COMPETITION AND ECESIS 253 



up of tuberous plants when gro^\^^ too closely, an actual struggle 

 between competing plants never occurs. Competition arises 

 from the reaction of one plant upon the physical factors about it, 

 and the effect of these modified factors upon its competitors. In 

 the exact sense, two plants, no matter how close, do not compete 

 with each other as long as the water content and the nutrient 

 material, the heat and light, are in excess of the needs of l)oth. 

 When the immediate supply of a single necessary factor falls below 

 the combined demands of the plants, competition begins. 



266. The factors involved. All of the factors essential to 

 one or more of the primary functions of the plant play some part 

 in competition. Such factors are water, humidity, light, and 

 temperature. Of these humidity and temperature are relatively 

 unimportant, while water content and light are decisive. In 

 wet soils the question of air content seems to be of importance, 

 but ordinarily this is apparently not true. 



Plants are sometimes said to compete for room. This view 

 is incomplete, and probably has resulted from the fact that plants 

 show the effects of competition the more the closer they grow. 

 The explanation is that the amount of water and light available 

 for each decreases as the plants become more crowded. The 

 moment that the roots of one enter the area from which the other 

 draws its water supply, or the foliage of one overshades the leaves 

 of the other, a change in factor results, which is unfavorable to 

 one or the other, and competition begins. 



267. Competition for water and light. Plants that grow in 

 close or crowded masses compete with each other for water or light. 

 In the majority of cases both factors are involved. Plants with 

 a larger, deeper, or more active root system react upon the habitat 

 and reduce the amount of water available for those with poorer 

 root systems. The stronger, taller, more branched or more leafy 

 plants receive the larger share of the sunlight, and the others can 

 obtain only what is left. This action of one competitor upon 

 the habitat, and of the habitat upon the other competitor is cunui- 

 lative. An increase in the leaf surface of a plant not only reduces 

 the amount of light available for the plant near it or beneath it; 

 it also renders necessary the absorption of more water and nutrient 

 salts, and correspondingly decreases the amount a^ailal)le. The 

 result is that the successful individual prospers more and more, 

 while the less successful one loses ground in the same degree. As 



