E. R. Saunders 359 



all the Fi ovules carrying singleness (XY) appear to carry whiteness 

 (W) like the ovules from which F^ is itself derived; while of those 

 F^ ovules which carry doubleness all (or almost all) lack W. If it 

 should be confirmed that only the two parental forms (singles with 

 white plastids and doubles with cream plastids) occur in F^, then 

 "all" will presumably be correct in each of the above cases; but 

 if, as analogy with other cases suggests, the factor W shows not 

 complete but partial coupling, of the same nature as that described 

 under (6 a) for the factors X and Y, then we may expect that of the 

 F^ ovules carrying doubleness almost but not quite all will lack W, and 

 that in a large sowing in F^ a few doubles with white plastids will 

 occur. In the event of this latter alternative proving true we should 

 be able to synthesise the sulphur-white form afresh from true-breeding 

 whites and creams. For the F^ single from d-non-cream x rf-cream is 

 formed from the union of the same combinations of factors as was ^i, 

 and will presumably therefore repeat the same gametic series. Hence 

 if the appropriate single white be selected in F, it may be expected to 

 behave like a pure-bred sulphur-white. 



(6) In the reciprocal cross where single F^ plants are derived from 

 the union XYw ovules and xyW pollen a corresponding but reversed 

 distribution of the plastid colour factor explains the observed results. 

 Here W is introduced into the pedigree on the male side and is 

 evidently borne by all (or almost all) the germ cells of one sex — no 

 doubt the male — in Fi. Since the presence of the dominant allelo- 

 morph in all or nearly all the germ cells of one sex produces a constant 

 or almost constant result in F^ whatever the distribution of this factor 

 among the germ cells of the other sex, we are unable merely from the 

 F2 result to infer the distribution of W among the ovules. But we 

 may suppose from analogy that all (probably) of the ovules carrying 

 creamness will carry singleness, and that nearly all those carrying 

 doubleness will carry whiteness. 



(c) When crossing occurs between two forms of unlike plastid 

 colour, one of which is an eversporting, the other a non-double- 

 throwing single, the distribution of the allelomorphs W and w appears 

 to be different in the Fi singles which are heterozygous in regard to 

 singleness from that in the singles which are homozygous in this 

 respect. In the heterozygous singles, which here have the constitution 



XYxyWw, recombination of the four components XY, xy, W, and w 

 occurs in the same manner as described above under (6 a) for the four 



24—2 



