Water 91 



turnip IS more watery still ; and a pumpkin contains 

 only five and a half per cent, of solid matter. 



And yet, when we have stored our potatoes and 

 turnips, or our pumpkins, we do not find it necessary 

 to water them. They do not shrivel ; they keep their 

 moisture. 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 can- 

 not 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 be- 

 come 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 there- 

 fore able to retain their moisture for a considerable 

 time. In a similar way, the stems of most woody 

 plants aad 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 growing parts, and the leaves 

 by which it is chiefly allowed 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; 



