292 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 inter- 
change occurring so long as the green parts are exposed 
to sunlight, carbon dioxide being absorbed and decom- 
posed, 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. 
The 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 experi 
ment the mixture of gases in the receiver will be found to 
be almost devoid of oxygen, that which was there originally 
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 practically 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 extinction can 
be made evident, just as in the case of the mushroom. 
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 ag in dark- 
ness. An apparatus which was originally devised by 
Garreau can be easily arranged to show the absorption 
of oxygen, even when a green plant is exposed to a bright 
sunlight. There are many forms of it, but a convenient 
