Winifred E. I^renciilky 



151 



The association between the amount of growth and the (juantity 

 of soil available is further demonstrated by the efficiency indices of 

 the barley with different rates of seeding (Table VI), the index decreasing 

 steadily with the increase in the number of seeds per pot. Fig. 3 shows 

 this even more strikingly. As the large pots contain exactly twice as 

 much soil as the narrow pots, two seeds in large pots each have as much 

 soil and therefore food available as one seed in the narrow pots, four in 

 large as two in narrow, etc. Consequently in the figure the efficiency in- 

 dices for both sets of pots are plotted together according to the quantity 

 of soil available for individual plants, and the curves are practically co- 

 incident, showing again how entirely dependent the growth is on the 

 available food supply. 



per cent. 



8 seeds per 20 kilos of boil. 

 4 seeds per 10 kilos of soil. 



Fig. 3. Black line shows the average efficiency indices for varying numbers of plants 

 grown in 20 kilos of soil (broad pots). 



Dotted Ime ditto in 10 kilos of soil (tall pots). 



2. EFFECT OF INDIVIDUAL FACTORS ON COMPETITION. 



The above experiments with mustard and barley show that if the 

 food supply in the soil is restricted this will be the chief limiting factor 

 in determining the amount of possible growth, and therefore the action 

 of other factors of competition will be less obvious. On the other hand, 

 these other factors are at work at the same time under these conditions 

 so that the results obtained are not necessarily solely due to competition 

 for food. In order, therefore, to ascertain the w^orking of the various 

 factors it is necessary to arrange experiments in which each may be 

 studied individually, without interference by the others. 



