Posted by
lazyguy
431 days ago
(http://spotlight.encarta.msn.com)
Category: Science Discovery Environment |
Deciduous comes from Latin words meaning "to fall or cut." When you "decide" something, you come down on one side of a question, cutting off the other. In describing plants that decide to lose their leaves by sensing the length of nights, we call it photoperiodism--from Greek words for a light/time system.
As we all notice, even during a warm autumn, sooner or later it gets cooler. In New England, where fall colors are famous, the air gets colder--often much colder. Along with cold air comes dry air. Cold air cannot hold as many water molecules per cubic meter or cubic foot as warm air. You've probably come across food from time to time that's been "freezer burned." It got dried out in the cold. So, deciduous trees drop their leaves as a means to keep moisture inside--to keep from drying out.
The evergreen alternative
Trees go to all the trouble to construct big, thin, high-efficiency, sunlight-gathering leaves, and then in the fall, they cut them loose. It seems like it would lead to a big management problem--a huge budget of energy and chemistry. Yet there they grow. It's fascinating to compare the alternate approach in the other style of large plant--evergreen trees. We call their leaves needles, and they're coated with wax to keep the water in all year. With long, narrow, needle-like leaf designs, they can conserve water, but they can't gather as much sunlight in each needle, or leaf.
So, trees have needles or broad leaves based on a tradeoff: the need to gather sunlight as efficiently as possible, all the while keeping enough moisture inside during both the moist-air summer and the dry-air winter.
Closing up shop
The process of a deciduous tree closing up its food-making shop for the winter is chemically complicated. When the days get darker, plants have complex biochemistry that sets a series of events in motion. As the amount of light per day goes down, photoperiodic plants produce ethylene and a substance called abscisic acid. That leads to a change in the ratio of the hormones auxin and cytokinin. Scientists named these compounds by their functions. Auxin is the hormone that makes cells elongate or grow, similar to the word augment. Cytokinin comes from words that mean "put division in motion." 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.
The corky, tough area you've probably observed at the base of a fallen leaf is the "zone of abscission." Abscisic acid? Abscission? Hmm. Tree cells are abandoned; they leave behind suberin (from the Latin for cork). It's a conversion of living cells into protective armor. In the winter, you can tell a deciduous tree's species just by examining its leaf scars.
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. Leaves become spectacular because photosynthesis requires not only green chlorophyll, but also chemical interactions with a few other specialized pigments. There's xanthophyll (xantho is Greek for yellow). There's carotene--orange or red like a carrot. Then there are terpenes, the chemicals that generally make up a plant's essential oils which give plants their smell, their "flavor."
Last call
If there are a few warm, sunny days in the fall, trees are able to gather the last few nutrients and bring them into their branches before fully abandoning the leaves. This process is apparently facilitated by anthocyanin, which not all trees can produce. It means "purple flower" in Greek. It's the chemical that gives a red or purplish color to leaves (and, incidentally, to red wine).
On the other hand, if there is a string of cloudy days, not enough anthocyanin is made to produce bright red fall colors. Along the same line, if the weather gets cold quickly, trees often abandon the leaves in place without cutting them off. The zones of abscission aren't fully formed. Leaves stay attached to the trees, and the dark brown pigments are left behind. There may be an evolutionary advantage for trees in this situation. Leaving some leaves as a bit of nourishment for animals caught in the cold may protect trees from these hungry cold-weather snackers. Foraging animals might go for the easily chewed dead leaves instead of chomping on a tree's still-moist hard, woody branches.
Fall is a chance to observe the problem-solving systems of evolution. For plants everywhere, water is a main ingredient in food production and their prime solvent for chemical transport. It's essential. Plants on land need to have especially good water management. Evergreen trees retain moisture with compact needle-shaped leaves. Deciduous trees developed a way to make hay--well, plant cells and plant sugars--only while the air is moist. Then these big, strong wooden plants found efficiency in abscising (cutting off) their leafy sunlight-gathering systems as a way to conserve. And is it ever beautiful!
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