645] ASSORTMENT OF CHROMOSOMES 343 



It is evident that the orientation of the trivalents in 

 the first metaphase must be nearly or quite a random 

 one, as was suggested in triploid (Enotheras (5, 6) and 

 mulberries (8), and as is the case in triploid Cannas (1). 

 (Nearly similar results were also obtained from a total 

 of 58 single-metaphase plates from this triploid Datura.) 



Detachment of Chromosomes.— Three buds yielded 

 62 pollen-mother-cells with both second-metaphase plates 

 countable, and among these there were six cells showing 

 that one chromosome had been detached at the first ana- 

 phase (Fig. 7), one cell showing detachment of two- 

 chromosomes, and one cell showing both one and two de- 

 tached chromosomes. Thus there were about 13 per cent, 

 of cases of detachment. These detached chromosomes 

 (8) form microcytes when the pollen-mother-cells con- 

 strict to form tetrads (Fig. 8). Table II shows the num- 



bers of microcytes seen in nearly 3,500 tetrads from 3 

 triploid plants. The average is 13 per cent, of cases of 

 detachment, but the variation in different buds appears 

 too great to be due to chance alone. In 100 pollen-grains 

 there would be about 5 microcytes. 



