CHAPTER XXIV 

 ABSORPTION FROM THE SOIL BY PLANTS 



249. The problem. In the chapters on soils we had frequent 

 occasion to refer to the close relation existing between plants 

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

 the ground the stem of some vigorously growing plant, 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, and a single tree may supply from 30 to 50 gallons 

 of the sap. The sap is mainly water. Since the water comes 

 out of plants, there must be some place from which the plants 

 secure it. Our problems are, what is the source and what are 

 the methods by which plants secure water ? 



250. Loss of water from the soil. 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 conclude that the plant must 

 have been taking up water from the soil about as rapidly as 

 it evaporated from the leaves. 



251. Osmosis. The first question is, How does the water get 

 into the roots ? In order to answer this we shall have to study 

 the peculiar behavior of solutions. This behavior may be shown 

 by the use of a dense sugar solution and pure water. A funnel 



234 



