30 PLANT LIFE ON THE FARM, 
in the form of carbonic acid gas—a combination of car- 
bon and oxygen—that it is found in the atmosphere, but 
only in small proportion compared with the other con- 
stituents. In the plant carbon exists in much larger 
proportion than any other ingredient, with the sole ex- 
ception of water. It forms, in fact, about fifty per cent 
of the dry matter of plants left behind after the water 
and gases have been expelled by heat. This large 
quantity of carbon has to be taken up in the form of 
carbonic acid by the leaves. It is a moot point whether 
any carbon is taken up by the roots, but, if any, it is 
only asmall proportion. In any given volume or quantity 
of air, the proportion of carbonic acid is very minute, so 
that the leaves must be very active in securing and util- 
izing all that comes within their reach. 
What Leaves do in the Light.—Direct experiments 
have shown that this appropriation of carbonic acid is 
effected by the agency of the green coloring matter or 
chlorophyll when exposed to the action of light. In the 
dark no such appropriation takes place. The plant feeds, 
so far as its carbon is concerned, on the carbonic acid of 
the air through the agency of sunlight and of chlorophyll. 
At least two-tlirds of the chlorophyll itself consists of 
carbon in association with a small proportion of oxygen 
and hydrogen, and a still smaller quantity of nitrogen. 
The carbonic acid thus introduced into the plant 
does not remain as such, but its constituent carbon is 
retained in the plant for its own purposes, while the 
oxygen gas is eliminated. The bubbles of gas that rise 
from a water weed in a pond when exposed to the sun 
consist of oxygen chiefly, and it has been shown that the 
amount of oxygen gas given off is about equal to that of 
the carbonic acid gas absorbed. Hydrogen and oxygen, 
the absorbed water, are, it is said, assimilated by the 
plant simultaneously with the carbon, 
