338 Doublciiess in Stocks 



heterozygous single, when crossed with an eversporting form, we might 

 well expect a proportion of the F^ plants to yield a higher percentage 

 of doubles than 1 d. : 3 s. Of such cases however we have as yet no 

 certain example. It is otherwise however as regards those F.2 families 

 which show a deficiency of doubles. Some of these cases, at least, seem 

 beyond question, but until we have a fuller understanding of the real 

 meaning of coupling it is difficult to obtain a clear insight into their 

 cause. 



The results seem to show that the lower proportion of doubles 

 obtained from some Fi cross-breds in matings where sister i^i plants 

 gave 3 s. : 1 d. cannot be considered in all cases as an effect due to either 

 one of the parents apart from the other, but must be regarded as due 

 to a combination of factors brought together by their union. It was 

 found, for example, that the same true-breeding individual may give 

 only the usual proportion of doubles (3 s. : 1 d.) in F^ when crossed 

 with one eversporting strain, but will show a marked deficiency of 

 doubles in some families when crossed with another d-strain (see 

 Table IV, where 6 F^ derived from the union no-d-cream x d-azure 

 all gave the usual proportion of double plants, while in F. from a 

 mating with rf-light purple, where the same cream individual had 

 been used as the seed parent, one or two families indicated a marked 

 deficiency of doubles). These facts suggest the possibility that the 

 conditions which result in the production of a single or a double may, 

 in some cases, be more complex than those represented in the simple 

 formula hitherto employed, according to which the occurrence of a 

 single is attributed to the presence of two factors (XY), the occurrence 

 of a double to the absence of either or both. This may remain true, 

 and yet it may also be that more than one such pair of factors exists, 

 and that the presence of the two members of one o?- other pair will lead 

 to the production of a single. The complementary distribution of the 

 two members of a second pair {X'Y') among some of the d- and 

 ?jo-d-strains respectively would lead to an increased production of 

 singles in F^ as the result of a union between a d- and a ?io-rf-strain 

 which happened to contain the complementary factors, if these factors 

 are borne by ovules and pollen alike ; whereas unions between two d-, 

 or between two no-d-strains would be unaffected by the presence of one 

 only of the second pair of factors. It is in fact difficult to see how 

 otherwise results such as those obtained in mating 8 (see p. 341) are 

 to be explained, since it seems hardly possible to suppo.se that the 

 discrepancies can be due to a mere chance variation. 



