34 PLANT STUDIES 
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 may also call the foliage leaf, therefore, an organ cf 
respiration, because so 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 
indicate both their peculiar work (photosynthesis) and 
the fact that they are doing other kinds of work as well. 
