A. H. Trow 273 



we should expect in each case (if the coupling occurs in both sexes 

 in the form 2:1:1:2) families of the constitution 2HR : iHr : IhR : 2hr. 

 If, however, coupling occurs in one sex only in the form 7:1:1:7, one 

 family should have the constitution 7HR : IHr : IhR : Ihr, and the 

 other the constitution \HR : \Hr : IhR : Ihr. 



In Senecio, the ^i $ plants would need isolation, and the bisexual 

 disc-florets would have to be removed while in bud. The P*$ plants 

 unfortunately have no $ flowers, and it is practically impossible to 

 remove the stamens without injury to, or accidental pollination of, the 

 stigmas. It would therefore be necessary to allow self- and cross- 

 pollination to take place, and to allow for the additional hr types 

 produced by self-pollination. The ratios to be looked for are : — 



2:1:1:2; 7:1:1:7; 1:1:1:1; 2:1:1:2-1-0;; 7:1:1 :7+a;; and 1:1:1 :l-f- a:, 



where x represents the additional hr types produced by self-pollination. 

 In Primula, where the necessary castrations are easily made, the three 

 ratios 2:1:1:2, 7:1:1:7 and 1:1:1:1 should alone occur. Small 

 families of 16 to 32 plants should suffice to determine the type of 

 ratio. I regret to say, however, that the pure strain of lanuginosus 

 has become so weak and unhealthy — perhaps by successive selfings — 

 that it is no longer available for experiment. The experimental proof 

 in the case oi Senecio cannot be furnished before the end of 1915 \ 



The assumption of dominance, although often of great assistance, 

 as in this case, in giving a clear and condensed view of an experimental 

 result, often tends to obscure facts of importance. The nine types 

 which result from the interaction of two pairs of characters are, I think, 

 recognisable in these cultures. 



The following table gives the numbers found and the expectation 

 on the assumption of coupling of the form 2:1:1:2 in both sexes. 

 Compare also Table X, p. 259. 



RR Rr rr 



Culture 23 

 Culture 2i 



Total (23 and 24) 



Expectation. Coupling of 

 the form 2:1:1:2 



^ I find that Gregory (Proc. Roy. Soc. B., Vol. 84, p. 14) has made the necessary crosses 

 and proved that coupling takes place in each sex. The exact form of the coupling, however, 

 still remains somewhat uncertain. Primula sinensis appears to be a somewhat difficult 

 type for such a determination. Upon the whole, the evidence clearly points to the 

 occurrence in this case of the 2:1:1:2 ratio in each sex. 



