E. R. Saunders 335 



cross-bred has been produced by the union of germ cells carrying XF and 

 xy respectively, it will again produce germs of XF and xy composition, 

 but the combinations Xy and xY apparently do not occur. X and F 

 behave in fact as though they were coupled, a condition which we may 



represent thus XY. It will be well to point out that inability to throw 

 doubles need not necessarily imply that an individual is homozygous in 

 regard to the condition in which the factors X Fare present. For we may 

 suppose that if at any time an XF ovule of an eversporting form were 

 by chance crossed with pollen from a pure-breeding single of XF com- 

 position — a possibility which might easily come to pass now and again 

 when the diflferent strains are grown side by side in the open — we should 

 at once get a zygote of XYXY composition. This zygote and all its 

 posterity would behave as any tnie-breeding single so long as self- 

 fertilisation or inter-crossing among the offspring continued. The 

 heterozygous nature of a certain proportion of the individuals would 

 remain undetected and would be perpetuated indefinitely under these 

 conditions. It would only become apparent if any of these individuals 

 were crossed with an eversporting form ; then the different proportion 

 of doubles occurring in F^ families derived from sister F^ plants, on 

 crossing with an eversporting form, would disclose the peculiar hetero- 

 zygous nature of the nevertheless true-breeding parent. 



Similarly it would seem that among true-breeding singles with 

 white plastids some may have the factor W coupled in some or all the 

 germ cells with the XF group — a condition which may be indicated 

 thus XYW — so that when such germ cells unite with an xyw ovule 

 or pollen grain of an eversporting form the resulting ^i cross-bred is 

 unable to form the combinations XYw or xyW. In other cases on the 

 other hand W appears not to be thus coupled, the results indicating a 

 redistribution of the factors in the ordinary way. Individuals breeding 

 true to singleness and to whiteness may therefore conceivably be of six 

 different kinds, according as the factors X, F, and W are wholly, 

 partially, or not at all coupled in the zygote, as shown below. 



Possible types of true-breeding singles with white plastids. 



Zygote Gametes 



Homozygons 



1 XYWXYW all XYW 



2 XYWXYW all XYW 



3 XYWXYW all XYW 



