1. In young endosperms of Trillium and Paris, the mitoses were synchronised. A comparison of the number of mitoses, determined from the number of nuclei, which had taken place in the first 5 weeks after controlled pollination within and between 10 species of Trillium and 2 of Paris, revealed that the rate of endosperm development varied according to the relationship of the plants used. 2. (i) Trillium species were self-sterile, although, in a few cases, slowly developing endosperms formed and later degenerated. (ii) Within species cross-pollinations of Trillium plants gave rise to rapidly growing regular endosperms in which at least 8 synchronised mitoses had occurred in 5 weeks. (iii) After inter-specific cross-pollination, endosperms developed more slowly and frequently showed nuclear abnormalities. These were particularly frequent in endosperms formed by crossing the Japanese T. kamtschaticum with the American T. grandiflorum. No viable hybrid seed was formed. 3. The results of reciprocal crosses between the more distantly related Trillium species differed, but in most cases the endosperm was more retarded than after crossing closely related species. 4. Mitotic abnormalities, which were not restricted to the most slowly developing hybrid Trillium endosperms, included bridges, acentric fragments, lagging chromosomes, “stickiness”, irregular disjunction, micronuclei, giant polymorphic nuclei, and aneuploidy. 5. After crosses within and between Trillium species, 5 wholly or partially aneuploid endosperms were observed. In each case, one maternal chromosome, either B, C, D, or E but not the large A chromosome, was missing or extra. 6. Paris quadrifolia was self- and cross-fertile. When hybridised with Trillium grandiflorum or T. cernuum, it produced no viable seed but when used as the female parent it formed endosperm which, in spite of nuclear abnormalities, developed fairly rapidly for a few weeks. The reciprocal crosses resulted in very little endosperm production. 7. Differences in the rate of development between reciprocal crosses of distantly related species indicate a strong chromosomal control. Failure to form normal endosperms is due to the interacting influence of the paternal genome introduced by the foreign pollen. The degree of interaction depends on how dissimilar the two genomes are and how strongly the male parent is represented in the hybrid nuclei. When the paternal genome is greatly outnumbered by the maternal ones, as in Paris quadrifolia × Trillium (PPPPT) endosperms, development is more rapid than when both are equally represented, as in Trillium x Paris quadrifolia (TTPP) endosperms.