While the leaves act as partial lungs they have two other most important functions. First, they must manufacture the food for the entire tree. "Starch factories" is the name that Uncle John gives to the leaves when he talks to children, and it is a good name. The leaf is the factory; the green pulp in the leaf cells is part of the machinery; the machinery is set in motion by sun-shine power instead of steam or water power; the raw materials are taken from the air and from the sap sent up from the roots; the first product is usually starch. Thus, it is well when we begin the study of our tree to notice that the leaves are so arranged as to gain all sunlight possible, for without sunlight the starch factories would be obliged to "shut down." It has been estimated that on a mature maple of vigorous growth there is exposed to the sun nearly a half acre of leaf surface. Our tree appears to us in an unfamiliar light when we think of it as a starch factory covering half an acre. Plants are the original starch factories. The manufactories that we build appropriate the starch that plants make from the raw materials. Starch is plant-food in a convenient form for storage; but as it cannot be assimilated by plants in this form it must be changed to sugar before it can be transported and used in building up plant tissues. Hence the leaves have to perform the office of a stomach in order to digest the food they have made for the use of the tree; they change the starch to sugar, and they take from the sap nitrogen, sulfur, phosphorus, and other substances which the roots have appropriated from the soil, and to these they add portions of the starch, and thus make the proteids which form another part of the diet of the tree. It is interesting to know that while these starch factories can operate only in the sunlight, the leaves can digest the food, transport it, and build up tissues in the dark. [Illustration: _Fig. 287. The bole of a sugar maple grown in a wood._] The autumn leaf, which is so beautiful, has completed its work. The green material which colors the pulp in the leaf cells is withdrawn, leaving there material which is useless, so far as the growing of the tree is concerned, but which glows gold and red, thereby making glad the eye that loves the varying tints in autumn foliage. It is a mistake to believe that the frost makes these brilliant colors: they are caused by the natural old age and death of the leaf, and where is there to be found old age and death more beautiful? When the leaf turns yellow or red it is making ready to depart from the tree; a thin corky layer is being developed between its petiole and the twig, and when this is finally accomplished the leaf drops from its own weight, from the touch of the lightest breeze, or from a frost on a cold night. OBSERVATIONS ON THE MAPLES. We want you to know the maples from actual observation.