186 



PHYSIOLOGICAL BOTANY. 



vegetable substances, are brought into a situation favour- 

 able for receiving a peculiar modification, which we 

 shall describe in the fifth period of nutrition. Of the 

 three elements more especially essential to the compo- 

 sition of all vegetable matter, we find that two of them, 

 the oxygen and hydrogen, may be furnished by the 

 water retained after the process of exhalation has been 

 completed. 



FOURTH PERIOD OP NUTRITION. 



(172.) Respiration. The first actual change pro- 

 duced in the sap is effected by a process analogous to 

 animal respiration. The air is inhaled by the leaf and 

 the fresh surfaces of other parts of the plant, and 

 its oxygen then unites with the carbonaceous matters 

 contained in the sap, and the result is the formation of 

 carbonic-acid. The greater part of this gas is then 

 held in solution by the sap ; and the whole or very nearly 

 all the azote which was separated from the oxygen, 

 is exhaled. Besides the carbonic acid thus formed by 

 the plant itself, the trifling proportion every where 

 found in the atmosphere is also inhaled ; and a still 

 larger quantity is introduced in the water absorbed 

 by the spongioles. Hence it appears that a threefold 

 provision is made for maintaining a supply of this ne- 

 cessary ingredient. So long as plants remain in the 

 dark, no fresh change takes place in this condition of 

 things ; the carbonic acid is retained, but is not fixed 

 in the form of an organic compound. This further 

 result requires the additional stimulus of light, and then 

 the decomposition of the carbonic acid is effected, the 

 carbon becomes fixed under the form of an organisable 

 compound, which we shall presently describe (art. 1 76.), 

 and all or nearly all the oxygen with which it was united, 

 is exhaled into the atmosphere. So long then as plants 

 continue to vegetate in the dark they tend to vitiate the 

 atmosphere by abstracting its oxygen, and also by the 



