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the cells. It has been calculated that an oak-tree may have 700,000 leaves, and that 111,225 kilogrammes of water may pass off from its surface in the five months from June to October, and that 226 times its own weight of water may pass through it in a year.

Now comes the question, What are the salts needed for that so much mechanism should be expended on their accumulation? To answer this, we must look at the mesophyll cells a little more closely. Each of these consists of a thin cellulose cell-wall, lined with colorless protoplasm, which incloses a large sap-cavity (vacuole); in the protoplasm are imbedded a number of bright-green, rounded chlorophyll corpuscles, a relatively large nucleus, and a few less conspicuous granules, etc. The cell-sap contains various substances dissolved in water. Some of these substances are salts and other materials ready to be made use of; others are, so to speak, waste products or worked-up materials that are going to be got rid of, or sent to places where they will be made use of, respectively.

In the colorless protoplasm which lines the interior of the cell-wall and surrounds the cell-sap we find a nucleus and the chlorophyll corpuscles, as said, and a few words must be devoted to the latter. Each chlorophyll corpuscle consists of a rounded mass of proto-plasmic substance of somewhat spongy texture, containing the peculiar green body, chlorophyll, imbedded in it as in a matrix. These chlorophyll corpuscles are living organs, and they require food materials—water, oxy-