68 The Great World's Farm 



upwards; but a great deal of this would be lost during its 

 passage before it reached the young shoots but for the 

 fact, already mentioned, that the trunks or stems through 

 which it passes are protected against the air, and moisture 

 can escape but very slowly through bark or cork, though 

 it does still escape to some small extent. 



When the sap reaches the green parts of the plant it 

 passes off into the air as invisible vapor; or, rather, the 

 water of the sap passes off in this way, and the food from 

 the soil, the mineral matter, is left behind. But even 

 from the green stems and leaves the water is not allowed 

 to escape quite unchecked, else it might pass off too fast — 

 faster than it could be supplied. 



For anything moist, whether it be moist earth or wet 

 clothes, dries when exposed to the air. The air sucks the 

 moisture out of all, and the drier the air, the more quickly 

 it sucks. Without some protection against this thirsty 

 air, therefore, leaves and green stems would also be 

 sucked dry, like anything else, and accordingly their outer 

 skin is more or less thickened; and it often contains, or 

 is covered by, a waxy deposit as well. We may perhaps 

 have noticed how drops of dew he upon the leaves without 

 soaking in, so that when the dew is shaken off, the leaves 

 are dry. This is especially noticeable on some shiny 

 leaves, but also on some mealy-looking ones, as, for 

 instance, cabbage leaves; and in both cases it is the waxy 

 substance in or upon the skin of the leaf, which not only 

 prevents water from soaking in, but also prevents all but 

 a very small quantity of the moisture from being drawn out. 



Then, if water is constantly passing off, and that in 

 considerable quantities, how does it escape.? A Httle, as 

 we have said, passes off through the whole surface, but 



