IN homomorphic self-incompatible flowering-plants, control of pollen behaviour is determined either sporophytically or gametophytically. On the basis of evidence from forty-two genera of twenty-three families, Brewbaker1 pointed out correlations between type of control, pollen cytology and site of the incompatibility response with some exceptions in the Gramineae. In general, species with sporophytic control have trinucleate grains, inhibition occurring in the stigma, while those with gametophytic control have binucleate grains, inhibition being delayed until the pollen tube has begun to grow. At least two other physiological differences characterize the classes2: binucleate pollen tends to show greater longevity in storage and a higher capacity for growth in vitro than trinucleate pollen.