1 68 THE STORY OF THE PLANTS. 



Well, the stem is well provided with a whole sys- 

 tem of upward distributing vessels in which water 

 may be conveyed to the various parts, just as 

 it is conveyed in towns through the pipes and taps 

 wherever it is needed. But what is the motive 

 power for this mechanical work ? How does the 

 plant raise so much liquid to such a considerable 

 height, without the intervention of any visible 

 and tangible machinery ? 



Two main agents are employed for this pur- 

 pose. The one is known as root-pressure ; the 

 other as evaporation. 



I begin with the former. The cells of which 

 roots are made up are most ingeniously constructed 

 so as to exert this peculiar form of pressure. 

 Each one of them has at its outer or free end, 

 where it comes into contact with the moist earth, 

 a wall of such a nature that it very readily ab- 

 sorbs water, and allows the water so absorbed to 

 flow freely through it inward. But once in, the 

 water seems almost as if imprisoned in a pump; 

 it cannot pass outward again, only inward and 

 upward. You may compare the cell in this respect 

 with those mechanical valves which yield readily 

 to the pressure of fluids from outside, but instantly 

 close when a fluid from inside attempts to pass 

 through them. In this way the outer cells of the 

 hairs on the roots, which come in contact with the 

 moistened soil, get distended with water^ and swell 

 and swell, till at last their walls will give no long- 

 er, and their own elasticity forces the water out 

 of them. But the water cannot flow back ; so it 

 has to flow forward. Again, each cell or vessel 

 which the stream afterwards enters is construct- 

 ed on just the same general principle as the ab- 

 sorbent root-cells ; it allows water to pass into it 



