E. R Saunders 323 



(b) Consideration of the relation of singleness and doubleness to 

 plastid character. 



All forms having colourless plastids, and breeding true in this 

 respect, may be supposed to contain a factor W, and to be homozygous 

 in respect of this factor. In true-breeding cream forms W is absent. 

 In the sulphur-white race the case is more complex. The factor W, 

 though present in some of the ovules, is evidently absent from the 

 pollen, since matings with pure cream as % yield only plants with 

 cream plastids (see later p. 352, also Evol. Rep. IV. p. 39). Further 

 there appears to be some complex relation between this factor W and 

 the factors for singleness and doubleness, since, as previously stated, the 

 sulphur-white i-ace, even when self-bred, always gives a mixture of single 

 whites and double creams with a small percentage only of double whites 

 and apparently no single creams ^ These results can be accounted for 

 if we assume that in this form W is either coupled with, or repelled by, 

 one of the two factors X or F. The assumption that repulsion occurs 

 can only be made to fit the results observed if other assumptions are 

 also made involving further complexities ; it will therefore be simpler 

 to proceed on the hypothesis that the relation is one of coupling. We 

 will suppose the coupling to be between W and X, we shall then 



^ More than 2000 plants have been recorded, and none of the singles among them had 

 cream plastids. The non-appearance of the single cream therefore seems hardly likely to 

 be due to accident. Should this however eventually prove to be the case it would be 

 unnecessary to assume the existence of any special relation between W and either X or Y 

 (as described above), since the scheme of coupling described for the two factors X and Y 

 extended to cover the case of three independent factors (X, Y and W) would give all four 

 forms in the required proportion, viz. many single white and double creams, a few double 

 whites and still fewer single creams. 



