THE INTER-RELATIONS OF PLANTS AND MAN 123 



which are excretions. Many of these secreted and excreted plant 

 substances have been found useful by man; this will be more 

 clear after reading Part Four. Latex is a plant secretion with 

 elastic qualities which, in the familiar form of rubber, has become 

 an important plant product upon which many modern indus- 

 tries depend. Other industrial uses of plants include the manu- 

 facture of plastics and cellulose products, of varnishes and resin 

 products, and of the various tannins, dyes and volatile oils. 



Plant life is beneficial to man in many indirect and rarely 

 appreciated ways. We have already discussed the importance of 

 green plants in keeping constant the oxygen balance in the air, 

 and of colorless plants in the cycle of carbon and nitrogen. Other 

 less obvious relationships are those in which vegetation controls 

 soil erosion, floods and water supplies. 



Recently the importance of plants in erosion control has been 

 vividly brought to our attention. Continuous high winds have a 

 surprising lifting power and can remove surface soil if it is un- 

 protected by vegetation. In the prairie states, where the dominant 

 plants are grasses, the level expanses are frequently subjected to 

 high winds. These have little eff'ect on the soil as long as it is 

 covered by a mat of grass, whose intertwined roots and stems 

 eff'ectively hold the soil in place. Farmers in this area, during 

 the first world war, plowed thousands of acres of land previously 

 covered with buff'alo grass in order to grow sufficient wheat to 

 supply the abnormal demand. When the overseas market for our 

 cereal grains ceased to exist, these surplus "marginal" lands were 

 abandoned. But man had upset the natural balance of plant life, 

 and the buff'alo grass did not come back onto these unused 

 fields. Unusually high winds at this particular time then took a 

 toll of the surface soil, removing it in clouds of dust which 

 darkened many states, making an uninhabitable dust-bowl of 

 what formerly was grasslands. Only a slow and laborious replant- 

 ing of these thousands of square miles with grasses capable of 

 living in such an environment will keep the soil where it belongs. 



Barren soil is also easily eroded by rain which removes the 

 topsoil. Rainfall, when it comes suddenly and in great amounts, 

 runs off the surface of the ground and, carrying with it soil 

 particles, forms gullies wherever a hillside is unprotected by 



