8 HOW PLANTS GROW 



of salt in a glass of water, and drop in the slice of potato. In fifteen 

 minutes it will be soft and shrunken, showing that some of its water 

 has been extracted by the denser salt solution. Now place the slice 

 of potato in pure water and it will soon recover its solid quality, due 

 to the absorption of pure water by denser sap of the potato. The piece 

 of potato can not continue to absorb, but with plants, the roots con- 

 tinue to take up water, as it is constantly being evaporated by the 

 leaves. 



FIG. 2. Root-hairs. On the right is a magnified section showing root-hairs in contact 



with soil grains. 



Evaporation of Plant Water. The soil water contains small 

 quantities in solution of all the minerals a plant needs, but the solu- 

 tion is so weak that a barrel of soil water would scarcely contain a 

 spoonful of minerals. Therefore, rapid evaporation of water is 

 necessary, as large quantities of water must be passed through the 

 plant in order that it may obtain sufficient mineral food. 



The amount of water evaporated varies with climate and soils, 

 but in general it requires from 300 to 500 pounds of water to each 



