OF THE LEAVES. 753 



carbonic acid. The former function they regard as a process 

 of assimilation, and the latter as a kind of real (i.e. animal) 

 rcsjpiration. Mr. J. Broughtou has more recently demonstrated 

 a constant evolution of carbonic acid from nearly all parts of 

 growing plants, and considers that this gas, though partly due 

 to previous oxidation, is mainly separated from the proximate 

 principles during chemical changes. 



Those who maintain Burnett's views 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 ; while the supporters of Pepys' views 

 regard the exhalation of oxygen gas as 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 opinions of dif- 

 ferent physiologists, that various ideas 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 green 

 leaves, is very doubtful ; though it certainly occurs in other parts 

 of the plant which are not green. (See page 763.) 



Whatever \\ew& 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 it is remembered 

 that it is the only known process by which oxygen gas, — so 

 essential to our existence, and which is constantly being re- 

 moved 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 that are 

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

 condition. Thus we see that, " the two great organised king- 

 doms of nature are made to co-operate in the execution of the 

 same design ; each ministering to the other, and preserving that 

 due balance in the constitution of the atmosphere which adapts 

 it to the welfare and activity of every order of beings, and which 

 would Soon be destroyed were the operations of either of them 

 to be suspended. It is impossible to contemplate so special an 

 adjustment of opposite effects without admiring this beautiful 

 dispensation of Providence, extending over so vast a scale of 

 being, and demonstrating the unity of plan upon which the whole 

 system of organised creation has been densed." 

 3c 



