Every autumn across the Northern Hemisphere, Mother Nature seems to wield a magic paintbrush, and the leaves of our deciduous trees begin to change color in anticipation of being shed as the trees prepare for winter.
The leaves change from their customary lush green to an insurgence of fiery hues. The richness and beauty of the fall foliage in the northern United States far surpasses that of most other areas of the country, and any artist would consider him or herself fortunate to have on their palette the colors exhibited by our fall foliage.
The green pigment in leaves during the growing season is due to the complex compound named chlorophyll (C55 H70 MGN4 O6) that absorbs the red and blue parts of the spectrum of light given off by the sun. The green portion of the spectrum is not absorbed by chlorophyll but is reflected, giving the leaf its green appearance when a considerable amount of the compound is present.
Our sumacs are usually the first to display, radiating magnificent shades of magenta and crimson. The brilliant oranges and yellows of the maples soon follow, and the unique scarlet of the sour gum is a sight one will not soon forget. The deep yellow of the sassafras and the oaks with their deep red, red and green, and brown round out the basic color schemes.
Contrary to popular opinion, frost does not directly affect the change in leaf colors, though a hard frost does serve to deepen the colors. The internal workings of a leaf in autumn are primarily affected by the gradual onset of cold weather, less light in the atmosphere, and drier climatic conditions.
As a result of these factors, a constricting band of cells called the abscission layer forms at the base of the leaf stem where it is attached to the tree. Beneath the abscission layer, another layer forms that is similar to scar tissue and which covers the wound when the leaf eventually falls.
These layers of tissue serve to obstruct the circulatory systems of the plant, and moisture no longer flows into the leaf. When the leafs water supply is cut off, chlorophyll breaks down, and the process of photosynthesis ceases. Now is the time when leaves begin to change color.
Photosynthesis is the extremely complex process by which green plants trap the radiant energy of sunlight in molecules of chlorophyll and use it to form energy-rich carbohydrates and free oxygen gas from the raw materials of carbon dioxide in the atmosphere and water from the ground via conducting vessels to the leaves.
When chlorophyll breaks down (dissociates), the other pigments in the leaves, which were masked by the dominant green pigment, make their appearance. Hidden by chlorophyll are orange pigments called carotenes and xanthophylls that are responsible for the yellows and gold. A group of compounds called anthocyanins account for the flaming reds and purples. Brown pigments known as tannins give the leaves of some oaks and the sycamore their rich brown color.
When young children ask why leaves turn to various colors in autumn, it is probably best not to mention such things, as chlorophyll dissociation, abscission layers, xanthophylls, and carotenes. The fairy tale of Mother Nature instructing her handyman, Jack Frost, to mix up all of the colors and dart about the countryside dabbing the leaves is much better received. Or, perhaps, reading them the following written by Henry David Thoreau will assist in the explanation:
Far in the woods, these golden days,
Some leaf obeys its makers call;
And through their hollow aisles it plays,
With delicate touch the prelude to fall.
Dr. Robert Hedeen is a former resident of Marylands eastern shore and resided in the Chicago area from 1960-1971. He is a retired professor emeritus of biological sciences in the University of Maryland system.