In this spring time of the year, the phenomenon of autumn colors is perhaps very far from our thoughts. As life begins anew, we are not too anxious to contemplate that wonder of plant life that awes us in the fall. The daffodils and crocuses (crocus’?) have already burst forth into yellow and purple; the grass has more than overcome the gray countryside with a rich carpet of green.
For the sake of this study, let’s turn our thoughts back to last fall. At one time or another, every one of us has taken a color tour and marveled at the variety and beauty all around.
It is the sugar maple that sets the standard for summer green tree color. It is not dark blue-green like the elms, nor yellow-green like the poplars, nor gray-green like the willow. Yet, about the tenth of October this dependable green will vanish. In its place, the same oval, composed of the same leaves, will appear flaming orange and yellow. In moister ground, where we can find the red maples, the change will be to bright crimson. All neatly in a row, a line of birches will become as yellow as butter, while an ash at some distance will turn as purple as prunes. The change comes as swift as a new window display and much more soundlessly and smoothly than the change of stage settings.
The key to this miracle is a tiny band of cells at the base of the stem of each leaf, where that stem is attached to the twig. As summer draws to a close, these special cells – and no others in the leaf system – begin to loosen and dry out. In just a short time they become so brittle that the leaf will break off and fall to the ground. Before this actually happens, the tree prepares to heal the scar where this break will occur by converting other cells just below these brittle cells into a tough corky tissue. The toughness of these cells interferes with the plumbing system in the leaf stem by stopping the pipelines that bring the sap into the leaf.
Thus, the supply of life-giving sap is shut off from the leaf a couple of weeks before the leaf is ready to fall; these are the two weeks of the fall foliage.
The procession of color begins with the little things like the strawberries and blackberries and various herbs. Little plants have autumn colors like the big ones. As the vanguard of the trees, comes the red maple; after them the sugar maples, elms, and ashes; and last of all, the hickories and birches.
As soon as a leaf has its supply of fresh sap cut off by this plugging of its pipelines, photosynthesis stops. This means that the leaves are no longer capable of manufacturing food and are no longer useful to the organism of the tree. Chlorophyll, which is so indispensable to this process, is an unstable chemical that must be constantly renewed to exist. Therefore, the green chlorophyll, isolated in the leaf, is destroyed by the sun’s rays and disappears. In this way, green color which is dominant in the leaf, is removed. What is left becomes a vivid demonstration of color chemistry.
One pigment which comes into its own when the chlorophyll disappears, is carotin. Carotin (the color of carrots) was in the leaf all the time, floating around in the cell sap in little specks. But, despite the fact that these carotine plastids are bright yellow or orange, they are masked by the powerful green of chlorophyll all summer long. This same pigment makes butter yellow after the chlorophyll in the grass has disappeared in the cow’s milk factory. Carotin is also the pigment of egg yolk. The fact that it survives all through the process of food making is proof of its stability. Sugar maple leaves are rich in the orange and chrome hues of carotin. Birches have pure yellow carotin.
What about the autumn crimson of red maples and Virginia creeper, the scarlet of oaks, the dark red of sassafras leaves, the plum coloring of the ashes? These red and bluish tones are caused by a sugar chemical in the cell sap. It is not a pigment at all, but rather a sugar in solution. It colors sap just as a drop of iodine colors water. If the sap of the tree in question is relatively alkaline, it turns blue or purple. Whether sap is acid or alkaline depends on the chemical make-up of the tree and not too much on the soil. The red maples are acid, so they turn red; ash trees are relatively alkaline, so they turn plum color.
Carotin and sugar still does not account for the brown colors which are so prevalent among oaks. When yellow or orange is toned down, it appears to our eyes as brown. After the yellow or orange of the leaves is exposed for some days, it loses its intensity and is keyed down to brown. Another cause of the brown color is a substance found in the food cells, a powerful astringent, called tannin. This acid stiffens protein in animal hides so that they become insoluble and incidentally turn to a rich tan color. This is the process of tanning hides. The same thing may happen in leaves. One of the most beautiful and distinctive effects in the fall is contributed by the American beech leaves. They turn to a duotone, tan streaked with yellow along the veins. Because of the wonderful variety in the chemical make-up of trees and plants, each species turns into its own distinctive color and we can often identify them by their hues in the fall.
It is interesting to note the reason that some trees do not contribute any new vivid colors to the landscape of fall foliage. For example, black walnut and butternut trees drop their leaves so swiftly when the brittle cells are formed that their branches are bare when other trees are flashing with colors. The locusts retain chlorophyll green until the leaves drop off. This implies that the corky cells which heal the scar may not cut off the supply of sap to the leaves. To verify this, examine the leaf scar of a locust in the winter, or any time after the leaves have fallen. You will find, instead of a clean scar, an irregular and broken callus, with a little hole into the bark, where the buds are buried.
Autumn colors and dropping leaves are part of the same phenomenon. To strip a tree completely of its leaves is a device of nature to preserve the life of that tree. Those handsome broad leaves are pouring out water in the form of invisible vapor into the air. This water is lifted out of the ground. But the supply of available water in the earth is curtailed when the ground freezes in the winter. Therefore, the leaves must be “shut off” before winter or they will dissipate vital sap when it cannot be replaced from the roots. The most efficient way to shut off the leaves is to knock them off. Nature does this by forming those brittle cells with which their adjacent corky cells plug up the sap system and cause the spectacle of fall foliage.
The brilliance of the fall foliage may be enhanced by the weather conditions. The intensity of light has much to do with the intensity of the red and purple of the sugar chemical. For this reason, the brightest colors are observed when the fall season is bright and clear. If there is an excess of overcast weather, the yellow and orange of carotin are not affected. They glow on moist and misty days with exquisite rich pastel hues. On the other hand, the reds and purples of the brighter varieties are dulled by cloudy weather.
Autumn colors do not depend on the frost. Some imaginative scientists are of the opinion that in some prehistoric age when the habits of the broad-leafed trees were being formed, the frost had a hand in the process. But we know better; God has given the seasons to man; spring time and harvest. Nor should we imagine that today this process is automatic; our heavenly Father, who numbers the hairs of our head, also exercises his paternal and creative power over the ever-changing trees. The perfect timing throughout this fascinating process is not a haphazard thing; but all-wise and purposeful.