496 



THE AMERICAN NATURALIST 



[Vol. L 



Ever-sporting Single Pure-breeding Single (as it is Found 



XYxy in Ordinary Commercial Material) 



XYXY 



Gametes Gametes 

 Ovules Pollen Ovules Pollen 



7XYorl5Xy all xy allXY allXY 



7 xy or 15 xy 



the presence of X and Y in the zygote always producing single- 

 ness, whether the factors are coupled or not. 



Now on this supposition the mating d-single ? X no-d single J 

 will give some plants of the constitution XYXY. These when 

 self -fertilized will presumably give some pure-breeding F 2 plants 

 of XYXY composition. If these pure-breeding singles in which 

 X and Y are not coupled are crossed back with the pollen of an 

 ever-sporting single we shall again have a plant with the consti- 

 tution XYxy. In this way we may hope perhaps to synthesize 

 the ever-sporting form. This experiment is already in progress. 

 For this purpose the ever-sporting strain known as sulphur- 

 white is particularly well suited if used as the female parent in a 

 mating with a pure-breeding cream. As the ovules of the sul- 

 phur-white appear to be of the four kinds, XYW, XyW, xYw, 

 xyw, we know that only F 1 single whites will serve our purpose. 

 We also know that we can disregard among the F 2 families de- 

 rived from the self-fertilization of F x whites those which contain 

 doubles, and proceed to cross individuals in the pure-single fam- 

 ilies with pollen of an ever-sporting type. By the choice of a 

 sulphur-white as the female parent in the first cross we are saved 

 much trouble in identifying those F x plants which contain XY 

 unlinked. In this way we may hope to obtain further light on 

 the different condition of the factors for singleness in the pure- 

 breeding and ever-sporting single, respectively. 



After dealing with the stock Frost suggests that his hypothesis 

 of selective sterility no doubt also explains the case of Petunia, 

 and adds that in any case my view that singleness is here domi- 

 nant is untenable. He further mentions that both Goldschmidt 

 and Belling 11 hold the view that doubleness and not singleness is 

 ii Belling 's statement is that "in the Petunia doubtless may be incom- 

 pletely dominant as in the greenhouse carnation." The American Nat- 

 uralist, Vol. XLIX, Xo. 578, p. 126, Note 1, 1915. 



