THE STEM AND BRANCHES. 183 



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-i^resszire ; the 

 other as evaporation. 



I begin with the former. The cells of which 

 roots are made up are most ingeniously con- 

 structed 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 absorbs water, and allows the water 

 BO 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 longer, and their own elas- 

 ticity forces the water out of them. But the 

 water cannot flow back ; so it has to flow for- 

 ward. Again, each cell or vessel which the 

 stream afterwards enters is constructed on just 

 the same general principle as the absorbent 



