40 Genetical and Cytological Studies on Pisum 



and pollen mother cells from an F^ plant of Early Giant rogue 615^/30 

 crossed Th. 10 (see Part I) were similarly investigated. Ten pollen mother 

 cells were studied in the latter two cases, and thirteen in the first. The 

 results are given in Tables I and II and in Fig. 3. The graphs indicate 

 a certain order in the number and distribution of the chiasmata — which 

 may be better understood when such statistical methods have been 

 appUed to many species. While the mean chiasma frequencies of 615^/30 

 and 532/30, 2-8 and 2-9 respectively, do not differ greatly, there is a 



12 3 4 5 6 



Fig. 3. Chiasmata in Pisum sativum. In the case of 696' only the five bivalents are included. 



considerable difference in the distribution of the chiasmata throughout 

 the chromosome complement, as shown by Fig. 3. 



696^/30 has a mean chiasma frequency of 2-55 per bivalent, which is 

 considerably less than that of the other two plants. It may be that one 

 of the original parents, Th. 7 or Duke of Albany, had a lower chiasma 

 frequency than Th. 10,' or it may be that the association of four chromo- 

 somes lowers the chiasma frequency of these chromosomes. Further 

 work may throw light on this subject. The terminalisation coefi&cient was 

 calculated by dividing the number of terminal chiasmata by the total 

 number of chiasmata. 



