Oct., 1923 1 STOUT — STUDIES OF LYTHRUM SALICARIA 44I 



on the agency of insects for their effectiveness in pollination. The relative 

 lengths of the different sets of stamens and of the pistils are unquestionably 

 provisions favoring crossing. 



It is, however, obvious that such differentiations are not fundamental 

 expressions of sexual antithesis, since they are all reciprocal in any pair of 

 flowers. Any two plants of any two forms are cross-compatible or cross- 

 incompatible according to whether the cross is legitimate or illegitimate. 

 The differentiations, at least in respect to maleness, that in dimorphic 

 plants are seen in individuals as such and which appear to have definite 

 genetic value are here seen equally well in the two sets of stamens in a single 

 flower. All this emphasizes the fact that the conditions are secondary and 

 acquired in contrast to the more primitive condition of homomorphism and 

 a more general compatibility of gametes. 



The generally accepted view has been that the differentiations in this 

 species are well established and very stable. The tendency has been to 

 emphasize, as did Darwin, the evidence that here there is adaptation favor- 

 ing crossing, and to pass the evidence, which has to some extent been noted, 

 that the adaptations are incomplete. It is to be recognized that such 

 evidence has a very direct bearing on questions of the origin of trimorphism, 

 of the nature of sex-differentiations, and of whether there is still opportunity 

 for further selection in the species either toward greater or toward less 

 restriction of fertilization. It is evidence along these lines that the writer 

 wishes to present in reports, of which this is the first, of investigations with 

 the species. 



The Efficiency of Self-pollination for Plants Grown in Isolation 



The writer's studies of Ly thrum Salicaria were begun in 191 7 in testing 

 the self-compatibility of plants grown in isolation from other plants of the 

 same or of related species. On such a plant hundreds of flowers open 

 daily during a rather extended period of time and insect visitors can go 

 from flower to flower, but with no chance, if the isolation is complete, of 

 bringing pollen from other plants of Lythrum Salicaria. A large number 

 of flowers are thus involved in the chance for self-fertilization (including 

 here autogamy and geitonogamy), and the results can be obtained for the 

 entire period of bloom. This test does not, however, determine the relative 

 fertility of a plant to pollen of its two sets of stamens, nor does it reveal 

 the need or the efficiency of particular species of insects in the self-pollination 

 of the various forms, which may indeed give results that are highly variable 

 from season to season or from year to year or according to location. How- 

 ever, if seed is produced there is positive evidence of self-compatibility, and 

 the negative results may be tested further by controlled pollinations. 



Short-styled Plants Grown in Isolation. Two large, well-developed plants 

 several years old were dug from a mixed population growing at the New 

 York Botanical Garden. One (S no. 2) was grown in the garden of the 



