RESPIRATION. 107 
It must be added, however, that at the same time that leaves 
transpire, they also reciprocate and absorb water by their own 
surface. is function of absorption does not appear to bear any 
proportion to the number of stomates, but seems to be more or less 
considerable according to the quantity of wax which covers their 
surface, absorption being most active where this coating is thin. 
REsPIRATION. 
If we place an entire plant or a leafy branch in a balloon filled with 
gas which cannot be renewed, and leave the whole in darkness for 
some ten or fifteen hours, we may assure ourselves at the expiration 
of this time that the atmospheric air contained in the balloon is no 
longer of the same composition as before the experiment. Carbonic 
acid will be there in greater abundance, and the quantity of oxygen 
will be less. But if in place of leaving the plant in darkness we 
expose the apparatus to the influence of the sun’s rays, the 
phenomena will be reversed; after a few hours the air in the’ 
balloon will have lost a noticeable quantity of its carbonic acid, 
and will be enriched in its oxygen. 
In order to test this phenomena, let us fill a bell glass with 
water, to which has been previously added a considerable propor- 
tion of carbonic acid gas, and place in it a branch or an entire 
plant covered with leaves ; expose the whole to the rays of the sun 
for some hours, as is represented in Fig. 134. The air, if analysed 
after the experiment, will be found to contain scarcely any carbonic 
acid, but it will contain a larger portion of oxygen than before the 
experiment. Ifa branch of a plant, with the roots fixed in soil, 
and consequently in its normal state of vegetation, is placed in a 
glass vessel, and by means of an air-pump a given quantity of air 
is caused to circulate round it, this air, which, before the experi- 
ment, contained from four to five ten-thousandth parts of carbonic 
acid, after the apparatus has been exposed to the influence of the 
sun’s rays for a certain time will not be found to contain more 
than from one to two. If, on the contrary, the experiment is made 
during the night, it will be found that the quantity of carbonic 
acid would be increased, and at the expiration of a certain time 
