capillaris, we must consider that the longest chromosome 

 represents a multiple. If we subtract the intermediate average for 

 capillaris (20.4) from the average of the longest chromosomes of all 

 three species in this group (36.3), the remainder, 15.9, is only 1.1 units 

 longer than the shortest chromosome of capillaris, indicating that an 

 intermediate and a short chromosome might have united end to end 

 to form an element averaging 36.3 units. Then if we average the two 

 shortest chromosomes of these three species with the chromosome of 

 20.4 units, which, we have supposed has united with a short element, 

 the average, 19.9, is so like the intermediate of capillaris as to suggest 

 that it may have been duplicated in the group under consideration. 

 When we look at the averages now, the figures compare very well. 



capillaris 26.2 20.4 14.8 



pulchra, dioscoridis, on r_|_2Q "^4-27 6 



and sifemca — '- '- -=29.1 19.9 15.9 



3 



+2.9 -0.5 +1.1 



These species obviously form a group by themselves, especially 

 since it has been shown that the great size of the chromosomes in 

 dioscoridis is maintained upon hybridization with a species like setosa. 



DISCUSSION 



For two reasons it is impossible to make any sweeping general- 

 izations at this time concerning the data presented here. First, we do 

 not yet know how species differing in chromosome number can arise, 

 and second, we know too little about the genetics of Crepis. There are 

 two known methods by which a single pair of chromosomes can be added 

 to a complex, non-disjunction and species-hyl^ridization, but in neither 

 case has it been proved that stable types would ever result; and the 

 formation of new species presupposes stability. It has been suggested 

 that it is very improbable that stability is to be expected of tetrasomic 

 individuals because the complex as a whole is unbalanced by the addi- 

 tion of chromosomes. This view seems to be borne out by observations 

 on the cytology of tetrasomic plants of Datura (Belling and Blakeslee, 



