37 



THE WHEAT PLANT 



From only one (r^ r^) out of the sixteen zygotes are all the red 

 factors absent : this will be white, the rest, having from one to four colour- 

 factors, will be of several shades of red. 



When a wheat with deep red chaff, due either to one or two factors, is 

 crossed with a white-chaffed variety, the glume colour of the heterozy- 

 gotes in F 2 is always an easily recognised red, and homozygous reds even 

 of the lighter shades are found to be distinct from white. 



Where the coloured parent of the cross is of a light reddish tint the 

 heterozygotes are very pale and sometimes difficult to distinguish from 

 the homozygous whites. 



Some of the F 2 individuals classed as white were found by Nilsson- 

 Ehle to be in reality heterozygotes which split in F 3 into red, pale red, 

 and white-chaffed plants. Owing to the confusion between these hetero- 

 zygote white and the pure white, the experimental results of segregation 

 may appear not to conform to the ordinary 3 : i or 15 : i ratios. 



In Nilsson-Ehle's crosses of red and white spring wheats the total 

 numbers obtained in F 2 were 2625 red : 987 white 2"j red : i white, and 

 in some of the individual crosses the apparent splitting was 2-3 or 1-5 red 

 to i white, the number of plants classed as white-chaffed being too high. 



In consequence of the cumulative effect of two red factors the cross 

 Rjra x R 2 r x would, of course, lead to the production of forms containing 

 both Rj and R 2 , and therefore darker in colour than either parent, 

 together with some white individuals (r^ r n r 2 ) lacking the colour factors 

 altogether. 



v. White x White. The F x of this cross has invariably white chaff, 

 but the occurrence of red-chaffed plants is recorded by Shirreff in the F 4 

 generation of the hybrid ShirrefFs Bearded white ? x Talavera $ , both 

 white- chaffed wheats. 



