62 Double Petunias 



9 families giving a total of 136 single, 31 double where a ratio of 

 3 s. : 1 d. would give 125 single, 42 double. 



16 families giving a total of 397 single, 307 double where a ratio of 

 9 s. : 7 d, would give 396 single, 308 double. 



2 families not included in either of the above groups giving a pro- 

 portion of 2 single : 1 double. 



As yet it is not clear whether the occun-ence of these different 

 ratios indicates that more than one factor is concerned in determining 

 singleness and doubleness, or whether it results from the fact that 

 the proportion of germ cells carrying singleness and doubleness varies 

 in different individuals. In view of the results obtained with Stocks, 

 the former explanation seems the more likely. 



The results given in Tables III and IV show that singles belonging 

 to the various type forms, whether self- fertilised or crossed with another 

 type yield only singles (see Table III); and further, that cross-bred 

 singles having one parent single and one double are equally unable 

 to produce doubles when self-fertilised or fertilised inter se (see 

 Table IV), although the same individuals yield both singles and doubles 

 when crossed with pollen from a double. 



It would therefore appear that the pollen of all the singles tested 

 (23) was homogeneous as regards the presence of some factor x which 

 is essential to the manifestation of singleness, and which is absent 

 from some at least of the ovules. Whether the female germs are 

 homogeneous in this respect, and are all thus deficient ; or whether they 

 are heterogeneous, some lacking the necessary factor and some not is at 

 present uncertain. Precisely the same may be stated in regard to the 

 pollen of the doubles. In some of the grains some necessary factor is 

 evidently wanting, but whether this is the case in all the male germs 

 is not yet clear. It may however be safely asserted that whichever 

 alternative represents the true condition as regards the ovules in the 

 single, the converse will be found to hold good for the pollen of the 

 doubles. For the results obtained would equally follow whether it 

 were the ovules of the single which were homogeneous and the pollen 

 of the doubles that was heterogeneous, or whether the reverse were the 

 case. Analogy with Stocks^ would suggest that the first-mentioned 



^ In the account of the results obtained with Stocks {Evolution Reports, loc. cit. ) it is 

 stated that the homogeneous pollen of the heterozygous (ever-sporting) singles carries 

 doubleness (i.e. absence of singleness), but that among the ovules some carry doubleness 

 and some singleness. This mode of expressing the difference in constitution between the 

 male and female germs is permissible if we suppose that the occurrence of singleness or 



