100 



HISTOLOGY AND PHYSIOLOGY. 



WATER. 



126. All living vegetable tissue is abundantly supplied 

 •with water. lu aquatic plants it may constitute as much 

 as ninety-five per cent, of the whole weight. In the terres- 

 trial plants it generally averages about seventy-five per 

 cent. In consequence of this quantity of water the cells 

 are rendered turgid, without which growth would be im- 

 possible. If green 

 plants be dried in 

 the air at ordinary 

 temperatures, they 

 lose only a por- 

 tion of their water. ' 

 Thus Eed Clover 

 contains seventy- 

 nine per cent, of 

 water, and when 

 air-dry (hay) it 

 contains seventeen 

 per cent. Fresh 222 



Pine-wood contains forty per cent, of water, and the same 

 dry contains twenty per cent. The first quantity in each 

 case may be called the free water of vegetation. The 

 second represents the ■water of organization. The root- 

 lets and root-hairs which penetrate the soil, growing between 

 the more or less minute soil-particles, where water is con- 

 tained (Fig. 222), are the organs of absorption. 



127. The tendency of the water in plants to assume a 

 state of equilibrium is disturbed by three causes ; namely, 



Fig. 222. Diagram representing a root, with root-hairs penetrating the soil; s, 

 soil-particles ; hf root-hairs ; a, air. 



