A. B. Stout 125 



to a conception of multiple factors directly concerned with the trans- 

 mission and expression of incompatibilities this must necessarily be a 

 result in inbreeding or line-breeding. East and Park (1918) assume 

 that this is true for cross-incompatibility. The evidence from chicory 

 shows conclusively that repeated self-fertilization in line-breeding does 

 not lead to an increase of self-incompatibility. The average self-fertility 

 of a race may be maintained very uniformly under repeated self- 

 fertilization. No tests for cross-incompatibility were made in those 

 families of chicory for which new data are reported above. It may be 

 stated, however, that all the self-incompatible plants produced an 

 abundance of seed to open cross-pollination which could only have been 

 between sister or closely related plants of the variety red-leaved Treviso. 



Thus far the studies of self- and cross-incompatibilities have been in 

 species in which the incompatibilities were already present as a character 

 variable in constitutional or genetical value. No one has observed the 

 origin of such a condition in a species. No one has produced such a 

 condition experimentally. Numerous excellent studies have been made 

 (see especially Kraus and Kraybill, 1918) of the influence of various 

 conditions of nutrition on vegetative and reproductive vigour. Plants 

 of highly self-fertile species have been rendered sterile and fruitless 

 but in such cases the plant was fully sterile. It not only failed to set 

 fruit to self-pollination but to all cross-pollination as well. The sterility 

 was not relative, it was indiscriminate and absolute. 



The evidence therefore that conditions of incompatibility are not 

 directly induced by repeated self-fertilization, and are not to be ascribed 

 to the condition of hermaphroditism as such, is further proof that variation 

 is operating in the physiological sex differentiation of sex organs. 



3. Variations in morphological sex differentiation, especially recog- 

 nized as phenomena of intersexualism, occur frequently in species 

 prevailingly either hermaphrodite or dioecious, and are quite analogous 

 to those variations in physiological differentiation revealed by incompati- 

 bilities. 



Concerning the relation between seed-sterility from incompatibility 

 arid sterility from various types of impotence there is much need of 

 further information. In general the two classes are distinct. Incom- 

 patibility operates between sex organs either of the same hermaphrodite 

 or of different individuals which are highly functional in certain relations. 

 It is characteristic of self-incompatible and cross-incompatible plants 

 that the respective sex organs may be fully developed and potent. 



Many cases of pollen and embryo sac development are associated 



