E. R. Saunders 63 



alternative may be likely to prove correct (viz. ovules of singles hetero- 

 geneous, pollen of doubles homogeneous as regards absence of the factor 

 x) ; but the fact that if this were so we should expect a certain pro- 

 portion of Petunia singles to be homozygous as to singleness, and 

 therefore incapable of yielding doubles when crossed with the pollen of 

 a double, whereas, as a matter of fact, no such singles were met with, 

 lends considerable support to the opposite view (viz. ovules of singles 

 homogeneous, pollen of doubles heterogeneous in regard to absence 

 of x). 



Thus we find in Petunia the same peculiar type of gametogenesis 

 which has already been shown to occur in Matthiola. In both cases 

 segregation proceeds in such a way that certain factors are distributed 

 differently among the ovules and the pollen grains. It may also be noted 

 that in both instances doubleness behaves as the recessive character, 

 singleness as the dominant, but in other respects the two cases present 

 an interesting contrast. In the double Stock, as is well known, the 

 flower is completely sterile, whereas in Petunia the male organs are 

 functional in the double though the female are not. Further it appears 

 that although both in the single Stock which constantly throws doubles, 

 and in the single Petunia which yield doubles when fertilised by a 

 double, the pollen is homogeneous in respect of some factor needed to 

 produce singleness, the homogeneity is brought about by the absence of 

 this factor in the Stock, by its presence in Petunia. Consequently 

 doubles are obtained in the Stock when heterozygous individuals are 

 self- fertilised, or fertilised inter se, but not in Petunia. Lastly, in the 

 Stock a heterozygous single fertilised with double-carrying pollen j'ields 

 an excess of doubles ; in Petunia on the other hand singles crossed with 

 pollen from a double yield a majority of singles. 



Summary. 



1. Single Petunias belonging to the following forms : P. violacea, 

 P. nyctaginiflora, P. hybrida grandiflora, and Countess of Ellesmere, 

 whether self-fertilised or crossed with each other, yield only singles. 



doubleness is determined by the presence or absence respectively of a single factor. Now 

 however that the accumulated evidence points to the probability that more than one factor 

 is involved this difference between the male and female germs is more correctly expressed 

 in terms of some factor the presence of lohich is essential to singleness (as above in Petunia) 

 than in terms of the character singleness itself. 



