No. 596] SHORTER ARTICLES AND DISCUSSION 497 



dominant in this case. In my account 1 - of the results obtained 

 with horticultural strains of Petunia viohtira and P. injclatjini- 

 flora and a hybrid double form I suggested that we might account 

 for the following facts : 



1. That the singles when self-fertilized or inter-bred gave only 



singles. 



2. That when crossed with the pollen of doubles they gave both 



singles and doubles in F x , the singles being in excess though 

 not always in the same ratio, 

 by supposing either 



(a) That some factor essential to singleness was absent from all 

 the ovules of the singles and from some of the pollen of 

 the singles, or conversely 



(6) That some factor was absent from all the pollen of the 

 doubles, but only from some of the ovules of the singles, 



(c) Also that more than one factor was concerned in determining 



the single-double character, and 



(d) That singleness is dominant. 



It will be seen that the above suppositions provide only a gen- 

 eral basis of explanation ; they do not constitute a full solution 

 even of the facts observed. Frost, putting (c) on one side on the 

 ground that it concerns only the deviation of the ratio from 50 

 per cent., argues that both (a) and (ft) are untenable. He re- 

 gards the facts as indicating that doubleness and not singleness is 

 dominant, and holds the view that if some of the pollen of the 

 doubles be assumed to carry doubleness and some singleness the 

 hypothesis of partial sterility will explain the rest. But though 

 the formulation of (c) was intended primarily to provide for the 

 occurrence of more than one ratio, it was essential also to the 

 suppositions (a) and (&). If (c) is negatived then (a) undoubt- 

 edly becomes impossible, but if (c) is true then (a) or (ft) might 

 represent a basic part of the explanation with which some further 

 complication was combined, as to the nature of which, however, 

 the data available afforded no clue. Owing to the complete ster- 

 ility of the double plant, it was impossible to make the reciprocal 

 cross. The singles employed might be, in fact almost certainly 

 were, of mixed descent. It was realized that at best either scheme 

 offered only a partial solution. Unfortunately the efforts made 

 m the course of the experiments and since, to obtain seeds of the 

 wild species, have only been partially successful. The position 



