stout: pollinations in cichorium intybus 427 



possibility that selective mating of gametes may occur in certain 

 cases and constitute a factor in fertility and in segregation is, 

 however, not to be excluded. The assumptions of those who 

 would ascribe sterility from incompatibility to hereditary line 

 stuffs or individual stuffs are plainly opposed to the general 

 Mendelian assumption as to chance mating and pairing of the 

 newly combined factors. 



Further there would seem to be a natural presumption in favor 

 of the view that chromosome incompatibility resulting in im- 

 potence is only the extreme stage of a series whose earlier stages 

 are shown in the phenomena of gametic physiological and morpho- 

 logical incompatibility and that when in operation it is a very 

 special grade of differentiation. 



It must be recognized that impotence involving various gfades 

 of development of only one of the sex organs is less directly 

 attributable to an intranuclear incompatibility than are the cases 

 of nearly equal impotence of both sex organs as seen in numerous 

 interspecific hermaphrodite hybrids. The various sex forms in 

 Plantago lanceolata (Bartlett '13), for example, show gradations 

 from fully potent hermaphrodites through various grades of anther 

 and pollen impotence to plants that may be considered as pistillate 

 only. Here the conditions suggest the operation of physiological 

 processes in association with the differentiation of male sex organs, 

 and that in respect to the degree of such impotence an entire plant 

 is differentiated, much as is the entire organism in the case of 

 strictly dioecious plants and animals. The occurrence of plants of 

 Plantago exhibiting various grades of impotence among the flowers 

 on a single plant (gynomonoecious) (Bartlett '13, p. 174), indicates 

 the sporadic nature of the physiological processes involved with 

 this sort of differentiation, as do, likewise, the development of 

 various grades of hermaphrodite forms in animals, as especially 

 reported by Goldschmidt ('16). 



The writer has simply suggested in the introduction and in the 

 foregoing discussion some of the most obvious types of impotence 

 of which degeneration of spores is a very marked type. Other 

 types involving the failure in the formation of flowers or sporo- 

 phylls, of one or the other kind or of both, or of a general physio- 

 logical debility are equally in contrast to the conditions prevailing 

 in the case of the incompatibility as seen in Cichorium, Cardamine, 

 Reseda, etc. 



