V INTERACTION OF FACTORS 51 



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 and hidden by 



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. 



the five 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 of the corolla. In most primulas the eye 

 is small, but there are some in which it is large 

 and extends as a flush over a considerable part of 

 the petals (Fig. 10). Experiments showed that 

 these two pairs of characters behave in simple Men- 

 delian fashion.short style ( = "thrum") being dominant 

 to long style ( = " pin ") and small eye dominant to 



