200 PIIYTOTOMY. 



plants consume as much as they give out when they art in 

 the shade. 



It has been observed, that a smaller quantity of carbonic 

 acid is always given out in the shade, than the quantity of 

 oxygen absorbed ; and, also, that tlie (quantity of the latter in- 

 creases when branches or leaves are cut off. Both circum- 

 stances seem to shew, that oxygen is not merely concerned 

 in the formation of carbonic acid, but is also appropriated as 

 a part of the plant : and the more so, because, according to 

 the latest experiments, juicy plants, and those with fleshy 

 leaves, consume the greatest quantity of oxygen, and form 

 the smallest product of carbonic acid. 



315. 



Which of the two surfaces of the leaves performs these 

 functions, or how they are divided between the two, is not 

 yet completely ascertained. Most experiments favour the 

 idea that the upper surface of the leaves is especially employ- 

 ed in exhalation. This surface is also the better adapted 

 for this purpose, as it is better exposed to the light of the 

 sun, and can give out the oxygen through the closed parti- 

 tions of its cells, just as easily as, in perfect animals, this same 

 substance forms a communication for itself through the shut 

 vesicles of the lungs, and the equally impervious partitions of 

 the other vessels. 



316. 



The exhalation of oxygen gas, during sunshine, is an ef- 

 fect of several conspiring circumstances. One necessary in- 

 ternal condition is the vital activity of the plant, which being 

 excited by the light of the sun, decomposes the carbonic acid 

 watei" in such a way, that while the oxygen is given out, car- 

 bon and hydrogen are fixed and become appropriated. The 

 exhaled oxygen gas is by no means derived from the decom- 

 position of water into its constituent partsj because, some- 

 times, the quantity of the oxygen corresponds exactly with 

 the quantity of carbonic acid which has been consumed ; and 

 because, sometimes, no oxygen gas is produced, when water, 



