26 PLANT LIFE ON THE FARM. 



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 car- 

 bonic acid is very minute, so that the leaves must be 

 very active in securing and utilising 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 colouring 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 sun light 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. 



The first result of this assimilation, chemists tell us, is 

 the formation of a soluble substance, "glucose," allied both 

 to starch and to sugar, and which, or a portion of which, 



