16 



A PRIMER OF FORESTRY. 



TRANSPIRATION. 



The leaves give out not only the oxygen derived from 

 the decomposition of carbonic acid gas taken from the 



air and carbonic acid 



gas produced inbreath- 

 ing, but also great quan- 

 tities of water vapor. 

 The amount of water 

 taken up bj^ the roots is 

 very much larger than 

 is required to be com- 

 bined with carbon and 

 the earthy constituents 

 in the leaves. In order 

 that fresh supplies of 



Fig. 14. — By comparing this diagram -with 

 Pis. VIl-IX and tig. 16, the place of eacL 

 cut in tbe tree ^vill be made clear. 



earthy constituents in solution may reach the leaves 

 rapidly, the water already in them must be got out of 

 the way. This is eifected by 

 transpiration, which is the evap- 

 oration of water from all parts of 

 the tree above ground, but princi- 

 pally from the leaves. Even 

 where the bark is very thick, as 

 on the trunks of old Oaks and 

 Chestnuts, transpiration goes on 

 through the lenticels in the bot- 

 toms of the dee^:) cracks. It 

 sometimes happens, especially in 

 spring before the leaves come 

 out, that transpiration can not 

 get rid of the water from the roots 



as fast as it rises, and that it falls in dro])s from the 

 buds, or later on even from the leaves themselves. 



Fig. 15.— Top of a common 

 cork, slightly moistened to 

 bring out the lines of annual 

 growth, which are rather 

 unusually plain in this speci- 

 men. 



