316 THE BIOLOGICAL BASIS OF INDIVIDUALITY 



(2) in the interaction of germ cells with each other, and (3) in the interactions 

 of germ cells and of structures associated with the germ cells. 



I. In higher plants, Jost (1907) assumed that the retardation in the growth 

 of the pollen-tube into the style, which is observed in certain cases of self- 

 fertilization, was controlled by substances given off by the tissues along which 

 the pollen-tube grew on its way towards the ovary, and which he assumed to 

 be specific for each individual. But, according to Jost, it was not necessarily 

 differences in the chemical constitution of these substances, but differences 

 in the concentration of the latter, which distinguished the different individuals 

 and might suffice to explain their specific effects. 



Subsequently, Correns (1912) interpreted the observed facts in accordance 

 with the concepts of Mendelian heredity, and in particular, with the concepts 

 of the pure lines of Johannsen. He investigated self-sterility in Cardamine 

 pratensis and concluded that the inhibiting substances, which in this case 

 do not permit the pollen-tube to penetrate into the style and which thus pre- 

 vent auto fertilization, are not characteristic of the individuals as such. Ac- 

 cording to this investigator, it would not be correct to assume that a certain 

 substance is unique and occurs only in one particular individual and that 

 it is lost when this individual dies, but he believes that there are substances 

 characteristic of certain pure lines, which are dependent upon the inherited 

 genetic constitution of these lines. However, the individuals in Cardamine 

 do not represent members of pure lines, because each individual is the result 

 of preceding fertilizations in which members of different lines entered. A 

 special combination of substances rather than one particular substance is 

 therefore characteristic of each individual. But according to the law of chance, 

 it is possible even for different nonrelated individuals to possess the same 

 combination of substances. These conclusions of Correns are based on the 

 analysis of the behavior of individuals belonging to the F x generation, which 

 develops after fertilization between two homoiogenous individuals of 

 Cardamine pratensis. He carried out back-cross fertilizations between the 

 F 1 hybrids and each of the two parents and thus he could establish the ex- 

 istence of four classes of individuals according to the character of the sub- 

 stances retarding the growth of the pollen-tube, which each one of these 

 individuals possessed. When the father had the factors Bb and the mother 

 the factors Gg, these multiple allelomorphs were transmitted to the offspring 

 according to the rules of Mendelian inheritance. Whenever either B or G, 

 or both together, are present in the pollen and in the female tissues, the pollen- 

 tube is inhibited in its downgrowth. Accordingly, there is only one of the four 

 classes of F 1 hybrids which is fertile with both parents, and it has the factors 

 Bg- 



The genetic constitution which causes self-sterility has been subsequently 

 analyzed by East in Nicotiana. East assumes that within the same organism, 

 in the male as well as in the female apparatus, which latter corresponds 

 genetically to the sporophyte and represents therefore diploid somatic tissue, 

 two characteristic substances, S a and S 2 , exist. In the female the stigma and 

 style retain both these substances, because their segregation is effected only 



