CHAPTER XXII 



The Descent of the Sap 



The rising sap in a fluid made up of a large 

 amount of water and a very small proportion 

 of nutritive substances in solution, which are 

 absorbed from the ground by the roots and 

 conveyed to the leaves by the sap-wood. It 

 is not as yet a fluid capable of feeding the 

 plant, and assumes this character after 

 reaching the leaves by a double process. In 

 the first place, being dispersed among the 

 leaves, which collectively provide a great 

 surface for evaporation, it gives out its excess 

 of water as steam and concentrates its service- 

 able materials. Then, under the influence of 

 the sunlight and through the action of the 

 green matter in the leaves, it experiences 

 changes which completely alter its nature. 



Among these processes one of the best 

 known is the decomposition of the carbonic 

 acid gas, absorbed from the air by the leaves 

 and from the ground by the roots. We have 



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