PLANT SOCIETIES. 479 



920. Longevity of the forest. The forest is capable of self- 

 perpetuation, and, except in case of unusual disaster or the action 

 of man, it should live indefinitely. As the old trees die they 

 are gradually replaced by younger ones. So while trees may 

 come and trees may go, the forest goes on forever. 



921. Autumn colors. One of the striking effects produced 

 by the deciduous forests is that of the autumn coloring of the 

 leaves. It is more pronounced in the forests of the United States 

 than in corresponding life zones in the eastern hemisphere because 

 of the greater number of species. With the disintegration of 

 the chlorophyll bodies, other colors, which in some cases were 

 masked by the green, appear. In other cases decomposition 

 products result in the formation of other colors, as red, scarlet, 

 yellow, brown, purple, maroon, etc., in different species. These 

 coloring substances to some extent are believed to protect the 

 nitrogenous substances in the leaf from injury. The colors 

 absorb the sun's rays, which otherwise might destroy these 

 nitrogenous substances before they have passed back through 

 the petiole of the leaf into the stem, where they may be stored 

 for food. The gorgeous display of color, then, which the leaves 

 of many trees and shrubs put on is one of the many useful adapta- 

 tions of the plants. 



922. Importance of the forest in the disposal of rainfall. The 

 importance of the forest in disposing of the rainfall is very great. 

 The great accumulation of humus on the forest floor holds back 

 the water both by absorption and by checking its flow, so that 

 it does not immediately flow quickly off the slopes into the drain- 

 age system of the valley. It percolates into the soil. Much 

 of it is held in the humus and soil. What is not retained thus 

 filters slow\y through the soil and is doled out more gradually 

 into the valley streams and mountain tributaries, so that the 

 flood period is extended, and its injury lessened or entirely pre- 

 vented, because the body of water moving at any one time is 

 not dangerously high. The winter snow is shaded and in the 

 spring melts slowly, and the spring freshets are thus lessened. 

 The action of the leaves and humus in retarding the flow of the 



