THE STRUGGLE FOR 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 the water- 
supply more favorable for the plants which man desires, 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 temporary, 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 
