J. Davidson 141 



The classes evidently show significant differences in the degree of 

 infestation in a given period. 



It would appear probable that owing to the influence of the cell sap 

 of the varieties concerned on the aphid metabolism, the rate of repro- 

 duction is considerably affected and thus under the same conditions of 

 environment, the chances of infestation occurring are greater with some 

 varieties than others. 



This in itself is an important economic consideration in that, with 

 the prospects of heavy rain or broken weather conditions (unsuitable for 

 aphids) within a two or three weeks' period of fine weather, the chances 

 of varieties in classes D, E and F recovering from aphis attack, are much 

 greater than would be the case with varieties in class A. 



Plant-breeding experiments would show whether susceptibiHty or 

 resistance is a specific mendelian character. It seems feasible to consider 

 tentatively, that the factor or factors (genes) which make for high 

 resistance as in Vicia narbonensis — w^hich as discussed below probably 

 represents the prototype of Vicia f aba — may have been present in the 

 original ^^^ld bean, and that this character has been lost or modified in 

 the process of selection in the cultivated varieties. 



A theoretical discussion of this question of varying susceptibiUty 

 would hardly be profitable at this stage. It would appear, however, that 

 the factor or factors concerned are associated with the general physiology 

 of the plant and have an influence on the cell sap. 



If one factor only is concerned, it should not be a difficult matter to 

 trace it by cross breeding experiments. On the other hand, if the character 

 of resistance is due to an interworking of a number of factors, the problem 

 becomes an extremely complex one. 



It is interesting to consider variety XVIII ( Vicia narbonensis) in this 

 respect. 



Some authorities consider this species as a prototype of the cultivated 

 Vicia f aba. 



According to De Candolle{2) the bean has been cultivated from pre- 

 historic times, and may have been distributed during the early migration 

 of man. Some thousands of years ago it was probably established wild 

 in two areas, namely, south of the Caspian sea and North Africa. 



Vicia 7iarbonensis most nearly represents the wild prototpye of the 

 modern cultivated race. It is found wild to-day in the Mediterranean 

 area; east towards the Caucasus; in Northern Persia and Mesopotamia^. 



^ I have tried Prolific Longpod ^ x Vicia narbonensis 9 in 1920 and have reason to 

 believe that three pods which came to maturity are successful crosses. The seeds from these 

 pods are being carried to the F^ generation. 



Ann. Biol, ix 10 



