104 THE NUTRITIVE FUNCTIONS. 



CHAPTEE VI. 



ON THE NUTRITIVE PROCESSES OP RESPIRATION AND 

 ASSIMILATION. 



The nutritive fluid wHch circulates through the tissues 

 of plants and animals, requires to be constantly brought 

 into immediate contact with the oxygen of the atmosphere, 

 which is the principal agent in efiecting those important 

 changes in its constituents, by means of which it is ren- 

 dered capable of ministering to their nutrition and deve- 

 lopment. For this purpose plants have been provided with 

 leaves and anjmals with lungs. 



The sap of plants is aerated by means of their vasiform 

 and tubular tissues, which communicate with the atmos- 

 phere through the stomata or openings in the epidermis of 

 their herbaceous and green parts. 



These vessels, contain sap during the first period of 

 vegetable activity in early spring, which is gradually 

 displaced by air. This fact is easily verified, by cutting 

 under water a young green shoot which has put forth its 

 leaves ; little bubbles of air will be seen to issue from the 

 orifices of these tubes. 



Respiration consists essentially in the' evolution of car- 

 bonic- acid and the absorption of oxygen. The process is 

 performed both by animals and plants, and has its origin in 

 the same general requirements. 



The first source of the demand for oxygen, common alike 

 to plants and animals, arises out of those changes which are 

 always going on in their interior, as a part of their nutrient 



