142 Ai)liis rumicis on Varieties of Vicia faba 



The improved conditions associated with good cultivation, manurial 

 treatment, etc., may to some extent influence the degree of susceptibility 

 to aphis attacks. In the case of wild and cultivated varieties, the grouping 

 of the 18 varieties used in these experiments (Table III) indicates that 

 these factors are not the only considerations. The infestation figures 

 obtained for the 10 varieties of broad beans experimented with in 1920 

 {loc. cit.) showed no significant difference in the degree of susceptibihty. 



IV. SUMiMARY. 



The reproduction of Aphis rumicis was tested on 18 varieties of field 

 beans and the results compared with the reproduction on Prolific 

 Longpod broad beans. 



The aphids used in the experiments were the offspring from one 

 Fundatrix. 



The experiments were all carried out under similar conditions. Five 

 plants of each variety were tested, and the total number of aphids 

 produced on each plant, from one a. v. ?, in 14 days was counted. 



The mean values of infestation for the varieties range from 37 to 1037, 

 vide Tables I and II. 



These mean values allow of the varieties being grouped into classes 

 representing various grades of susceptibility, ranging from 98 per cent, 

 to 3 per cent, vide Table III. 



Vicia narbonensis has a very low susceptibility. The results obtained 

 indicate that resistance or susceptibihty may be largely determined by 

 genetic factors in the plant. 



REFERENCES. 



(1) Davidson, J. (1921). Ann. Appl. Biol, viii, 51-65. 



(2) De Candolle, a. (1884). Origin of Cultivated Plants, 316-321. 



APPENDIX 



STATISTICAL CONSIDERATIONS INVOLVED IN TABLES 

 I AND II OF THE ABOVE PAPER. 



By R. A. FISHER, M.A. 



Fellow of Gonville and Caius College; Statistician, Rolhamsted 

 Experimental Station, Harpenden. 



The discussion of the probable error to be attached to Dr Davidson's 

 aphis infestation numbers, involves points of statistical interest, which 

 have hitherto not, in print at least, received adequate treatment. 



