140 Aj^his rumicis on Varieties o/Vicia faba 



The regular grouping of the figures showing the total number of 

 aphids produced in 14 days (column 6) indicates that there is a significant 

 difference in the degree of infestation of different varieties over a given 

 period of time. 



If we take the arithmetic mean of the total numbers produced on the 

 five plants in each variety (column 7) we obtain for each variety a mean 

 figure of infestation resulting from one apterous mother in a 14-day 

 period. To this is attached its probable error calculated as explained in 

 the Appendix. 



The varieties therefore may be grouped into six distinctive classes, 

 each class having a mean figure of infestation as shown in Table III. 

 The mean figure of infestation for each class is taken as the arithmetic 

 mean of all the means of the varieties included in the class. 



Referring each class to the variety Prolific Longpod (XIX), the mean 

 infestation figure for which is 1037, the relative degree of susceptibihty 

 of the varieties in each class may be expressed in percentages as shown 

 in Table III. 



Conversely by subtracting these percentages from 100, one gets the 

 relative degree of resistance to infestation of the varieties in the different 

 classes. 



After 14 days reproduction on varieties in class A, the plants may be 

 considered as fairly heavily infested, while the varieties in class D have 

 only a moderate infestation and those in class E less so. Class F has 

 almost a neghgible infestation. 



In 21 days the varieties in class A would be practically destroyed by 

 the aphid infestation, those in classes D and E less so and in class F 

 almost neghgible. 



There is no class here representing complete immunity from attack 

 but the aphids were compelled to stay on these plants. In nature winged 

 migrants would select their host and probably not reproduce on unfavour- 

 able varieties. 



