Stalk Talk(Planets of Plants.)
Do plants communicate with each other?
Something’s happened. You’ve shrunk to the size of a peanut. You have wings and long, floppy antennae. You are teetering on a potato leaf in a vegetable jungle. Corn, squash, and potato plants tower over you. As you can see, it’s lunchtime. And the potato plant just happens to be your favorite snack. You’re a Colorado potato beetle! Farmers and gardeners don’t like you very much.
The garden is hopping-like a school cafeteria. You hear the crunch of insects chewing, and the hum of “conversation”-buzzing, chirping, and chattering. A mingling of smells is in the air-the scents of tasty foods, a whiff of sweet perfume.
You move on to the next potato plant. But its leaves taste bad. “Bleech!” you say and spit it out. Sounds weird, but sometimes you feel like the plants get the word that you’re hanging around. They seem to make themselves taste awful before you have a chance to chow down. You squat on a leaf and scratch your head.
Plant Talk Hypothesis
Scientists are scratching their heads, too. They’re asking themselves: Do plants really “talk” back and forth? Do they communicate and warn each other, as this cartoon suggests? Does one plant “say” to its neighbor, “Hey, the bug that likes potato leaves is heading your way”?
Scientists have found that certain plants have a way of protecting themselves from attacking insects. These plants have “buzzers” that go off when insects take nips out of them. When the chomping begins, an alarm signal travels through the plant. This signal causes the plant to make chemicals that are bad news for the munching insects. The anti-insect chemicals poison the insects or stunt their growth.
Now scientists are testing a related idea about alarm signals-the “plant talk” hypothesis. Some scientists think that a plant can do more than just protect itself. They think a plant can also send out an alarm signal that warns its neighbors of danger.
The hypothesis goes like this: After an insect chews a leaf, the injured plant releases a chemical that floats through the air. Remember that in the Discover Activity you learned how chemicals travel through air. You also found out that different smells cause you to respond in different ways. For example, some of the odors might have caused you to turn away or hold your nose.
In the laboratory, scientists found evidence to support the plant talk hypothesis. Sagebrush plants contain a chemical that some scientists think is a powerful alarm signal. The scientists tested their idea by doing the experiment pictured here. They put sagebrush plants and tomato plants together in air-tight jars. Guess what happened? Tests showed that the tomato plants began to make anti-insect chemicals. The scientists concluded that the alarm signals given off by sagebrush traveled through the air and caused this response in the tomato plants. To further test their hypothesis, the scientists put tomato plants in the air-tight jars without sagebrush plants. These tomato plants did not produce the ant insect chemicals.
Do scientists need more evidence for the plant talk hypothesis? Of course they do! Remember, they did their tests in the laboratory. Plants don’t normally grow in jars. Now scientists are doing new experiments that are testing the hypothesis out-of doors in the plants’ own territory.
Plant Signals
Scientists don’t know for certain whether chemical signals travel from plant to plant. But they are quite sure about one thing. Chemical signals do travel inside a plant-from one part of a plant to another part.
When you plant an apple seed, what causes the seed to crack open? What causes a tiny root to come out of the seedcase first, and push downward into the soil? What causes a young shoot to poke through the ground and grow upward?
Chemicals called hormones control all of these activities. A hormone is a chemical messenger with a precise list of instructions.
A single plant contains a crew of hormones, each with its own assignment. One hormone is in charge of making roots grow downward. Another directs a stem to grow toward sunlight. When the time is right, other hormones make an apple ripen and turn red, or a maple leaf change color.
Hormones control all the activities connected with the growth and development of plants, such as this apple tree. In turn, the long days of summer, the cold nights of winter, and other signs from nature control hormones. Environmental signals-which you call the changing seasons-help, control how hormones are made and released within plants.
Most hormones are produced in one part of a plant, but they have their effect in another part of the plant. Hormones move through tubes that carry food throughout a plant. And hormones are constantly on the march inside a plant. For example, one hormone is in charge of making a plant bend toward light. This hormone is produced in the very tip of the stem, but it travels down into the stem with its instructions.
As chemical messengers, hormones don’t waste words. A tiny amount of a hormone can bring about a big change in a plant. For example, an extremely small quantity of a kind of growth hormone causes the shoot and root of an apple seedling to grow longer.
Besides controlling activities in different parts of the plant, hormones sometimes control each other. When it’s time for a plant to reproduce, the hormone in charge of fruit-ripening has control over the hormone that keeps the fruit green
