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Budding genius

Anyone who thinks plants are dummies hasn't seen a cabbage fight off loopers.

The plants pushing up everywhere have no calendar to tell them spring begins this week, but they can sense the world changing around them.

Winter's thick shell of frozen earth has melted into mud, and the ground is slowly warming. The sun's light, after slanting low and weak for months, is gaining strength, lasting longer, and beaming down from higher in the sky. The wind is losing its stinging edge. Water is flowing easily again.

And as the season rolls on toward summer, plants will keep time for us: The canopies above will fill with leaves, and the crocus blooms will be replaced by daffodils, then tulips, then Dutch irises and Spanish bluebells in colorful progression. This is how we have come to think of plant life: splendid and methodical.

But plants, researchers are finding, are surprisingly nimble in their daily lives, responding to changes with an animal-like speed. Plants have their own versions of our senses - sight, touch, taste, and smell - which they use to reach out for better food sources, for example, or to fend off an attacking insect.

"Plants are magnificently dynamic," says Steve Kay, a plant biologist at the Scripps Research Institute in La Jolla, California. "They measure their environment on a minute-by-minute basis."

In December, scientists announced they had completed the first genetic sequencing of a plant, giving them the DNA rule book for a weed called thale cress. As science discovers more secrets of the plant world, it is finding deep similarities with human biology.

Hormones and steroids course through plants, just as they do through animals. Plants send electrical signals in a primitive analogue to our nervous system. Even though plants are stuck in one place, they have evolved ways to survive in a quickly changing environment.

The Venus' flytrap is a spectacular example. It senses when a fly has landed on it, and its spiked jaws come slamming shut. But many plants, researchers now think, have a sense of touch. Farmers have noticed that crops grown during a windy season tend to be shorter, stockier, and less productive. The reason: A stalk of corn knows when it is being pushed around by the wind and devotes more of its resources to bolstering its stems, less to producing succulent kernels.

There is no evidence, alas, that plants respond to rock music, but there is plenty of evidence that they can smell trouble coming. When a cabbage plant, for example, is under attack by dread insects known as cabbage loopers, it gives off a sweet-smelling chemical called methyl jasmonate. When nearby plants smell this, they will make other chemicals to help fight off the strike.

Using the thale cress, or Arabidopsis thaliana, researchers have also identified a sense of taste in plants. Beneath the ground, roots draw a class of chemicals called nitrates out of the soil and use them to make energy. But the roots don't just branch out in all directions. If one part of the soil is richer in nitrates, the roots will grow in that direction. They taste the difference.

Of all the vegetative senses, though, it is "sight" that seems the most fine-tuned. Of course, a strawberry plant can't perceive the outlines of an approaching harvester, but plants do have incredibly sophisticated systems for detecting light and responding.

Anyone who has seen his houseplants reach longingly for the window has seen what botanists call phototropism, the ability to grow to the light. Throughout the stem are biological light detectors, which can inhibit growth. If one side of the plant is getting more light, that side will grow more slowly, pulling the entire plant toward the sun, the same way that holding a paddle in the water will pull a canoe to that side.

Sun-hungry plants can also tell when other plants are growing over them by comparing different wavelengths of light. If they sense they are losing the battle for sun time, they will direct their stems to grow faster.

A sunflower will track the sun over the course of a day. Its broad yellow flower faces east, like some great antenna, as the sun climbs the sky in the morning, then the plant tips westward to catch the sunset.

More amazing, perhaps, is the discovery that plants, like humans, have a circadian rhythm, a 24-hour cycle of biological changes that mirrors the cycle of night and day. In humans, for example, body temperature dips slightly during the night, conserving energy while we sleep. Then, as dawn approaches, the temperature comes back up in preparation for the work of the day.

With plants, dawn brings a surge in production of chemicals that protect them from damaging ultraviolet radiation from the sun. At midday, plants produce more of the chemicals they need to help handle photosynthesis. Then, as evening approaches, a plant will produce more of the chemicals it needs to resist the cold of night.

"Plants put on sunscreen in the morning and pull up their molecular bed-socks at night," says Kay.

According to Kay, plant research has helped scientists understand how humans function. Medicine has devoted increasing attention to our circadian rhythms and how, for example, to fight jet lag.

Much more poorly understood, though, is how we respond to the seasons, to lengthening days and shorter nights. The sun glitters, the leaves unfurl overhead, and the air grows heavy with new perfumes. We just call it spring fever.