350 Douhleness in Stocks 



doubleness, the result would be entirely in agreement qualitatively 

 with that actually observed, and not very different from it quantitatively. 

 For where 



would have been an exact result, 

 were actually observed. 



But it seems doubtful whether the case is in reality quite so simple 

 as this, for the proportion of the three forms occurring in the different 

 F^ families was not as uniform as on the above scheme we should expect 

 it to be, the deficiency of plants with cream plastids in some cases being 

 too great to be reasonably regarded as an accidental variation from the 

 ratio 3 white : 1 cream. On the supposition that the repulsion in F^ 

 between XFand W is complete for both kinds of germ cells, we get 



Ovules Pollen 



XYw XYw 



xyW ccyW 



It follows that all the F^ singles with white plastids should be 

 heterozygous both as regards singleness and doubleness and also as 

 regards plastid colour. Out of 108 such F^ singles derived as follows : 



70 from the mating between no-d-cveBxa % x c?-light purple ^ 

 18 „ „ $ X c?-white (/* 



20 „ „ $ X cZ-red ^ 



which were tested, 90 (viz. 61, 15 and 14 from the three matings 

 respectively) have already proved to be heterozygous as regards 

 singleness and doubleness. In the other 18 families doubles were 

 not recorded, but the numbers, none of which exceeded 11, are too 

 small to be regarded as decisively indicating that the F.2 parent was 

 unable to produce doubles. So far, then, the evidence presents no 

 difficulty in the way of the above supposition. But only 79 of these 

 families (viz. 55, 12 and 12 from the three matings respectively) 

 included plants with cream plastids ; and although the numbers in 

 the other 29 families in which they were lacking were mostly too small 

 to be conclusive, the fact that in one case as many as 40 singles were 

 recorded, all with white plastids, leaves it doubtful whether some of 

 the Fi plants may not be breeding true to whiteness, and consequently, 

 whether some F^ gametes may not be carrying the combination XYW. 

 If any such are formed they must evidently however constitute only 



