82 - VEGETABLE RESPIRATION. 
animal species, the evolution, namely, from the human — 
lungs, of carbonic acid during the night, and oxygen during — 
the day. 
279. It has been supposed that the expulsion of carbon 
may be for the purpose of relieving the plant from an excess — 
of this substance ; and if it be allowed that the nature of the 
action which goes on in the leaf be determined, not by the 
demands or condition of the plant, but by the presence or 
absence of an external agent, light, it is by no means impro- 
bable that there may frequently be a necessity for expelling 
a superfluous quantity of carbon. And this leads us to ob- 
serve, that, a priori, we should be inclined to doubt that so 
important a part of the functions of the plant should depend 
solely on the influence of an external agent, and be attended — 
with such opposite results according to its presence or ab- : 
sence. 
280. This would be a case to which we have nothing ana- 
logous in the animate works of nature. | We would have 
plants of the same species, varying in a most important 
function, producing results diametrically opposite, accord- 
Britain, the plants must give ‘out perhaps many times a$ 
much carbon as oxygen, while in tropical climes they must 
be almost constantly giving out oxygen. ; 
281. It must be observed, also, that there are other dif- 
ferences between the nocturnal condition of plants and their 
condition in the day-time, besides the presence and absence 
of light. During the day the sap is flowing freely and rapid- 
ly through the plant; large quantities pass through the 
___ leaves, and are modified by the great evaporation which takes 
_ consequently a much smaller quantity of the 
a the leaves. The pores are closed or 
