30 PLANT STUDIES 
bubbles in a test tube, as in an ordinary chemical experi- 
ment for collecting gas over water, and testing it in the 
usual way. 
Some very important things are learned by this experi- 
ment. It is evident that some process is going on within 
the leaf which needs light and which results in giving off 
oxygen. It is further evident that as oxygen is eliminated, 
the process indicated is dealing with substances which 
contain more oxygen than is needed. The amount of 
oxygen given off may be taken as the measure of the work. 
The more oxygen, the more work ; and, as we have observed, 
the more light, the more oxygen; and no light, no oxygen. 
Therefore, light must be essential to the work of which the 
elimination of oxygen is an external indication. That this 
process, whatever it may be, is so essentially related to 
light, suggests the idea that it is the special process which 
demands that the leaf shall be a light-related organ. If so, 
it is a dominating kind of work, as 'it chiefly determines 
the life-relations of foliage leaves. 
The process thus indicated is known as photosynthesis, 
and the name suggests that it has to do with the arrange- 
ment of material with the help of light. It is really a pro- 
cess of food manufacture, by which raw materials are made 
into plant food. This process is an exceedingly important 
one, for upon it depend the lives of all plants and animals. 
The foliage leaves may be considered, therefore, as special 
organs of photosynthesis. They are special organs, not ex- 
clusive organs, for any green tissue, whether on stem or fruit 
or any part of the plant body, may do the same work. It 
is at once apparent, also, that during the night the process 
of photosynthesis is not going on, and therefore during the 
night oxygen is not being given off. 
Another part of this process is not so easily observed, but 
is so closely related to the elimination of oxygen that it 
must be mentioned. Carbon dioxide occurs in the air to 
which the foliage leaves are exposed. It is given off from 
