50 MENDELISM chap. 



the mask of albinism is all the while occurring that 

 segregation of the different colour factors which would 

 result in all the varieties of coloured forms, if only 

 the essential factor for colour development were 

 present. But put in the developer by crossing 

 with a pure coloured form and their variety of con- 

 stitution can then at last become manifest. 



So far we have dealt with cases in which the 

 production of a character is dependent upon the 

 interaction of two factors. But it may be that some 

 characters require the simultaneous presence of a 

 greater number of factors for their manifestation, 

 and Miss • Saunders has shown that there is a 

 character - in -ten - week stocks which is unable 

 to appear except through the; interaction of three 

 distinct factors. Coloured stocks may be either 

 hoary with -the leaves and stem covered by- small 

 hairs, or "they may lack the hairy covering, in which 

 case they are termed glabrous. Hoariness is dominant 

 to glabroUstiess ;- that is to say, there is a definite 

 factor which can turn the glabrous into a hoary plant 

 when it Jis^ present. But in families where coloured 

 and white' stocks occur the white are always glabrous, 

 while the coloured plants may or may not be hoary. 

 Now -colour in the stock as in the sweet -pea has 

 been pt'eved to be dependent upon the interaction of 

 two separate factors. Hence hoariness depends 

 upon three separate factors, and a stock cannot be 

 hoary unless it contains the hoary factor in addition 

 to the two colour factors. It requires the presence 

 of all these three factors to produce the hoary 

 character, though how this comes about we have not 

 at present the least idea. 



