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. 



FIG. 166. a, Sandcherry, Primus besseyi; B, Wyant plum, P. americana; C, D, Ft hybrids 

 from Sandcherry X Wyant. (After Beach and Money, Iowa A. E. S.) 



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- 



