Winifred E. Brenchley -ioS 



Dry Weight 'per plant m grams. 



Unlimited food supply Restricted food supply 



March— May K. 9-2 L. 1-85 



May— July M. 7-2 N. 2-6 



(6) Effect of restricted food supply on efficiency indices. 



The general form of the curves (Fig. 11) of the efficiency indices for 

 the various series throughout the year is nuich the same whether the food 

 supply is abundant or restricted, showing that on the whole there is a 

 similar response to the environmental conditions that are common to 

 both sets. In detail the correspondence is less close, indicating the in- 

 fluence exerted by the variable factor of food. In most cases during the 

 earlier weeks the rates of growth go up or down together in both cases, 

 as at this stage the environmental conditions of temperature and light 

 common to both are more potent than the variation in food supply. 

 Later on, when food begins to be much restricted in the one set, the 

 efficiency index curves no longer r^n together, and they become much 

 more irregular and erratic in the starved series. Under the conditions 

 of restricted food supply, therefore, changes in the common environ- 

 mental conditions produce exaggerated effects on the rate of growth. The 

 foregoing applies more particularly to the spring and autumn months, 

 but matters are rather different during the period of most rapid growth 

 in the summer. At this time the conditions of light and heat are so 

 favourable to growth that the plants can take advantage of a very large 

 supply of food, and any restriction in this respect is felt more severely 

 than when growth conditions are less good. Both sets of plants make 

 the most of the available food immediately the first period of growth 

 is over, and rise almost at once to a maximum efficiency index. This 

 greatly depletes the food store, and when no more is supplied the rate 

 of growth cannot be kept up and a rapid fall occurs, in contrast to the 

 prolonged period during which a high rate of growth is maintained in the 

 presence of abundant nutrients. In some cases the restricted plants 

 never attain such a high efficiency index as the unrestricted, and the 

 fall in the rate is less sharp, but in others the maximum index is as 

 high or even higher and then an exceedingly sharp fall occurs, on one 

 occasion (set P) the drop being from 11 to zero in three weeks. 



A statistical comparison of the figures relating to the "changed" and 

 "unchanged" series bears out the results obtained from observation of 

 the curves of total growth, but shows more clearly the course of events 

 in the later part of the life of the plant. The mean differences between 



