136 PLANT BIOLOGY 



ther study : maple sugar, rubber, tar, turpentine, wood pulp, alcohol, 

 charcoal. Consult Bailey's " Cyclopedia of American Agriculture," 

 Vol. II, "Crops," or any encyclopedia. Determine (1) the -parts 

 of the United States (or of the world) in which the product is 

 obtained in large quantity, (2) the amount and value of a year's 

 crop, (3) methods of preparing the product for market. 



143. Uses of forests in regulating rainfall and flow of 

 streams. We turn now from a consideration of our forests 

 as a source of lumber and manufactured products to a dis- 

 cussion of their effect -on the fall of rain and the flow of 

 streams. It is probably true, in the first place, that the 

 destruction of large tracts of forest lands means a lessened 

 rainfall, at least so far as local showers are concerned. We 

 saw in our study of the functions of the nutritive organs of a 

 plant that great quantities of water are absorbed by the roots, 

 carried up through the woody bundles of the stem, and given 

 off through the stomata of leaves. It has been estimated 

 that a single oak tree of average size gives off in a single 

 season over one hundred and twenty-five tons of water. If 

 we were to multiply this amount by the number of trees in a 

 forest, we would get some idea of the enormous amount of 

 water lifted into the air by this agency. 



Not only do trees help to produce rain ; they also conserve 

 the rain when it falls by holding it in the soil, and preventing 

 disastrous floods. Let us see how this is brought about. 

 When the raindrops fall upon the tree tops, the water drips 

 from leaf to leaf, and finally reaches the ground. Here it 

 trickles down through the floor of the forest, which is formed 

 of thick layers of decaying leaves, interlacing roots, and earth 

 particles (see frontispiece) . All these form a porous sponge 

 which absorbs and holds back the water. Suppose, now, the 

 trees are removed from the hillsides. When the rains come, 

 there is no means of absorbing the water ; instead, it flows 



