r,6s 



THE AMERICAN NATURALIST 



[Vol. XLIX 



the hybrids with rubricalyx — a not improbable hypothesis 

 — then the dilution effect will be the same in ¥ x or in cross- 

 ing back, whenever an R chromosome is present in the 

 next generation; and when such a chromosome is not 

 present there will of course be complete absence from the 

 buds of the rubricalyx pigment. On this hypothesis, in an 

 original cross between rubricalyx and grandiflora a cer- 

 tain (observed) reduction in pigmentation occurs. When 

 the F t hybrid is crossed back with grandiflora the addi- 

 tional grandiflora chromosomes thus introduced dilute or 

 inhibit the color still further, while the presence or ab- 

 sence of the diluted R will depend upon whether or not 

 the R chromosome from rubricalyx is present. It would 

 thus appear to be unnecessary to assume that this chromo- 

 some is itself modified by its different nuclear and cyto- 

 plasmic environment. 



In other words, the grandiflora chromosomes may be 

 supposed to exert a mass effect in inhibiting the influence 

 of the R chromosome. It is, of course, possible that in 

 these circumstances the R chromosome itself may be 

 permanently modified, but it seems possible to explain all 

 the facts without making this assumption. In any case, 

 whatever the modus operandi, there can be no question 

 that the R character is permanently diluted by crossing 

 with grandiflora, and the degree of dilution is increased 

 every time the hybrid is again crossed back with that 

 species. 



Another noteworthy fact is that as the pigmentation 

 becomes more dilute its morphological expression is more 

 irregular. The color pattern of the bud begins to break 

 up, and instead of continuous pigmentation of the whole 

 bud a patchy effect will be produced. This spotted condi- 

 tion of the buds is very marked in certain families, e. g., 

 in the second generation of offspring from {rubricalyx X 

 grandiflora) X grandiflora (see Gates, 1915a, Fig. 113, p. 

 280). When it appears it is found to persist in later 

 generations. To account for this condition through the 



