OP THE LEAVES. 733 



diametrically opposite results to that of animal respiration, upon 

 the atmosphere we breathe. Others, such as Mohl and Hen- 

 frey,say that here we have two distinct functions going on, — one, 

 taking place hy day, and consisting in the consumption of car- 

 bonic acid, with fixation of carbon and evolution of oxygen ; 

 and another, only occurring by night, in the leaves and other 

 green parts, but also by night and day, in those not green, and 

 which consists in the absorption of oxygen and evolution of 

 carbonic acid. The former function they regard as a process of 

 assimilation, and the latter as a kind of respiration. 



Those Avho maintain Burnett's views, also regard the constant 

 exhalation of carbonic acid by day and night, as constituting 

 vegetable respiration ; and the exhalation of oxygen by day, as 

 connected with assimilation 



The supporters of Pepys' views regard the exhalation of 

 oxygen gas as a vegetable respiration. Pepys says that oxygen 

 is given off by the leaves both by night and day, but in a greatly 

 accelerated degree during the day ; but by most observers no 

 evolution of oxygen has been traced at night. 



It will be seen from the above abstract of the views of diffe- 

 rent physiologists, that various opinions are entertained by them 

 as to the action of the leaves and other green organs under 

 different degrees of light ; and also upon the character of such 

 changes. Generally, it may be stated, — that all agree as to the 

 evolution of oxygen by the green parts of plants under the 

 influence of solar light, and that most authors call this vegetable 

 respiration ; that the evolution of carbonic acid by night is ex- 

 tremely small as compared with the opposite change by day, and 

 is altogether denied by some authors ; and that the constant 

 exhalation of carbonic acid by day and night, in healthy leaves, 

 is very doubtful. 



Whatever views we may entertain, all admit that this evolu- 

 tion of oxygen gas by day has a most important influence in 

 Nature. This will be at once evident when I state, that it is 

 the only knowm process by which the oxygen gas, which is so 

 essential to our existence, and which is constantly being removed 

 from the atmosphere we breathe, by the respiration of man and 

 other animals, by the process of combustion, by oxidation of 

 mineral matter, and by other processes which are constantly 

 going on upon the globe, — is restored to it in a free condition. 

 Suppose that plants had not this power of restoring oxygen to 

 the air by their respiration, it would necessarily happen that, the 

 proportion of oxygen in it would gradually diminish, and in its 

 place we should have a coiTesponding amount of the poisonous 

 carbonic acid gas, and the effect would be ultimately, that the 

 air we breathe would become unfit for animal life, precisely as 

 the air of a close room where chai-coal is burnedwould, from simi- 

 lar changes, destroy life. But such is not the case, for chemical 

 3 B 3 



