114 THE PRINCIPLES OF AGRICULTURE 



amount of space. Owing to the denser cell -sap 

 of the root -hairs these hairs absorb water by 

 osmosis. There are in solution in the soil 

 water minute quantities of food substances, and 

 these are absorbed independently of the relative 

 amounts present. The absorptive activity, or 

 pull, of the root- hairs is so great that water may 

 be extracted from a soil apparently dry. 



186. Plants contain much water; but it re- 

 quires oven temperatures, about 222 F., to 

 drive out all the water from plant substance. 

 The total water in some plants, as determined 

 by the chemist, is as follows: 



Dry clover seed 6.4 per cent. 



Dry beans 12.5 " " 



Green apple twigs 50.0 " " 



Potato tubers 80.0 " " 



Green clover tops 85.0 " " 



187. Water is absorbed in greater quantit} 7 

 than can actually enter into the composition of 

 the living plant; and the surplus water is thrown 

 off by a process of evaporation known as tran- 

 spiration. The water is rapidly transpired from 

 certain plant surfaces, especially from the leaves 

 and green stems. The water current is import- 

 ant; for example, it promptly distributes foods. 



188. Leaves are provided with thousands of 

 minute pores in the epidermis, connecting with 

 the delicate tissues within. These pores, or 



