CHAPTER IX. 



THE STRUGGLE FOR EXISTENCE. 



100. Definition. — The phrase " struggle for existence " 

 has come to mean, so far as plants are concerned, that it is 

 usually impossible for them to secure ideal relations, and 

 that they must encounter unfavorable conditions. The 

 proper light and heat relations may be difficult to obtain, 

 and also the proper relations to food material. It often 

 happens, also, that conditions once fairly favorable may be- 

 come unfavorable. Also, multitudes of plants are trying 

 to take possession of the same conditions. All this leads 

 to the so-called "struggle," and vastly more plants fail 

 than succeed. Before considering the organization of plant 

 associations, it will be helpful to consider some of the 

 possible changes in conditions, and the effect on plants. 



101. Decrease of water. — This is probably the most com- 

 mon factor to fluctuate in the environment of a plant. 

 Along the borders of streams and ponds, and in swampy 

 places, the variation in the water is very noticeable, but the 

 same thing is true of soils in general. However, the change 

 chiefly referred to is that which is permanent, and which 

 compels plants not merely to tide over a drouth, but to 

 face a permanent decrease in the water supply, 



Around the margins of ponds are very commonly seen 

 fringes of such plants as bulrushes, cat-tail flags, reed- 

 grasses, etc., standing in shoal water. As these plants 

 partially decay, their bodies and the entangled silt from 

 the land presently accumulate to such an extent that there 

 is no more standing water, and the water supply for the 



