THE PLANT COVERING OF THE EARTH 321 



322. Cultivated plants. Where the natural plants have 

 been removed, it has usually been done in order to substitute 

 for them our cultivated plants, for these supply products 

 which we need for food, clothing, and other uses. There is 

 no industry in all the world so important as farming. Its 

 products are of more value than those of any other industry, 

 and all industries depend upon it for their prosperity. Where 

 agriculture thrives it makes a need for transportation facili- 

 ties; on the other hand, many fertile regions have had their 

 production stimulated through the introduction of railways, 

 since without railways farm products find only a local mar- 

 ket. In similar ways the other industries of the entire country 

 are related to agriculture. 



323. Plants and soil. In the chapters on soils we had fre- 

 quent occasion to refer to the close relation existing between 

 plants and the water content of soils. If we were to cut off 

 the stem of some vigorously growing plant close to the 

 ground, we should find that the sap would appear from the 

 cut end of the remaining stub. A maple tree or a grapevine 

 will show this escape of sap, or " bleeding," particularly well 

 if a branch is cut off early in the spring. Maple trees are 

 often tapped for the sap, which is evaporated to secure the 

 sugar which it contains, sometimes a single tree supplying 

 from thirty to fifty gallons of sap in one season. The sap is 

 mainly water. Since the water comes out of the plants, there 

 must be some place from which the plants secure it. 



It is not at all difficult to prepare an experiment which will 

 show that water is lost by the soil when a plant is growing 

 in it. If a pot with a growing plant is wrapped in a sheet of 

 rubber so that no water can escape excepting from the plant, 

 it will be found that the soil becomes drier and that weight is 

 lost. A similar pot of soil without a plant in it and completely 

 inclosed in rubber does not lose in weight. We should con- 

 clude that the plant must have been taking up water from 

 the soil about as rapidly as it evaporated from the leaves. 



