402 MEMOIRS OF THE NEW YORK BOTANICAL GARDEN 



on the phenomena of physiological self- and cross-incompatibility. 

 To what degree the incompatibilities involve pollen-tube growth, 

 irregular fusions of gametes, or embryo abortion has not been 

 adequately determined. 



DISCUSSION 



There can be no doubt that in Cichorium Intybus the failure to 

 set seed in the numerous cases of self- and cross-pollinations is 

 due to some sort of physiological incompatibility operating in the 

 interactions between the cells concerned with the processes of 

 fertilization. Such sterility is to be sharply distinguished from 

 that involving different types of impotence or anatomical incom- 

 patibility as defined above. It appears also that the sterility 

 observed in chicory is quite the same as that exhibited by such 

 plants as Cardamine (Correns '12, '13), tobacco (East' 15a), 

 Reseda (Darwin '77, Compton '12, '13), Eschscholtzia (Darwin 

 '77, Hildebrand '68, '69), and in cultivated varieties of the pear 

 (Waite '95), plum (Backhouse '11), apple (Lewis and Vincent '09), 

 cherry (Gardner '13), and in strains of the blueberry (Coville '14, 

 '15). Besides these cases which have been studied in some detail 

 there is evidence which indicates the operation of similar phe- 

 nomena in a considerable number of other species. 



It seems that the causes of this sort of sterility are to be sought 

 in the physiological processes which are operating in an organism 

 quite independently of the anatomical differentiation of the sex 

 organs. Upon this broad generalization practically all students 

 of the subject are agreed. Many have been inclined to assume 

 also that in the higher plants the processes are concerned chiefly 

 with that phase of fertilization which invokes the completion of 

 the growth of pollen tubes through the pistil to the eggs. In the 

 more specific analysis, however, of the conditions, processes and 

 causes, there is the widest diversity of opinion. 



The more comprehensive of the theories, already discussed in 

 this paper in some detail, fall quite naturally into three main 

 classes as follows: 



I. The view especially held by Darwin that "self-sterility" 

 (physiological incompatibility), not including that in dimorphism 

 and trimorphism, is incidental and a result of the direct influence 

 of external environment, producing too great similarity in the 

 germ cells. 



