Winifred E. Brenchlfa' 



147 



On the whole, therefore, it is apparent that where a number of plants 

 are growing in close association with a limited supply of food the quan- 

 tity of available nitrogen sets a definite limit to the possible p;rowth, and 

 so is a very important factor in the competition between plants. Other 

 factors are in play, however, and later experiments were designed to 

 isolate the more important of these (see p. 151). 



per cent. 



1 seed 



2 seeds 



3 seeds 



4 seeds 



5 seeds 



Fig. 2. Black line shows the efficiency index of dry weight production (per cent.) 

 when varying numbers of mustard plants were grown in 10 kilos of soil (tall 

 pots). 



Dotted line shows the relative amount of food available per plant with each 

 rate of seeding. 



V. H. Blackman has recently put forward an important proposition 

 indicating that the growth of an annual plant, at least in its early stages, 

 follows approximately the "compound interest law^," the rate of interest 

 or the percentage rate at which plant material accrues being termed the . 

 "efficiency index" of dry weight production. In the present experiment 

 the reduction in the amount of dry matter produced by each individual 

 plant with a decrease in the supply of available food is an expression 



^ Blackman, V. H. (1910). "'I'lie compound interest law and jilant growth." Ann. 

 Dot. XXXIII, p. 357. 



