xi] Double Stocks 203 



The numbers are irregular and obviously need a further 

 analysis. 



The crosses which are instructive are those which were 

 made between the double-throwers and the pure single 

 strains. Whichever way the cross is made, F l is always 

 single. When F n _ is raised from these plants the curious 

 fact appears that the result differs according to the way in 

 which the original cross was made. When the cross is in 

 the form pure single $ x double-throwing $, all* the F^ 

 plants give a mixture of doubles and singles in F,, ; but 

 when the reciprocal cross is made, namely double-thrower 

 used as $ x pure single used as , it is found that the F^ 

 plants are of two kinds, (i) those which throw doubles 

 mixed with singles in F v and (2) those which throw only 

 singles in F z . 



The conclusion to be drawn is evidently that the pollen- 

 cells of the double-throwers are all (perhaps nearly all) 

 bearing the double character, but that the egg-cells of these 

 same plants are of two kinds, those which bear singleness 

 and those which bear doubleness (see 22, pp. 5 and 36). 



For the first time therefore we have a proof that, in a 

 hermaphrodite form, there may be a substantial difference 

 between the factors borne by the male and female cells of 

 the same plant. It is the existence of this instance which 

 leads me to hazard the suggestion introduced in the discus- 

 sion of the facts of sex-inheritance in Bryony, that the 

 female cells of Bryonia alba may be of two kinds and male 

 cells of one kind (see p. 168). 



This discovery, though it may prove to be the clue to 

 the problem of the double Stocks, leaves the main difficulty 

 still unsolved. We have to find the scheme whereby it 

 comes to pass that the doubles, though bred from singles, 

 are nevertheless as a rule in the majority, and the difficulty 

 of offering a plausible suggestion is not diminished by a 

 knowledge of the fact that the male cells may be all double, 

 for this fact implies that a majority of the egg-cells must 

 also be bearing doubleness, and ultimately that part of the 

 singleness must have, as it were, disappeared in gameto- 

 genesis. We are at present quite unable to offer any 



* No exception has yet been met with. There are however some 

 reasons for anticipating that exceptions may exist. 



