AMERICAN 
JOURNAL OF BOTANY 
Vol. X November, 1923 No. 9 
THE PHYSIOLOGY OF INCOMPATIBILITIES 1 
A. B. Stout 
In general survey, physiological incompatibilities in fertilizations include 
two groups of phenomena: 
1 st. There are the very general and characteristic failures of cross¬ 
fertilization between different species, long considered as the most adequate 
evidence of specific distinction; and 
2d. There are those physiological limitations to free and general 
fertilization within species exhibited best in the failure of certain homo¬ 
morphic hermaphrodites to self-fertilize, but also in the cross-incompati¬ 
bilities among seed-grown individuals of the same species or race. 
Certain aspects of the physiology of these incompatibilities are clear. 
They exist and are in operation when the sex organs and sex elements are 
in a condition for proper fertilization; the elements do not function in 
certain relations but do in others; fertilization is discriminative. 
It is not, therefore, a question of what brings the spores or gametes to 
ripening, or of the mere production of those egg or stylar secretions or 
chemotactic influences which make fertilization possible. It is rather a 
question of a very special kind of development or physiological condition 
which discriminates between fertilizations when they are otherwise possible. 
Inter-specific incompatibilities are very generally considered to involve 
species specificity. They are expressed in the interaction between egg 
secretions and sperms, in mechanical and chemical resistance of the cortical 
layers of eggs to the entrance of sperms, in the extrusion of sperm chromatin 
after fertilization, in the death of the heterogenetic hybrid, or in the sexual 
impotence of such hybrids. In all these ranges of expression the incom¬ 
patibilities appear to be deep-seated and inherent in the physical and 
chemical differences in the organization of species. 
Turning now to intra-specific incompatibilities, there is apparently 
a very different physical basis. Here there is self-incompatibility involving 
the germ cells of a single individual. Here also there is cross-incompati¬ 
bility between individuals of the same parentage. Such cases are frequent 
among homomorphic hermaphrodites. They are so widely distributed in 
the families of flowering plants that it would seem that there must be some 
1 Read in the symposium on “Sterility in Plants,” at the joint meeting of Section G 
of the American Association for the Advancement of Science, the Botanical Society of 
America, and the American Phytopathological Society, at Cambridge, December 27, 1922. 
[The Journal for October (10: 399 - 457 ) was issued October 23, 1923]. 
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