CHAPTER XXIII 



DESCENDING SAP 



" " a SCENDING sap, a liquid composed of a large 



A' 



quantity of water and a very small proportion 

 of dissolved nutritive substances, is absorbed in the 

 ground by the roots and carried to the leaves through 

 the sap-wood. It is not yet a nutritive fluid for the 

 plant; it becomes so in the foliage by a double proc- 

 ess. First, on being distributed to the leaves, which 

 furnish a vast surface for evaporation, it exhales 

 its superabundant water in the form of vapor and 

 thus concentrates its usable ingredients. Then, un- 

 der the influence of the sun's rays and through the 

 medium of the green matter contained in the leaves, 

 it undergoes modifications that work a fundamental 

 change in its character. Among the processes here 

 taking place, one of the best known is the decompo- 

 sition of the carbonic acid gas taken from the air 

 by the leaves and from the soil by the roots. 



"We have seen that this gas, the plant's chief 

 source of nourishment, is composed of carbon com- 

 bined with the breathable part of the air, or oxygen. 

 Under the action of the sun's light the leaves decom- 

 pose this gas, liberating the oxygen in a condition 

 henceforth fit for the respiration of animals and 

 for combustion, while the carbon remains in the 



112 



