94 ABSORPTION OF WATER AND MINERALS 



by this are made stronger and better protected against parasites, 

 but less able to take in water and solutes. Probably all of the 

 cells of the root epidermis are, up to a certain age, capable of 

 growing forth as root hairs, and only need the stimulus to do so, 

 for we find that the number of root hairs varies with the character 

 of the environment, more being formed where more are needed. 

 For instance, when grown in a moist atmosphere, where only 

 that water can be absorbed which the air holds, the hairs are very 

 numerous and long, while in a moist soil where the hairs can 

 become partly submerged in the water films about the soil par- 

 ticles shorter hairs in less number are formed; and where roots 

 are grown in water fewer and shorter hairs grow forth or none at 

 all. 



Method of Intake of Water and Solutes. — The water and 

 solutes enter the root hairs by osmosis and diffusion. The sap 

 of the root hairs holds in solution osmotic substances such as 

 sugar and acids which cause the inflow of the water, and since 

 this is continually passed on to the conducting tissues the con- 

 ditions causing its intake are more or less constant. The sub- 

 stances in solution in the soil water (the solutes) pass into the root 

 hair by diffusion, and the speed of their onward movement 

 through the membranes is by no means necessarily the same as 

 the rate of the inflow of the water. If the intake of water is 

 accelerated because increased evaporation from the leaves is 

 creating larger demands for water, it does not follow that the 

 entrance of solutes into the root hairs is hastened to the same 

 extent. The movements of water and solutes are governed by 

 different conditions. The solutes keep going in so long as there 

 is less of them in a unit of volume inside the cell than outside; 

 while the water continues to enter while there is a greater con- 

 centration of osmotic substances inside than outside. And the 

 same thing is true in the interchange of water and solutes between 

 different cells of the plant body, namely the water passes from 

 the cell having less into the one having the greater concentration 

 of solutes, while the solutes pass from the cell having greater into 

 the one having the less concentration of solutes. Of course, 



