86 THE OAK 
other living parts of the cell—e.g. the colourless proto- 
plasm and nucleus. They obtain these from the cell-sap, 
through the agency of the colourless protoplasm in which 
they reside. 
In order that they may perform their functions 
properly, however, it is essential that they be exposed 
to light; this is effected by their being in cells which 
are disposed in thin layers, such as we have seen the 
mesophyll of the leaf to be. In fact, the flat, thin, 
expanded form of the leaf is a direct adaptation to the 
end that these chlorophyll corpuscles shall be properly 
illuminated by the sunlight; moreover, the large in- 
tercellular passages which communicate by thousands 
of stomata with the atmosphere ensure their being 
thoroughly aérated. In addition to allowing the free 
access of the oxygen of the air, moreover, these inter- 
cellular passages admit of the small quantities of carbon 
dioxide in the atmosphere also reaching the chlorophyll 
corpuscles. Oxygen and carbon dioxide, therefore, are 
found dissolved with the other materials in the cell- 
sap which saturates the protoplasm and reaches the 
chlorophyll corpuscles. 
These facts premised, we are in a position to follow 
generally the astounding transformations which go on 
in these millions of chlorophyll corpuscles in the oak- 
leaf. Carbon dioxide and water exist side by side in 
the protoplasm of the chlorophyll corpuscle, and rays of 
sunlight—.e. energetic vibrations of the ether which 
pervades the universe—penetrate into the system. By 
