gen, etc.—for the support of their life processes, just as do the other living parts of the cell—e.g., the colorless protoplasm and nucleus. They obtain these from the cell-sap, through the agency of the colorless 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 intercellular passages which communicate by thousands of stomata with the atmosphere insure their being thoroughly aerated. In addition to allowing the free access of the oxygen of the air, moreover, these intercellular 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—i.e., energetic vibrations of the ether which per-
