J. Davidson 137 



maximum and minimum daily temperature in the glasshouse throughout 

 the experiments is shown in Fig. 1. 



There were six pots of each variety of field beans and six pots of 

 garden Prohfic Longpod broad beans. In all cases each pot contained 

 a single plant. The latter variety of beans, which gave a high figure of 

 infestation in the 1920 experiments, is taken as the standard to which 

 the other varieties are referred in order to get relative figures of infesta- 

 tion. One plant of each variety was the "stock" plant for the variety, 

 and the remaining five plants were utihzed for the infestation tests. 



The seeds were all sown on 25. iii. 21. 



The aphids used in the experiments (Aphis rumicis) were derived 

 from one egg, all being the offspring of one Fundatrix. Oviparous 

 females, which developed in the colonies in the 1920 experiments, laid 

 eggs in October 1920 on Euonymus europaeiis. Some of the eggs com- 

 menced to hatch out on 8. iii. 21, and one Fundatrix was isolated on 

 Euonymus europaeus on 25. iii. 21. This individual was the "stem- 

 mother" of all the individuals used in the present experiments. As the 

 individuals in each generation became adult they were isolated. The 

 Fundatrix produced a. v. ?? of 1st v. gen. These gave rise to a mixed 

 progeny of w. v. ?$ and a. v. $$ in the 2nd v. gen. Two winged migrants 

 were transferred on 26. i v. 21 to each of the "stock" plants referred to 

 above. On these they produced a. v. $? of 3rd v. gen. When these 

 a. V. $$ were almost adult and before they actually began to reproduce, 

 the five plants of each variety were separately infected on 11. v. 21 

 with an a. V. ? derived from the stock plant of the variety concerned. 

 The date and time when the a. v. ? on each plant began to reproduce 

 was recorded (vide Tables) and reproduction was then allowed to go 

 on for 14 complete days. The total number of aphids produced on each 

 plant at the end of that period was then counted. 



It will be noted that the infections of the different plants were made 

 on the same day and that the 14-day reproduction period extended over 

 practically the same period for all the varieties, thus ensuring that 

 factors of temperature, humidity and sunshine were the same for all. 



Further points which should be considered in experiments of this 

 kind have already been given in the paper referred to above. 



III. DISCUSSION. 



Tables I and II shows the results obtained for the 18 varieties of field 

 beans. These may be compared with the results obtained for Prolific 

 Longpod (XIX) which is taken as the standard in order to fix the relative 

 values of susceptibility of the other varieties. 



