Experimental Studies of Self-Incompatibilities 53 



widely different in such qualities as color of flowers, type of 

 branching, shape of leaves, etc., are either self-fertile or self- 

 sterile, and plants of a sister series quite similar in all respects 

 are either self-fertile or self-sterile. When an F x plant of hybrid 

 origin is self-fertile in any degree, the evidence indicates that any 

 of the sex cells may function in any recombination ; on the other 

 hand, in self-sterile sister plants whose sex cells must, it would 

 seem, be of much the same diversity none are compatible. Also 

 all the sex cells of an F 4 plant which must have much the same 

 germ-plasm constitution may fail to function together, while those 

 of a sister plant may be highly functional. Two self-sterile plants, 

 sisters of an Fi cross or sisters of any generation, may be cross- 

 fertile or cross-sterile quite indiscriminately. 



6. The development either of self-compatibility or of self- 

 incompatibility occurs in both cross-bred and inbred races, the 

 latter often being highly constant races. Both self-fertile and 

 self-sterile plants occurred among sister plants that were r 1 

 hybrids of rather wide crosses; they also appeared among inbred 

 strains derived by crossing self-sterile parents for as many as 

 three generations, and they occurred among the progeny of self- 

 fertile plants even after four generations of self- fertile parentage. 

 The evidence at hand makes it clear that self-compatibilities do 

 not necessarily decrease as a result of inbreeding. 



7. The results obtained in the cultures of chicory make it clear 

 that self-incompatibility and self-compatibility are here not to be 

 described as dominant and recessive characters, or paired allelo- 

 morphs, and that there is no simple Mendelian formula that fits 

 the results. The evidence at hand for the behavior of similar 

 phenomena in other species is also quite in agreement with this 

 conclusion. 



8. The conditions controlling sex-fusions, judged by the be- 

 havior of compatibilities and incompatibilities in such species as 

 Cichoriutn Intybus, arise in connection with the development of 

 the sex organs and sex cells as such. In this sense the controlling 

 factors are of epigenetic and individual development. 



9. The factors which determine or prohibit successful fertiliza- 

 tion in chicory, whatever their essential nature may be, are highly 

 variable as to degree, specificity, and transmission in heredity. 



