106 THE NUTRITIVE FUNCTIONS. 



their own organism or that of animals, until it is first oxy- 

 dized. 



Now the sap is digested or de-oxydated in the leaves of 

 plants, that is to say, the carbonic acid of the sap is decom- 

 posed under the influence of light, and all the superfluous 

 oxygen is evolved into the atmosphere. The remainder 

 of the oxygen, required for nutrition, is drawn through 

 the vasiform tissue and ducts into the interior of the plant. 

 The sap in all parts of the plant is thus brought into com- 

 munication with the gaseous fluids necessary to its elabora- 

 tion, and those various organic products developed, in all 

 of which oxygen enters into composition as an element. 

 These pneumatic vessels are most admirably situated, for 

 this purpose, in the midst of the woody tissues of plants, 

 through which the sap continues to flow until their vital 

 activity ceases. 



The sap is therefore first digested or de-oxydated in the 

 leaves of plants, and the food is changed first to chyme and 

 then to chyle in the stomach and intestines of animals, res- 

 piration or oxydation giving the finishing change to the sap 

 in the interior of the plant, and rendering the chyle identi- 

 cal with the blood. Respiration is absolutely essential to 

 the growth of plants as well as of animals. 



Dutrochet ascertained by experiments, that the air con- 

 tained in the ducts and spiral vessels in the interior of the 

 stem, was altered in its composition, in proportion to its 

 distance from its original source of supply in the leaves. 

 He found that the air in these vessels was gradually de- 

 prived of its oxygen, which was absorbed by the sap in the 

 neighboring vessels. 



That the interior sap-cells in the neighborhood of the 

 air-tubes absorb oxygen and give out carbonic acid, is evi- 

 dent from what we know of the economy of those opposite 



