152 Some Factors in Plant Competition 



A. Influence of food supply on competition. 



Proof of the close relation between the quantity of available food and 

 the amount of growth made was obtained from several sets of water 

 cultures carried out with barley and wheat in 1915. The plants were 

 grown in four food solutions of different strengths, the usual culture 

 solution^ (called N for convenience) and others one-fifth, one-tenth, 



N N N 



and one-twentieth as strong [N, - , z-^ . ^ j . Each experiment 



(three in all) was carried on for six weeks, three series being run at the 



same time. In one the nutrient solution was changed every four days, 



in the second it was changed once (half-way through the test), and in the 



third no change was made at all. The plants were spaced at equal 



distances apart, so that each received a similar light supply. In every 



case it was found that the dry weights of the plants decreased coin- 



cidently with the strength of the nutrient solution. Even where the 



N N 

 solutions were constantly changed the ^^^ or r^^ plants were very 



much behind the N plants, showing that the total quantity of available 

 food had a direct bearing on the amount of growth. When the solutions 



were changed but once, or not at all, the ^ plants were considerably 



weaker than the N, as they had apparently not received as much food 



as they could deal with. When the solutions were frequently changed, 



N 

 however, the -^ plants did not fall nearly so much behind the A\ 



indicating that the total amount of food supplied in the solution with 

 constant changing enabled the plants to make almost their maximum 

 growth, while the N plants probably received more food than they could 

 profitably deal with'^. (Fig. 4.) 



Thus, when the competition of one plant with another for food is 

 removed, and when the water supply is adequate and all plants receive 

 a similar and plentiful supply of light, it is seen that up to a certain 

 limit the amount of growth is proportional to the supply of food, though 



1 I'olas.siimi nitrate 1 gm. 



Ma<;iiosiiiiii sulphate '5 ,, 



Cal<"ium siilpluito '5 „ 



Potns.-iuni (li-hydrogon phosphntf ••! „ 

 Soflium c'hloiiflo o ,, 



Ferric clilorirlc -04 ,, 



Distilled Wi»ier — tf) make u]) I litre. 

 * Bnnrhley, W. E. (1916). "The effect of the concentration of the nutrient solution 

 on the growth of barley and wheat in water cultures." Ann. Hot. xxx, 77-90. 



m 



