1. Plants of Rubus laciniatus, a pseudogamous apomictic blackberry, and of Merton Thornless, previously regarded as being entirely sexual, were pollinated with different male parents, and auxin was applied to a half of the fruits seven to nine days after pollination.
2. In some crosses with R. laciniatus auxin application improved the development of fruits but in others the average numbers of large nutlets per fruit and their germinability were slightly reduced.
3. In the R. laciniatus series, the proportion of non-hybrids differed with different male parents and was positively correlated with the ploidy of the male parent. In two of the crosses auxin treatment significantly increased the proportion of hybrids. Unreduced or diploidised gametes frequently functioned successfully, and in one cross auxin increased the production of hybrids with high chromosome number. Many non-hybrids occurred in the Merton Thornless series also, and here too their proportion was influenced by the male parent used.
4. The survival of different arrays of genotypes in different crosses was explained by postulating that the selection pressure on the embryo sac is determined by its relative compatibility with the genetic constitution of the pollen parent. The concept of the 'genetic strength' of a genome is of value in explaining the results.