26 GENERAL BOTANY 



Now caustic potash absorbs carbon dioxide very 

 easily, but not air, so that the rise of water in A 

 must mean that the air in the tube is being changed 

 into carbon dioxide, that is to say, the seeds or buds 

 are absorbing the active part of the air, oxygen, and 

 giving out carbon dioxide. But since only one-fifth 

 of the air is oxygen, and the rest practically all the 

 very inert gas nitrogen, only one-fifth of the air in 

 the tube can be changed to carbon dioxide and there- 

 fore the water does not rise further. In the other 

 tube, B, the level of the water does not change much, 

 which means that the volume of the air and of CO^ 

 in the tube remains about the same, that is to say, 

 the amount of CC>2 given out is about the same as the 

 amount of oxygen absorbed. This action goes on 

 wherever there is actively living substance, but only 

 where it is living, as shown by the fact that there 

 is no change in the tube C. It does not go on, for 

 instance, in the centre of a large tree-trunk, for the 

 wood there is not alive but dead. It is this dead 

 heart-wood, often coloured red or brown, which alone 

 is of any use as timber. But everywhere else in 

 the plant where there is actual life, there respiration 

 goes on. 



The COs which is formed makes its way along 

 minute crevices and passages, not tubes, in the tissues, 

 till it gets to the outer surface, either through special 

 cracks in the bark of the smaller branches, or more 

 generally through the stomas. 



From the roots it also passes out through the root- 

 hairs, and, dissolved in the water of the soil, helps 

 in the natural breaking up of the mineral substances. 



