I93I The secondary association of chromosomes 367 



is especially true in the absence of 1) any comment as to fixation and 

 2) comparison between different stages of meiosis. 



Assuming that fixation has not produced clumping, in many of the 

 examples listed in the table secondary association is quite obvious. In 

 others it is less obvious, but in this connection we must remember that 

 the cytologist always chooses the " best" plate to draw, i. e. the plate 

 which shows the most even distribution of chromosomes in polar view. 

 Thus an unconscious selection is made of plates showing the least 

 grouping of bivalents. It may be taken for granted that when associa- 

 tion is seen in these plates, then it most certainly will be more pro- 

 nounced in others not figured. 



It is realised that no final conclusion can be drawn from inspection 

 of the drawing of other workers. From experience of secondary as- 

 sociation in Dahlia however, the orientation, disposition and grouping 

 of bivalent chromosomes in the many drawings examined strongly 

 suggest that this association does occur in a great number of cases. 



I would emphasise therefore that the comments on the references 

 listed in Table 2 and the table itself are offered as a basis for further 

 investigation rather than at the expression of a final opinion. The 

 following brief notes are designed to direct attention to points of 

 special interest in the papers quoted in Table 2. 



Saccharum. 



Bremer reports that at diakinesis it is usual to find the full number 

 of bivalents. Attention is drawn to the association of bivalents of 

 similar size. Multivalent association is, at the most, rare in the species 

 and varieties studied, yet pronounced groups of 2, 3 and 4 bivalents are 

 clearly shown (e.g. Fgs. 54). 



Cotton (Denham 1924). , 



Association in the n=13 forms suggests that these, like Pyrus, are 

 secondary polyploids in which certain chromosome types have been 

 duplicated. It is possible that Brassica sativa and oleracea (Winge, 

 Karpechenko) along with related forms are likewise secondary diploids 

 (see Lawrence, 1930, pp. 296). 



Senecio (Afzelius 1924). 



The basic number of this genus is 5 (cf. S. discifolius, Emilia 

 sagittata and E. sonchifolia). The drawing of Othonna carnosa shows 

 association of bivalents ; note similarity in size of associated bivalents 



