THE StBUGGLE FOE EXISTENCE. 143 



bulrushes and their associates has permanently decreased 

 below the favorable amount. In this way certain lake 

 margins gradually encroach upon the water, and in so 

 doing the water supply is permanently diminished for many 

 plants. By the same process, smaller lakelets are gradually 

 being converted into bogs, and the bogs in turn into drier 

 ground, and these unfavorable changes in water supply are 

 a menace to many plants. 



The operations of man, also, have been very effective in 

 diminishing the water supply for plants. Drainage, which 

 is so extensively practiced, while it may make tlie water- 

 supply more favorable for the plants which man desnx's, cer- 

 tainly makes it very unfavorable for many other plants. 

 The clearing of forests has a similar result. The forest 

 soil is receptive and retentive in reference to water, and is 

 somewhat like a great sponge, steadily supplying the streams 

 which drain it. The removal of the forest destroys much 

 of this power. The water is not held and gradually doled 

 out, but rushes off in a flood ; hence, the streams which 

 drain the cleared area are alternately flooded and dried up. 

 This results in a much less total supply of water available 

 for the use of plants. 



102. Decrease of light. — It is very common to observe 

 tall, rank vegetation shading lower forms, and seriously 

 interfering with the light supply. If the rank vegetation 

 is rather temjDorary, the low plants may learn to precede or 

 follow it, and so avoid the shading ; but if the over-shading 

 vegetation is a forest growth, shading becomes permanent. 

 In the case of deciduous trees, which drop their leaves at the 

 close of the growing season and put out a fresh crop in the 

 spring, there is an interval in the early spring, before the 

 leaves are fully developed, during which low plants may 

 secure a good exposure to light (see Fig. 144). In such 

 places one finds an abundance of "spring flowers," but later 

 in the season the low plants become very scarce. This 

 effective over-shading is not common to all forests, for 



