Water 67 



water them. They do not shrivel; they keep their moist- 

 ure. Why does not a plant do the same.-* 



The only answer to this question is, simply because it 

 can't. It cannot shut the many mouths by which it is 

 constantly losing moisture. We human beings cannot 

 prevent the escape of water through the pores of our skin, 

 or in the breath which we breathe; and the plant is in 

 similar case. It is constantly giving off water, and if the 

 loss is not made up it must needs become dry and shrivel. 



Almost every part of a plant which is exposed to the 

 air, and not covered by a layer of cork or of thickened 

 skin, is constantly losing moisture in ordinary air; and 

 unless the roots can suck up enough to make the loss 

 good, it droops, flags, withers, and dies. 



The potato and the pumpkin are protected — the one 

 by cork, the other by thick skin — and they are therefore 

 able to retain their moisture for a considerable time. In 

 a similar way, the stems of most woody plants and trees 

 are protected by layers of cork, and often of fibrous bark 

 as well, which almost, though not altogether, prevent the 

 escape of water. It is the young, green stems, the grow- 

 ing parts, and the leaves by which it is chiefly aUowed to 

 go off into the air; and these are just the parts which 

 especially need the mineral food, the food derived from 

 the soil, which the roots are constantly preparing. 



How are the roots to convey this food to the growing 

 parts of the plant? Of course, they cannot do so; they 

 can only make it ready, and then it must be pumped up 

 to where it is wanted. Accordingly, as the water is drawn 

 off, so to say, above, the sap from below — that is, water 

 containing food from the soil— rises to supply its place. 



A constant current, therefore, rises from the roots 



