RESPIRATION. 107 



changes whicli take place in the exterior sap-cells in the 

 parenchyma of the leaves. Kecent experiments have 

 shown that the evolution of oxygen gas from the leaves of 

 plants only takes place when the sun shines directly upon 

 them ; that the process ceases when his rays are intercepted 

 by clouds, and, as the light of day gradually fades, is ac- 

 tually reversed, oxygen being absorbed and carbonic acid 

 eliminated. Light is therefore absolutely necessary to the 

 de-oxydizing process, which varies in rapidity with the dif- 

 ferent degrees of illumination, and ceases altogether when 

 this influence is withdrawn, oxydation taking place during 

 the night, j Therefore the cells in the interior of the plant, 

 in proportion as they are withdrawn from the solar in- 

 fluence by intervening layers of tissue, must necessarily 

 become more and more absorbent of the oxygen, which is 

 communicated to them through the air-tubes from the leaf- 

 laboratory, and the air in those tubes must also become im- 

 pregnated with carbonic acid in proportion to the amount 

 of tissue which they have traversed in the dark portions 

 of the interior of the plant. 



The process of de-oxydation or digestion in the leaves, has 

 masked and disguised the true respiratory process in the in- 

 terior of plants, which is carried on by the pneumatic vessels 

 of their fibro-vascular system. In what does this differ from 

 the process of respiration in animals ? The principle is pre- 

 cisely the same. There is the same oxygenation of the nutri- 

 ent fluid in the interior of both organisms ; but the sap is 

 oxygenated in the plant by a system of pneumatic vessels 

 which of5ciate as conducts of oxygen from the leaves, and 

 bring it into direct communication with the sap-cells in all 

 parts of the vegetable fabric ; whereas in animals, the pneu- 

 matic vessels, which oxygenate the blood, are confined to 



