PLANT COVERING OF THE EARTH. 165 



the water enters into a thick spongy layer, com- 

 posed of partly decayed leaves together with trunks 

 and branches which are constantly dropping from 

 the trees upon the surface of the earth. Through 

 this sponge the water moves but slowly on its way 

 to the streams, and when it is actually in the 

 brooks its progress downward is retarded by numer- 

 ous dams made as we have just described by fallen 

 timber and drift-wood. The result is that instead 

 of pouring swiftly to the sea, the flood-waters may 

 slowly creep away, requiring weeks in place of 

 hours for their discharge to the greater rivers. 



There is another effect which forests have upon 

 the soil, an effect which is not exercised by any 

 plants less in size than our trees. The strong 

 roots of trees, penetrating far down into the crev- 

 ices of the rocks and into the subsoil, draw upward 

 above the surface and build into their trunks the 

 solid matter which we find in the ash remaining 

 after the wood is completely burned. This valua- 

 ble nutritive matter drawn from the depths is 

 returned to the earth when leaves and branches 

 decay, and is thus stored in the upper part of the 

 soil, and becomes accessible to the growing crops. 

 Furthermore the trees in their growth gather, as is 

 the case with all plants, a large part of their sub- 



