STATING THE CASE. 



29 



open, and the atmospheric air is free to enter 

 therein, carrying with it the carbonic acid gas which 

 pollutes it. The air circulates in and out of the 

 loose cellular interspaces. As soon as the carbonic 

 acid it carries comes into contact with the chlorophyll 

 or " leaf- green," a magic chemical change ensues. 

 Carbonic acid is a molecule, or cluster of three 

 atoms, two of which are oxygen, and the single one 

 carbon. These are temporarily connected in a very 

 remarkable manner. But the chlorophyll has the 

 power of unpick- _ __ _ 



ing this cluster of 

 three atoms, so as 

 to get at the car- 

 bon it wants. The 

 carbon is seized 

 upon and ab- 

 sorbed, and the 

 oxygen atoms are 

 then turned loose 

 into the atmos- 

 phere again. That 

 very carbon atom 

 was perhaps once in our blood. It was set free at 

 the lungs, where it combined with the oxygen, and 

 thus became part of the carbonic acid. Now the same 

 carbon atom has been absorbed by the " leaf-green." 

 By and by it will be transformed into sugar, starch, 

 wood, or some other vegetable substance. 





Fig. 10. — Magnified Section of a Leaf, showing the 

 Cells within, full of ChloropJiyll, or Green-colour- 

 ing Matter. 



