92 Water 



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 

 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 vapour; 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 lie 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 



