BREEDING DISEASE-RESISTANT PLANTS 409 
crosses between American species and vinifera. The data are not given 
but he believes the observed numbers of resistant and susceptible vines 
favor the assumption of two factors that condition immunity when either 
is present alone or when both are present together. 
Vic. 166.—a, Sandcherry, Prunus besseyi; B, Wyant plum, P. americana; C, D, FP: hybrids 
from Sandcherry X Wyant. (After Beach and Maney, Iowa A. E. 8.) 
Resistance to aphis in the stone fruits is thought to be a heritable 
character from the result of crosses made at the Iowa Experiment Station 
(Fig. 166). The data permit no reliable conclusions regarding the geno- 
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