THE STRUGGLE FOR EXISTENCE 143 



bulrushes and tlieir associates lias perniauently decreased 

 below the favorable amount. In this way certain lake 

 margins gradually encroach upon the water, and in so 

 doing the water supjily is permanently diminished for many 

 plants. ]5y the same i)roccss, smaller lai<elets are gradually 

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

 ground, and these unfavoral)le changes in water su])ply are 

 a menace to many plants. 



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

 diminishing the water supply for plants. Drainage, which 

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

 supply more favoraljle for the plants which man desires, cer- 

 tainly makes it very unfavoralde 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, Init rushes off in a flood ; hence, the streams whii'li 

 drain the cleared area are alternately flooded and dried up. 

 This results in a much less total supjily of water available 

 for the use of plants. 



lO'i. Decrease of light. — It is very common to observe 

 tall, rank vegetation shading lower f(jrms. and seriously 

 interfering with the light supply. If the rank vegetation 

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

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

 vegetation is a forest growth, shading becomes permanent. 

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

 close of the growing season and put (nit a fresli crup in the 

 spring, there is an interval in the ea,rly spiing, before the 

 leaves are fully developed, during whicli low plants may 

 secure a good exposure to light (see Pig. 14-1). In such 

 places one finds an abundance of "spring flowers," 1nit later 

 in the season the low jDlants become very scarce. This 

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



