VARIETAL RESISTANCE 



281 



stage of the work, but they indicate that the hereditary behaviour 

 of immunity is complex. It seems to be determined by and 

 dependent upon a certain balance of genetical factors and governed 

 by a number of genes of somewhat varied action — in part com- 

 plementary and in part cumulative. Monzen (1926) lays stress 

 upon the physico-chemical nature of resistance, and gives certain 

 data lending support to his contention. He determined the ^H 

 values of the cell-sap of eleven varieties of apple by the colorimetric 

 method, and ascertained that it ranged between 4-4 and 5. The 

 variations, therefore, are very limited, and from these deter- 

 minations it appears that the ^H of susceptible varieties ranged 

 from 4-5 to 5, while that of the immune varieties (including 

 Northern Spy) was 4-4. The acidity of the immune varieties 

 he regards as being due to the larger amounts of malic acid 

 present. With the object of further testing this conclusion, he 

 carried out a series of experiments with apple trees grown in 

 carefully washed sand and nourished with Pfeiffer's solution. 

 The latter medium was rendered alkaline or more acid in different 

 degrees, and the fecundity of the aphis was determined during 

 a definite period on trees grown in the different solutions. The 

 results, shown below, indicate that acidity appears to markedly 

 reduce the fecundity of the aphid, while alkalinity favours it. 

 Monzen further adduces evidence that manuring with acid 

 fertilisers will tend to increase the resistance of the trees to 

 infestation. 



Table XIV 



It is improbable that relative acidity or alkalinity is the whole 

 explanation of the questions involved, and Monzen's conclusions 



