The Fall Extravaganza As Leaves Change Color

The word `deciduous` comes from Latin, meaning "to fall off." What is it that makes a trees leaves change color, and eventually to fall off? Although the leaves do it, the trees themselves start it, but they are doing this in order to survive. However it comes about, the performance dazzles the eye and stirs the heart of those who see it. And even as the extravaganza reaches its climax, next year's performance is waiting in the wings. When cells divide, they're growing. As the ratios of these specialized plant hormones change, trees stop making the famous energy-producing chemical chlorophyll. They pull in all the nutrients they can from the leaves, and cut the leaves off from their main stems.

In September/October in the Northern Hemisphere, the curtain opens on the show, quietly and without fanfare. A tiny band of cells where the leaf's stem is attached to the twig begins to loosen and dry out. Between these cells and the twig, a layer of corklike cells begins growing. It is scar tissue forming even before the amputation of the leaf takes place.

It is the season for brightly lighted days and cool, crisp nights-requirements for the colorful extravaganza that is to follow.

                                             

As the layer of corky cells toughens, the tiny pipelines that bring sap to the leaves are stopped up. All the while the other layer of cells is continuing to loosen and dry out. The flow of sap to the leaves has been cut off, but it is still weeks before they will fall. These are the days of autumn's blaze of glory. Without sap, photosynthesis in the leaves stops and the green chlorophyll in the leaves is destroyed by the sun's rays.

With the passing of the green, pigments that have been in the leaf all summer now take the limelight. Outstanding is carotene. Sugar-maple leaves have orange and chrome hues of carotene. Birches have pure yellow carotene. As nights lengthen in the fall, the chlorophyll that was used to convert sunlight to food isn't produced anymore. It breaks down, and the other colors show through.

But what about the crimson of the red maple, the scarlet of the oak, the deep red of the sassafras, the plum color of the ash? Those colors are newcomers to the leaves. Only after the corky layer has cut off the flow of sap from the leaf do these dynamic colors usher in their dramatic finale to the fall show. If the weather is cool and bright, the leaf continues for a time to make sugar, which is now trapped in the leaf and is turned into a chemical called anthocyanin. If the sap is acid, the anthocyanins turn red; if alkaline, they turn blue or purple.

It isn`t the approaching cold of winter that causes the leaves to fall. The tree itself does it to conserve its water. Cold air cannot hold as many water molecules per cubic meter or cubic foot as warm air. During winter, very little is available from a frozen earth, and the broad leaves of deciduous trees give off large amounts of it. Without new supplies of water, these leaves would soon dehydrate the tree. So to forestall this, the tree sheds its leaves and seals the open wound with a layer of corky scar tissue.

The tree must retain its water, or the show will not go on next year. There would be no spring green, no summer shade, and no fall foliage to dazzle eyes and stir hearts. The buds of spring that burst open and send out green shoots are not newcomers. They have been there all year, waiting in the wings for warm sunshine to thaw their plumbing and start the sap flowing. Now they grow rapidly, getting the major share of available food.

But at the same time tiny buds no bigger than the head of a pin are being formed, packed with leaves, flowers, twigs, and stems. Only by midsummer, however, do these tiny buds get the food they need to grow larger and develop further. By the end of summer, they contain next spring's leaves and flowers, stems and twigs, all tightly packed inside waterproof wrappings. Protected from drying and freezing, they wait without stirring for months, waiting for spring. In this state of suspended animation, they are called winter tree buds.

                                                       

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