CONSEQUENCES OF CHANGE 



sequence the distribution of the odd set is a binomial one with halt 

 the triploid number expected to be the most frequent type. The 

 mean and the mode, however, are usually below 3X/2 on account 

 of the loss of a varying number of chromosomes, usually 10 or 

 20 per cent. The loss is, of course, highest in species with the 



x=3 -y<^ ADC 



x+A ^J^ 



+c vf 



-D 



-C 



2x = 6 



AADDCC 



Fig. 28. — Mitosis in types of balanced and unbalanced pollen grain produced by 

 a 3.V Crcpis capillaris in the proportions shown in Tabic 12 (after Chuksanova, 1939). 



smallest chromosomes, and for the smallest chromosomes in a set, 

 since these have the fewest changes of partner, the fewest chiasmata, 

 and therefore the most failure of pairing. Loss is also higher in the 

 larger embryo-sac mother-cell than in the smaller pollen mother- 

 cell. 



The course of events at mciosis in triploids is reflected in the 

 frequencies of the pollen grains having different chromosome num- 

 bers (Fig. 28 and Table 12). At mitosis in the pollen grains of 

 Crepis capillaris each of the three chromosomes can be recognized. 



124 



