354 Doubleness in Stocks 



together with partial coupling of the factors for singleness and double- 

 ness and for plastid coloui-, giving, as previously stated (p. 346), both the 

 rarer forms in the proportion of only about 1 in 33 if ?i = 16, or 1 in 65 

 if H = 32, may well account for their apparent absence iu a large number 

 of families in the present experiment. There seems in fact no reason 

 to doubt that, so far as the factors X, Y, W are concerned, the same 

 relation holds in both matings. But in the present case it seems 

 probable that in two of the unions another pair of factors come into 

 play, one member of this pair being present iu the sulphur-white, the 

 other in no-d-Aesh, and also apparently in no-rf-white, but not probably 

 in either incana or the Brompton strain. The efifect of the presence 

 of these additional factors in any F^ cross-bred will be to raise the pro- 

 portion of singles in the F.. family derived from this cross-bred as 

 described in Mating 11. Only in this way does it seem possible to 

 explain the frequent high percentage of singles in the ^2 families 

 where the two ten-week strains were employed, when other forms 

 such as the two biennials gave the expected 3 s. : 1 d. The genuine- 

 ness of these results is confirmed by those obtained in the reciprocal 

 union. 



Mating 12. Reciprocal cross, no-rf-non-cream % x rf-sulphur- 

 white ^. (For details see Table IV.) 



Two kinds of mating of this form were carried out, no-c?-hoary 

 Brompton white being used as the seed parent in the one case, 

 ?io-(^-glabrous flesh in the other. All four forms appeared in F.,, 

 singles with white plastids and doubles with cream plastids greatly 

 preponderating; in many families in fact only these two forms were 

 recorded. The proportion of singles and doubles in the large F., families 

 derived from the Brompton white was evidently 3 s. : 1 d. The families 

 derived from the «o-rf-flesh are mostly of small size. In some no doubt 

 the ratio is also 3 s. : 1 d. but in others there appears to be a distinct 

 excess of singles as in the reciprocal cross (see above). Altogether 

 48 F.. families were raised and doubles were recorded in 46 ; the larger 

 of the two remaining families consisted of 17 singles, the smaller of only 

 8; both were derived from the no-rf-flesh parent. As there is every 

 i-eason to suppose that every F^ derived from the union no-d J x rZ (/" 

 will produce doubles, and as the proportion of doubles in one or two 

 sister families is even less than 1 in 17, we may reasonably conclude 

 that a larger sowing would have given the expected mixture in these 

 cases also. 



