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It is quite commonly supposed that the coloring of autumn leaves and their fall is due to frost. It is true that leaves may fall a little earlier than usual if there are early frosts, or the weather has been unusually dry, but you can see by consulting your Tree Diary from year to year that the leaves fall about the same time, regardless of wind or weather. The fact is the leaves "deliberately" prepare themselves for the return to Mother Earth, much as you may some day get ready for a winter journey back to the old home place by fastening down all your windows, turning off the water, and then stepping outside and locking the front door! Let me explain: In the leaves, as in little kitchens, is prepared the food for the entire tree. The "little people," the cells, who do this work, pass on the food to the tree, only keeping back enough to supply their own modest needs. The leaves may be said to be "closing the windows" when, with the approach of winter, they gradually stop absorbing the sunlight; for, near the upper surface of the leaf are certain cells, standing side by side like long rows of tall windows, and their special function is to let in the sunlight through the green curtains for the work of the household. While the "windows" are being closed the water-supply from the "mains" -- the innumerable little bundles of pipes that carry the water from the roots to all parts of the tree -- is being gradually lowered and finally entirely shut off, at the same time the "front door" is being closed and locked. This is how the "doors" are closed: Quite a while before they fall the leaves grow a little wedge of cork between themselves and the parent tree, and before they finally drop, the openings in the twigs to which they are attached, and which would otherwise be left exposed to the invasion of injurious fungi, are carefully closed. The skin of the twig grows over them! |
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