RESPIRATION 



119 



cult 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 is placed in a closed receiver containing air, 

 and is 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 

 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 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 interchange in ques- 

 tion 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 to a bright sun- 

 light, is shown in fig. 82. It consists of a glass vessel 

 which can be closed by a cork through which a bent glass 

 tube of small calibre is passed. The tube is carried over and 

 made to dip into a small dish containing mercury. The 

 bottom of the vessel is covered with finely broken glass, 

 upon which is poured a strong solution of caustic potash. 



FIG. 82. APPARATUS TO SHOW 

 THE ABSORPTION OF OXYGEN 

 BY A GREEN PLANT. 



