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 



