286 VEGETABLE PHYSIOLOGY 
oxygen taken in. It is always accompanied or followed 
by the formation of a certain amount of watery vapour. 
The universality of this process is not always easy to 
demonstrate. It can be ascertained without difficulty in 
the case of almost all animal organisms, and of such of 
the vegetable ones as possess no chlorophyll. In the case 
of those plants which are green, however, there is, as we 
have seen in a preceding chapter, a converse gaseous 
interchange occurring so long as the green parts are 
exposed to sunlight, carbon dioxide being absorbed and 
decomposed, and an equal amount of oxygen exhaled. 
This interchange is usually more vigorous than the first 
one, and the latter is therefore difficult of detection under 
conditions which allow both to take place simultaneously. 
Ths absorption of oxygen can be easily observed in the 
case of a large fungus, such as a mushroom. If one of 
these plants be placed in a closed receiver containing air, 
and left there for several hours, at the conclusion of the 
experiment the mixture of gases in the receiver will be 
found to be almost devoid of oxygen, that which was there 
orginally having disappeared. An almost equal amount 
of carbon dioxide will be found to have replaced it, so that 
the volume of gas in the receiver will be unaltered. 
It is possible to devise an experiment which will show 
that a green plant has the same absorbing power. If the 
light is excluded from one placed in a similar vessel, 
no evolution of oxygen will take place from it, and that 
the oxygen present in the air at the commencement 
of the observation will diminish to the point of extinc- 
tion can be made evident, just as in the case of the mush- 
room. 
We have evidence, however, that this is not caused 
by the exclusion of the light, but that the gaseous inter- 
change in question proceeds in the light as well as in 
darkness. An apparatus which was originally devised by 
Garreau, and which can be easily arranged to show the 
absorption of oxygen, even when a green plant is exposed 
