Studies of pollen-tube growth on diploid, triploid and tetraploid styles with the same range of pollen have shown: 1. The tetraploid cherryPrunus cantabrigiensis is self-compatible. 2. Self-pollination of triploids fails entirely, in part because of the high sterility of the pollen and in part from the deformation of the styles and the presence of opposing factors for incompatibility. 3. The style as well as the stigma takes an active part in resisting incompatible pollen tubes. 4. Almost all the cross-pollinations examined are partially compatible, but the proportion of effective pollen tubes ranges considerably. 5. The growth rate of the compatible pollen tubes accelerates on the third and fourth day after pollination, whilst that of the incompatible tubes is greatly decreased and in many cases may be completely arrested in the stylar tissue at this time. 6. Cross-pollinations between diploids and tetraploids are favoured when the tetraploid is used as the female parent. 7. The complexity in behaviour increases with polyploidy and hybridity.