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 as 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 



