HETliROSTYLY 



difterciit plants borne on the same level (Fig. 63). All others are 

 sterile or nearly so. Thus a short stigma accepts pollen from short 

 stamens of both long and mid-styled flowers. It will not accept 

 pollen from the other stamens (mid or long) of these same flowers, 

 any more than from its own or any other mid and long stamens. 



LONG 



MID 



SHORT 



Fig. 6^. — Tristyly in LYthruin salicaria. Oiily the pollinations indicated by arrows 

 are compatible. AH others fail or virtually fail. Thus the two tiers of anthers in one 

 flower give pollen having dift'erent properties in incompatibility, while anthers from 

 tiers at the same level in different flowers give pollen having the same properties 

 in incompatibility (after Darwin, 1877). 



Before we return to our general genetic problem we must note 

 three highly instructive physiological properties of this system. First, 

 each parental genotype has two paths along which it can drive its 

 own pollen. Secondly, pollen from parents of diflerent genotypes 

 may be driven along the same path. And thirdly, the genotype of 

 the pollen itself never comes into the question. 



Tristyly in Lytlimm (or in O.xalis or Ndrcissus) could scarcely be 



251 



