STUDIES OF LYTHRUM SALICARIA 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 
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