12 THE HUMAN SIDE OF TREES 



tree can be explained by ordinary atmospheric pres- 

 sure, but many trees are four times that height. 



The "hydrostatic paradox" gives the answer. 

 This tells us that pressure of the air will elevate 

 a liquid to any height providing that there is no 

 continuity of mass. This condition is obtained in 

 the trees by numerous transverse septa in the water 

 ducts which prevent the transmission of air and 

 water in bulk, but permit a very free molecular dif- 

 fusion of water and everything dissolved in it. 

 This is much aided by the fact that many air bub- 

 bles in the water greatly reduce its proportionate 

 weight and make it virtually foam. Sap will al- 

 ways bubble out of a tree wound like so much froth. 

 Much aid comes from the leaves, which absorb the 

 water as it reaches them and transform it into 

 starch, thus creating a vacuum at the top of this 

 internal water system. When a tree is stripped of 

 its leaves for any reason, the ascent of water stops 

 and it is liable to "bleed" at some low point on its 

 trunk. 



The gas system is much simpler. The surround- 

 ing atmosphere is a great reservoir on which the 

 trees draw for the carbon dioxide so essential to 

 their internal processes. As a waste product, their 

 leaves exhale oxygen. From a man's point of view, 

 trees are atmospheric purifiers. 



