INTRODUCTION 5 



kind that no method of fighting them can ever be completely 

 successful. The only thing is to circumvent them temporarily, 

 keeping up a constant battle to prevent them getting the 

 upper hand. 



When a number of plants of either the same or dissimilar 

 species are grown in association the individuals of the com- 

 munity act and react on one another and determine the ulti- 

 mate nature of the plant association. The interaction between 

 the species is very much affected by other factors such as 

 moisture, sunlight, season, and interference by man, and under 

 the specialised conditions of cultivation the interplay tends to 

 become obscured to a great extent. When a large number 

 of plants are growing in close association keen competition 

 exists between them, and the weaker plants are apt to 

 be smothered out, while the stronger continue to struggle 

 on together. This competition takes place both below and 

 above ground. The supply of moisture and available plant 

 food is not inexhaustible, and if a large number of roots are 

 drawing from the same area at the same time semi-starvation 

 threatens the plants. Above ground there is usually an ade- 

 quate supply of air and sunshine, but the vegetation is not 

 always in a position to take full advantage of it. If the plants 

 are closely crowded together the leaves overlap and shade one 

 another, so that a large proportion of them cannot be reached by 

 the sunlight, and in this way the assimilation of carbon-dioxide 

 is hindered and the nutrition of the plants suffers accordingly. 

 Respiration is probably influenced less adversely under these 

 circumstances as the diffusion of the air carries an abundant 

 supply of oxygen even to the shaded leaves, and light is not 

 essential to breathing. The harmful effect of overcrowding is 

 constantly to be seen in gardens. If a number of young 

 seedlings are allowed to grow on too long in a crowded seed- 

 box they become "drawn up and delicate, and if left undis- 

 turbed die without making any satisfactory growth. Even if 

 they are planted out they never make the same robust and 

 healthy growth as do similar seedlings transplanted before 

 the little plants have time to injure one another. 



If the roots of the associated plants feed at different depths 

 in the soil, as for instance in the case of wheat and poppy, the 

 underground competition is far less severe, but the aerial 

 competition is so strong that the plants may suffer as much as 

 though all the roots were at the same level. Water culture 

 experiments, in which each barley plant had its own individual 



