34 PLANT RELATIONS. 



of oxygen and the giving out of carbon dioxide. It will be 

 noted at once that this is exactly the reverse of what takes 

 place in photosynthesis. During the day, therefore, carbon 

 dioxide and oxygen are both being absorbed and evolved. 

 It will also be noted that the taking in of oxygen and the 

 giving out of carbon dioxide is just the sort of exchange 

 which takes place in our own respiration. In fact this pro- 

 cess is also called respiration in plants. It does not depend 

 upon light, for it goes on in the dark. It does not depend 

 upon chlorophyll, for it goes on in plants and parts of plants 

 which are not green. It is not peculiar to leaves, but goes 

 on in every living part of the plant. A process which goes 

 on without interruption in all living plants and animals 

 must be very closely related to their living. We conclude, 

 therefore, that while photosynthesis is peculiar to green 

 plants, and only takes place in them when light is present, 

 respiration is necessary to all plants in all conditions, and 

 that when it ceases life must soon cease. The fact is, 

 respiration supplies the energy which enables the living 

 substance to work. 



Once it was thought that plants differ from animals 

 in the fact that plants absorb carbon dioxide and give off 

 oxygen, while animals absorb oxygen and give off carbon 

 dioxide. It is seen now that there is no such difference, 

 but that respiration (absorption of oxygen and evolution of 

 carbon dioxide) is common to both plants and animals. 

 The difference is that green plants have the added work of 

 photosynthesis. 



We must also think of the foliage leaf, therefore, as a 

 respiring organ, because very much of such work is done 

 by it, but it must be remembered that respiration is going 

 on in every living part of the plant. 



This by no means completes the list of functions that 

 might be made out for foliage leaves, but it serves to indi- 

 cate both their peculiar work (photosynthesis) and the fact 

 that they are doing other kinds of work as well. 



