34 PLAiSTT RELATIONS. 



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

 noted at once that this is exactly the reverse of what take.s 

 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 aijd 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 respirafion 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. AVe 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. 



<)nc(! 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 ofE carbon 

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

 but that respiration (absorjDtion of oxygen and evolution of 

 carbon dioxide) is rommon to both plants and animals. 

 The difference is that green plants have the added work of 

 photosynthesis. 



We may also call the foliage leaf, therefore, an organ of 

 ir.yiiriifioii, because so mucli of such work is done l)y it, 

 but it must lie remembered that respiration is going on in 

 evei'y living joart of the plant. 



This by no inea.ns completes the list of functions that 

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

 indicate both their peculiar work (photosynthesis) and 

 the fact that they are doing other kinds of work as well. 



