1. The theory of incompatibility in polyploids is discussed in detail, with special reference to thebehaviour of incompatibility in polyploids and the segregation of genetic types in polyploids of different constitution. 2. Cytological evidence is adduced for the tetraploid nature ofVerbascum phoeniceum. A new interpretation ofSirks' work on this species is shown to give a better approximation to observation when the incompatibility ofVerbascum is considered on an allotetraploid basis. 3. Cardamine pratensis is a polyploid (2n=ca.30). The work ofCorrens on this species is discussed on the same basis as forVerbascum and again a polyploid interpretation is found to be more satisfactory. 4. A list is given of the chromosome numbers of some plants exhibiting incompatibility, and the behaviour of certain of these is briefly discussed. 5. The polyploid theory is shown to imply that like factors in pollen and stylepositively inhibit andunlike factorspositively promote pollen tube growth, but the potencies of these two opposite reactions are unequal.