1907.] Flower Colour in Antirrhinum majus. 291 



also in the mating of two striped forms. In the two latter cases the results 

 are not yet complete. 



The original wild Antirrhinum appears to be of the magenta type. 

 ** Eeversion " to this old form, which so often happens when crimson, ivory, or 

 yellow types are crossed with white, is explained by the introduction into the 

 zygote of the factors I, L (and T) by the white parent. The introduction of 

 the factor I alone will bring about " reversion " when L is already present in 

 the zygote. 



The experimental evidence is not yet sufficient to determine whether the 

 phenomena in Antirrhinum are fundamentally similar to those observed for 

 Sweet Peas and Stocks. In the two latter cases,* production of colour depends 

 on the simultaneous presence in the zygote of two factors C and E; the 

 absence of both or either of these factors from the zygote renders it white. The 

 colour formed by the meeting of the two factors is red. 



To treat the case of Antirrhinum on parallel lines, we might suppose the 

 production of yellow, the base-colour, so to speak, to be due to the presence 

 in the zygote, not of one factor only, but of two factors. Any zygote con- 

 taining both would be yellow ; one only or neither would be white. If such 

 is the case, whites should be found, which, when crossed together, give yellow. 

 Whether this is the case or not cannot be decided yet, since the results of 

 crosses between whites are not known. 



All phenomena so far observed in Antirrhinum could be equally well repre- 

 sented on the two-factor theory, provided we assume that the whites, originally 

 mated with coloured types, contained one of the factors which constitute 

 colour ; this is quite possible, since so few white individuals (only three) 

 were employed in the matings. 



The question, then, as to whether colour in Antirrhinum depends on the 

 meeting of two complementary factors or not, must remain open for the 

 present. If two factors be necessary, albinos should be found, which, when 

 mated, give colour (here yellow), as in the cases of Sweet Peas and Stocks; if, 

 on the other hand, only one factor is necessary, then no two albinos when 

 mated will ever produce colour, and the case will be analogous rather to the 

 phenomena observed in animals. 



In the Sweet Peas and Stocks, moreover, the colour of the zygote may be 

 further determined by the presence of a third blue factor B, which changes 

 the red colour to purple ; but B has no effect unless C and E are also present, 

 and thus may be carried by an albino without producing a sensible effect. 



Quite comparable to this third factor in Peas and Stocks are the factors I, 



* < Third Eeport. 



