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 a small 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-thirds 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. 



