252 



PLANT PHYSIOLOGY AND ECOLOGY 



When a plant is carried into a group of otlier plants, or is sur- 

 rounded by its growing offspring, the struggle which results between 

 the individuals is competition. In all cases where a migrant is 

 carried into the vegetation of a very different habitat, it must 

 meet both tests. In consequence, ecesis usually includes adjust- 

 ment to competition as well as to physical factors. Competing 

 plants are really trying to obtain certain necessary amounts of 

 physical factors. Properly speaking, the struggle for existence 



Fig. 101. Competition in a family of Corispermum seedlings. The com- 

 petition is much keener between the seedlings that have sprung up 

 densely beneath the dead parent plant than between those arising 

 from the seeds scattered between the parents. 



in the plant world is a struggle between each plant and its habitat, 

 the latter being changed by competition in consequence of the 

 demands made upon it by other plants. The only exceptions to 

 this rule are furnished by plants which serve as hosts to parasites. 

 Between host and parasite there is a struggle not very different 

 from that between two animals. 



265. The nature of competition. Competition is purely a 

 physical process. With a few exceptions, such as the crowding 



