VARIETAL RESISTANCE 277 



other of these attributes are well known, but the real problem 

 involves their judicious selection and combination, which has, as 

 a rule, proved extremely difficult of attainment. 



Much has been written with reference to the nature and meaning 

 of resistance. Absolutely resistant plant varieties are of great 

 rarity, and resistance, more often than not, implies little more 

 than a low degree of susceptibility to attack. The factors which 

 control, or influence, this susceptibility are extremely varied, and 

 have been classified in various categories. In many cases it is 

 probable that no single factor is in itself responsible for the 

 observed effect, and that multiple factors are involved. Among 

 these contributing causes, thickness of cuticle, development of 

 sclerenchyma or other mechanical tissue, general hairiness or its 

 absence, acidity or alkalinity of the cell-sap, silica content, presence 

 or absence of certain glucosides, date of maturity, power of 

 recuperation and other attributes have been regarded as account- 

 able in particular cases. 



Another type of so-called resistance is brought out by the 

 work of Davidson (1922) at Rothamsted, which concerns the 

 reproductive capacity of Aphis rumicis on different varieties of 

 culinary and field beans {Vicia faha). Discrimination on the part 

 of the insect in the selection of one variety in preference to another 

 was eliminated in these experiments, which were conducted 

 throughout under identical conditions. His observations show 

 that the insect exhibits a wide range of fecundity on different hosts. 

 These differences are probably due to differences in the constitution 

 and nutritive values of the cell-sap in the respective varieties. 

 There appears to be a direct inhibiting effect of the cell-sap upon 

 the reproductive capacity of the aphid when grown on certain 

 varieties of beans, and the opposite effect when grown upon 

 other varieties, as is shown in the table on p. 278. 



It is probable that varietal differences in the properties of the 

 cell-sap of many other species of plants are reflected, in the 

 manner described, in the relative abundance of numerous kinds 

 of sucking insects feeding upon them. Much the same kind of 

 reaction, it may be added, has been observed by Monzen with 

 reference to the fecundity of Eriosoma lanigera on different 



