CHAPTER IX 
THE STRUGGLE FOB 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 
societies, 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 drought, 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 
grow close together, silt from the land is entangled by them, 
and presently it accumulates to such an extent that there 
is no more standing water, and the water supply for the 
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