••I. 

 Reprinted from the American Journal of Botany, X: 440-449, October, 1923. 



STUDIES OF LYTHRUM SALIC ARIA I. THE EFFICIENCY OF 



SELF-POLLINATION 



A. B. Stout 

 (Received for publication December 8, 1922) 



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 incom- 

 patibilities 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 constitu- 

 tion, they decidedly limit and restrict free fertilization and full productivity. 



The combination of morphological trimorphism with physiological in- 

 compatibilities 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 mor- 

 phological 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 (1865, 

 1877) showed that there is marked or even complete sterility to (1) self- 

 pollinations, to (2) intra-form cross-pollinations, and to (3) the inter- 

 form 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 



