CHAPTER XIII 



COMPETITION AND ECESIS 



263. Competition. In consequence of aggregation, two or 

 more individuals come to occupy the space previously occupied 

 by one, or the group of plants already in possession of an area is 

 greatly increased in number. Usually the immediate result is 

 competition between the various individuals, though this is not 

 always the case. The number of individuals may be so small 

 and the distance between them sufficiently large, so that the}- do 

 not compete with each other. Actual competition begins only 

 when one plant encroaches upon the space occupied by another. 

 Moreover, it sometimes happens that species are so different in 

 their nature and their demands upon the habitat that the}' may 

 be crowded together without being in actual competition. 



Competition occurs only between plants that meet each other 

 on terms more or less equal. It is impossible to speak of competi- 

 tion between an oak and the tiny herb that grows beneath it, or 

 between a puffball and the prairie grasses which surround it. 

 Likewise, there is no competition between a host-plant and the 

 parasite upon it, though two or more parasites upon the same 

 host may compete \\ath each other. Parasite competes with 

 parasite and host with host, though a rust, for example, may 

 often be a decisive factor in the competition between two wheat 

 plants. 



264. The struggle for existence. This popular phrase contains 

 two different ideas. As Darwin pointed out, a plant may struggle 

 against adverse conditions in the habitat, or it may stniggle with 

 other plants for things necessary to it. A ])ioneer migrating into 

 a new or denuded habitat must for a time make its way against 

 conditions more or less unfavorable to it. Establishment, or 

 ecesis, can take place only when it succeeds in adjusting itself. 



L'.")l 



