The conditions that exist in species with trimorphic flowers permit the investigation of the problems regarding the nature of sex-differentiation and the degrees of compatibility between male and female organs under very favorable circumstances. In these forms the morphological adaptations for cross-pollination are often decidedly correlated with physiological incompatibilities which make cross-fertilization more certain by excluding the functioning of the chance self-pollinations which occur. In general it is to be recognized that sexual fusions are favored by similarity of the gametes both in genetic constitution and in immediate origin, and that such conditions as trimorphism and self-incompatibility are to be regarded as, in a high degree, secondary and acquired. While these conditions secure the advantages of bringing together gametes of different origins and in greater or less degree of different genetic constitution, they decidedly limit and restrict free fertilization and full productivity. The combination of morphological trimorphism with physiological incompatibilities as seen in species like Lythrum Salicaria may well be regarded as the highest degree of specialization in sex-determination and fertilization that exists in flowering plants. For this species there is the obvious morphological differentiation giving three lengths of pistils and three sets of stamens of lengths corresponding to those of the pistils, with the stamens bearing pollen differentiated as to size, color, and nature of the reserve food material in storage. For the individual, the flower of any particular plant has a pistil of one of the three lengths and a set of stamens for each of the other two lengths. This gives differentiation of forms as such, and in the single plant there is the differentiation that gives two kinds of stamens. Furthermore, this morphological differentiation is decidedly correlated with physiological differentiation. The noteworthy researches of Darwin (I865, I877) showed that there is marked or even complete sterility to (i) selfpollinations, to (2) intra-form cross-pollinations, and to (3) the interform cross-pollinations that are illegitimate (i.e., those that involve different lengths of pistil and stamen). Seed-production was hence found to be more or less limited to legitimate pollinations, which are necessarily crosses. The specializations in these forms allow no doubts as to their significance such as have often been raised in regard to the colors of entomophilous flowers, for here the adaptations are morphological and depend directly