arranged somewhat loosely, so that there are spaces between them through which air can freely pass.  Over this mass of cells there is a skin, or epidermis as it is called, the green surface of the leaf.  In this there are multitudes of minute openings, or breathing pores, through which air is admitted, and through which also water or watery vapor passes out into the surrounding atmosphere.  In the leaf of the white lily there are as many as 60,000 of these openings in every square inch of surface and in the apple leaf not fewer than 24,000.  These breathing pores, called stomates, are mostly on the under side of the leaf, except in the case of leaves which float upon the water.  There is a beautiful contrivance also in connection with these pores, by which they are closed when the air around is dry and the evaporation of the water from the leaves would be so rapid as to be harmful to the tree, and are opened when the surrounding atmosphere is moist.

The green color of the leaves is owing to the presence in the cells of minute green grains or granules, called chlorophyll, which means leaf-green, and these granules are indispensable to the carrying on of the important work which takes place in the leaves.  They are more numerous and also packed more closely together near the upper surface of the leaf than they are near the lower.  It is because of this that the upper surface is of a deeper green than the lower.

Such, then, is the laboratory of the leaf, the place where certain inorganic, lifeless substances such as water, lime, sulphur, potash, and phosphorus are transformed and converted into living and organic vegetable matter, and from which this is sent forth to build up every part of the tree from deepest root to topmost sprig.  It is in the leaves also that all the food of man and all other animals is prepared, for if any do not feed upon vegetable substances directly but upon flesh, that flesh nevertheless has been made only as vegetable food has been eaten to form it.  It is, as the Bible says, “The tree of the field is man’s life.”

But let us consider a little further the work of the leaves.  The tree is made up almost wholly of oxygen, hydrogen, and carbon.  It is easy to see where the oxygen and hydrogen are obtained, for they are the two elements which compose water, and that, we have seen, the roots are absorbing from the ground all the while and sending through the body of the tree into the leaves.  But where does the carbon come from?  A little examination will show.

The atmosphere is composed of several gases, mainly of oxygen and nitrogen.  Besides these, however, it contains a small portion of carbonic acid, that is, carbon chemically united with oxygen.  The carbonic acid is of no use to us directly, and in any but very minute quantities is harmful; but the carbon in it, if it can be separated from the oxygen, is just what the tree and every plant wants.  And now the work of separating the carbon from the oxygen