Leaves & the Secret of Life

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But chlorophyll is remarkable not just for its color o- appetite for light. The real magic is its ability to use the light energy it and the other pigments absorb, as well as use carbon dioxide and water from the air and soil, to power a series of chemical reactions that ultimately converts solar energy to chemical, consumable and edible forms: glucose sugar, starches and other high-energy carbohydrates. These substances are piped through the leaf and into the tree's tissues, where they're used as food to help the tree grow.

At the same time, for every molecule of carbon dioxide inhaled by a leaf for photosynthesis, a molecule of pure oxygen is exhaled, breathing new life into our atmosphere.

Leaves release excess water as vapor through their stomata. Eventually that water will vaporize again—and again, and again, and again -gather in a cloud, fall to Earth and make the journey through a plant's tissues to its life-sustaining leaves.

Only green plants are capable of this photosynthetic feat, converting the sun's energy into food. Animals, insects and all other living creatures on Earth either must eat green plants or eat organisms that eat green plants, to obtain the stored energy necessary for survival. Each of us lives on a diet of sunbeams, delivered to us by leaves. Without the alchemy that each green leaf performs from sunup to Sundown during the growing season, life simply could not he.

COMES THE FALL

Fall's show actually begins, though in green disguise, on the first day of summer, June 22, the longest day of the year. With each ensuing clay the sun takes a lower, more southerly path across the sky.

The steadily diminishing daylight triggers all manner of winter-preparation activities in nature. Birds fly south to find warmer weather and a reliable food supply. Mammals add fat and fur to stave off the coming cold. And trees shed their leaves to keep from dying of thirst.

Deciduous trees lose vast amounts of water by evaporation through their soft, porous leaves. In a single summer, a typical beech tree sprays nearly 4,600 gallons of vaporized water through its 200,000 leaves.

In winter, tree roots can't easily draw new water in from cold or frozen soil to replace lost water. This is no problem for conifers, whose narrow, waxy needles give up relatively little moisture. But a fully leafed, vapor-breathing deciduous tree would quickly use up all the water stored in its tissues. The foliage would catch snow and ice, too, breaking branches with the weight.

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