PLANTS IN THEIR RELATION TO HUMAN WELFARE 137 



rapidly over the surface, swelling the streams into torrents, 

 which bring destruction and death as they flood the valleys 

 and fields along their course. As the water flows over the 

 surface of the land from which the trees have been cut, it 

 carries along the richest part of the soil,' thus causing loss of 

 fertility in the uplands. The material thus carried away 

 fills up the river beds and harbor mouths, and in many cases 

 a heavy expense is entailed in its removal. 



144. Dangers to forests. We have already called atten- 

 tion to the threatened destruction of our American forests 

 by careless lum- 

 bering. (See 141.) 

 This means not 

 only the whole- 

 sale cutting of 

 large areas of 

 trees, but the lack 

 of forethought 

 which lumber- 

 men show in 



leaving dead tree FIG. 69. Excessive erosion of land caused by de- 

 struction of forests. (Bailey.) 



trunks and 



branches to become the prey of destructive forest fires, 

 which, when once started, devastate wide areas. The annual 

 loss of property from this cause is conservatively estimated 

 at more than one hundred million dollars. 



This forest debris of dead tree trunks and branches also 

 furnishes breeding places for insects, which, when hatched, 

 prey upon healthy trees. Another source of danger, espe- 

 cially to young forest growth, comes from permitting large 

 flocks of grazing animals like cattle and sheep to feed upon 

 and trample down the small trees. If we are to preserve the 



