CHAPTER XX 

 RESPIRATION 



In all the experiments previously described we have assumed 

 that the shoot of the plant is supplied with atmospheric air. 

 The green parts can take carbonic acid from the air. The free 

 nitrogen of the air can be dispensed with. The question now 

 arises " Does the plant require or use the free oxygen of the 

 air?" 



If we put a number of moistened germinating seeds, or young 

 flowers (not green),' into two bottles which we close up by 

 means of air-tight corks, after a lapse of some (say twenty- 

 four) hours changes have taken place in the air inside the 

 bottle. We open one bottle, pour in some lime-water and 

 shake it up. The lime-water becomes milky : thus showing 

 that there is more carbonic acid in the contained air than there 

 was at first. At the commencement of the experiment the 

 lime-water would not become milky. We open the other 

 bottle and thrust a lighted taper into it. The taper is at once 

 extinguished : a fact which proves that the free oxygen inside 

 the bottle has, wholly or partially, disappeared. These experi- 

 ments illustrate the fact that growing flowering plants consume 

 oxygen and give out carbonic acid. Another simple experi- 

 ment proves the same. Before closing the bottle containing the 

 seeds, we place in it a vessel containing a solution of potassic 

 hydrate ; we also fit a bent glass tube into the cork (as in fig. 

 246) and pour a coloured liquid into the tube. The liquid will 

 stand at the same level in both arms of the tube. We then fit 

 the cork into the bottle. The carbonic acid present in the air 

 of the bottle will be constantly absorbed by the potassic 

 hydrate. Now, if the seeds merely gave out this gas with- 

 out taking in any other gas from the air, the amount of air 

 inside the bottle would remain the same throughout the 

 experiment and the liquid in the two arms of the tube would 

 remain at the same level. (For, the decrease of the amount of 



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