beginning is inorganic and the end organic compounds like sugar. Something is known also of the wear and tear on the chlorophyll, its waste products, and how it keeps itself not only fit but provides for its own constant renewal. One of the excess or by-products in this initial manufacture of sugar is oxygen. This is either used in other ways by the plant, or more generally it is thrown off through the stoma into the outer air. Oxygen, as one of the necessary constituents of the air that man breathes _in_, is thus thrown off, while, as we have seen, carbon dioxide, a poisonous gas which we breathe _out_, is a necessity for this manufacturing process in all green plants. Hardly any trick of nature so completely fulfills the wants of animal and plant life as this mutual exchange of by-products--in the case of animals it is the waste of respiration, in plants it is the wastage of sugar making and some other changes that go on in the plant just after this stage. The amount of sugar made, carbon dioxide taken in, and oxygen given off by this process suggests that while leaves may be very tiny factories they are among the most efficient in the world. Assuming an area of leaf surface equal to about a square yard the amount of sugar made would be about one-third of an ounce in a day or nearly three pounds in a single growing season. Carbon dioxide withdrawn from the air would average from the same area of leaf surface about two gallons a day or over three hundred gallons for the season. As an equal amount of oxygen is given off by the leaf, it becomes clear that as all of this interchange must go through the stoma the functioning of these and their guardians must be nearly one hundred per cent perfect. As we shall see a little later, they perform still other duties with even greater perfection. When we stop to reflect what an absurdly minute fraction one square yard of leaf surface is to the total leaf surface in the world, we come to some realization of the gigantic proportions of this process of manufacturing sugar and exchange of gases mutually useful to animals and plants. While in the United States most of the leaves fall in the autumn, the great bulk of the vegetation of the world holds the greater part of its leaves all the year, notably in the vast evergreen forests in the north, and of course practically all tropical vegetation. Chlorophyll in such places works continually and what the total of sugar production may be no man can even guess.