rilE ROLE OF THE CVTOFLASM 



229 



An instance of cytoplasmic influence enduring for a longer })eri()(l is 

 afforded by the protozoan, Paramecium. When individuals of two 

 races unlike in size are allowed to conjugate and then to multiply by 

 fission, the individuals of both exconjugant lines graduall}^ come to be of 

 the same size after about 22 fissions. The inference is that there is here a 

 Ungering cytoplasmic influence slowly being overcome by genes affecting 

 size. This inference is supported by the further observation that the size 

 eventually attained is not the same after different pairs of individuals 

 of the same clones have conjugated, for this indicates the formation of 

 various genie combinations in the meiosis and syngamy occurring at the 

 time of conjugation. 



Fui. 168. — Diagram illustrating cytoplasmic inheritance in species of Epilohium 

 (willow herbs). Rectangles represent plant.s; circles represent their nuclei. L, luteum 

 cytoplasm; H, hirsutum cytoplasm; I, luteum genome; h, hirsutum genome. The third 

 plant in the second row is a "nucleocytoplasmic hybrid" (compare Fig. 166, 7). Further 

 explanation in text. {Based on data of P. Michaelis.) 



In another class of cases the story is a different one: the cytoplasmic 

 effect does not disappear, but persists indefinitely even in spite of attempts 

 to increase the influence of the genes. In Epilohium (Fig. 168) the 

 hybrids derived from reciprocal crosses of E. luteum and E. hirsutum are 

 unlike in various vegetative characters and fertility. The hybrid type 

 containing luteum cytoplasm retains its distinctive characters even after 

 back crossing to hirsutum for 14 generations. After such a number of 

 backcrosses the nuclei in all probability have only hirsutum chromosomes, 

 yet the effect of the original maternal cytoplasm persists. When such 

 plants containing hirsutum chromosomes and luteum cytoplasm are 

 crossed reciprocally with E. luteum, the hybrids obtained are alike in 

 spite of the fact that the nuclear relations are the same as in the original 

 cross between the two species, where the reciprocal hybrids w;ere not alike. 



