stout: pollinations in cichorium intybus 339 



DISCUSSION OF THE LITERATURE BEARING ON 

 PHYSIOLOGICAL INCOMPATIBILITY 



All the more recent conceptions of the nature of steriHty in- 

 volving physiological incompatibility in non-dimorphic species 

 are but modifications of the general view of Darwin that its 

 causes are to be sought in conditions of differentiation in the consti- 

 tution of the sexual elements. Some of Darwin's most extensive 

 experimental work was done on the problem of the relative fertility 

 resulting from selling and crossing as bearing on the origin of 

 variations and their perpetuation and accumulation in evolution. 

 His publications reveal a wide knowledge of the facts of fertility 

 and sterility, which with his discussions have a direct bearing on 

 such important points as, {a) occurrence of self-sterility, {b) nature 

 and causes of self-sterility due to self-incompatibility, (c) relati'^n 

 of fertility to vigor of plants and to genetic relationship (whether 

 inbred or cross-bred), {d) the nature of differentiation involved in 

 sexual reproduction as seen in sterility and fertility. 



(a) Darwin's conceptions of self-sterility (due to physiological 

 incompatibility) were based on a comprehensive knowledge of the 

 occurrence of such phenomena. As early as 1868 he reports in 

 over thirty species the occurrence of "self-impotent" plants which 

 according to the literature and from his own observations could 

 not be fertilized with pollen from the same plant. He says re- 

 garding them: "They are sometimes so utterly self-impotent that, 

 though they can readily be fertilized by the pollen of a distinct 

 species or even distinct genus, yet, wonderful as the fact is, they 

 never produce a single seed by their own pollen" ('68, 2: 164). 



Darwin ('77) later discusses under the term "self-sterile" plants 

 the cases of physiological self-incompatibility which he has previ- 

 ously ('68) designated as "self-impotent," adds several species to 

 the list of such plants, and from his own data on Eschscholtzia cali- 

 fornica, Abutilon Darwinii, Senecio cruentus, Reseda odorata, and 

 Reseda lutea shows that self-sterility is present not as a fixed and 

 uniform behavior, but that it is fluctuating in its expression among 

 different species and also fluctuating among individuals of the 

 same species. Darwin obtained seed from a strain of Eschscholtzia 

 calif ornica which Miiller (Hildebrand '68; Miiller' 69; Darwin 

 '68) had found to be completely self-sterile in Brazil for six genera- 



