INTERACTION OF FACTORS 



55 



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. 



A somewhat different and less usual form of inter- 

 action between factors may be illustrated by a case in 

 primulas recently worked out by Bateson and Gregory. 

 Like the common primrose, the primula exhibits both 

 pin-eyed and thrum-eyed varieties. In the former the 

 style is long, and the centre of the eye is formed by the end 

 of the stigma which more or less plugs up the opening of 

 the corolla (cf. Fig. 9, A) ; in the latter the style is short 



FIG. 9. 



Sections of primula flowers. The anthers are shown as black. A, " pin " form with 

 long style and anthers set low down ; B, " thrum " form with short style and anthers 

 set higher up ; C, homostyle form with anthers set low down as in " pin," but with 

 short style. This form only occurs with the large eye. 



and hidden by the four anthers which spring from higher 

 up in the corolla and form the centre of the eye (cf. Fig. 

 9, B). The greater part of the "eye" is formed by the 

 greenish-yellow patches on each petal just at the opening 



