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Plant Behavior
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Plant Behavior and Nonliving Things

            Some parts of a plant grow up and some parts grow down. The things a plant does are called its behavior. When a seed starts to grow, a tiny root comes out first. And somehow it always grows in the right direction. It seems to know where “down” is. A plant knows which way is down because of the pull of the earth on certain chemicals in the root. These chemicals help the root grow down. Plant stems play tug-of-war with the earth. The earth pulls roots toward it. It also pulls stems toward it. But stems grow up, not down. Growth chemicals in the stems help the stems pull harder. The stems win the tug-of-war over the earth’s pull. Green plants need light to grow. Some plants can grow so they get the amount of light they need. Suppose a plant were getting light from only one side. The stem might then grow so that its leaves would be turned toward the light. The stem of many green plants can do this because of growth chemicals in the stem.
           
Most plants get water from the soil. The roots help the plant get the water it needs. But suppose the soil right around a plant is very dry. If there is water somewhere nearby, a little bit of the water may reach the root. That will cause the root to grow toward the place where more water can be found.
           
Plants “breathe” out water vapor. Plants have openings on their leaves for doing this. But when the air is very dry, the plants give off very little water vapor. The dry air causes the openings on the leaves to close up.

 

Plant Behavior and Living Things

            Deserts get very little rain. Some desert plants have many roots. These roots help them get as much water as possible. But if other plants get too close, there may not be enough water for all of them. One desert plant, the creosote bush, takes care of this. The roots of this bush give off a poison. Suppose another plant, even a young creosote bush, started to grow near an older creosote bush. The poison from the older plant would kill off the younger plant. This takes care of the problem of crowding for the creosote bush!
           
Some plants use poisons to protect themselves from hungry animals. Poison ivy makes a poison that bothers animals if they rub against it. Animals soon learn to stay away from poison ivy.
           
Some plants have a different way of protecting themselves. A certain grass is found where animals graze. If animals get near this grass, the grass droops. Then the grass doesn’t look good to eat. The animals may pass it up. Another plant, the mimosa, also droops at times. As soon as someone touches it, it draws its leaves close together.
           
Some plants eat animals such as ants and flies. These plants can grow in very poor soil. The soil does not have to supply all the minerals these plants need. These plants get some of the minerals they need from the animals they eat. One plant, the sundew, has sticky hairs all over its leaves. When a fly lands on a leaf, the fly gets stuck. Nearby hairs bend toward the fly. More sticky stuff pours over the fly. The fly cannot get away. Juices from the plant turn the fly’s body into a “soup.” The plant takes in this “soup” through its leaves. Another kind of animal-eating plant is the pitcher plant. Certain leaves of the pitcher plant catch rainwater. Ants or other small animals may crawl into these leaves looking for nectar. They often slip and fall into the water. That’s the end of the ants! Juices from the plant soon digest them. Another plant also eats animals. It is called the Venus’s-flytrap. When a fly lands on a leaf of this plant, the leaf closes up quickly. The fly cannot escape. Soon it is digested.
           
Some plants take care of themselves in still another way. They have weapons. The cactus has spines all over its stems. Spines help the kapok tree, too. Sometimes birds have mistletoe seeds stuck to their feathers. They rub the seeds onto some plants. The mistletoe then starts to grow on these plants. But the mistletoe can be harmful to these plants. It steals the water these plants take in from the soil. But, because of the kapok’s spines, birds don’t rub mistletoe seeds off on the kapok tree. This protects the kapok tree from mistletoe.
           
Animal “friends.” At certain times some plants have flowers on them. Some flowers have bright colors. The colors attract insects like bees. The flowers also make sweet nectar that many insects like to drink. In return, you might say the insects do a favor for the flowers. The insects carry pollen from a flower to another flower of the same kind. Pollen is the white or yellow dust made by certain parts of flowers. Pollen is needed by the flowers to make their seeds. One plant, the yucca plant, has a special friend. It is the yucca moth. The yucca flower opens up just once. It opens one evening. It stays open all that night. The next morning it closes, never to open again. Because it stays open only at night, day insects, like bees and butterflies, cannot carry its pollen. But the yucca moth flies at night. At night the yucca flower, which is white, shines in the moonlight. It also gives off a sweet smell. These things attract the yucca moth. The yucca moth then carries pollen from one yucca flower to another. At the same time, the yucca moth lays its eggs in the yucca flower.
           
Very often, insects eat plants. People often spray the plants with insecticides. Insecticides kill the insects. But they often kill other living things, like birds and mammals. Sometimes the insecticides stay on the plants. They can get into the vegetables you eat. There is a better way to keep insects and worms away from some plants. Certain plants can be used to chase away insects and worms from other plants. Marigold plants make a poison. Because of this poison, insects and worms leave marigolds alone. This poison is also helpful to rosebushes growing near marigolds. The roots of rosebushes are often destroyed by roundworms. But the poison from the marigolds chases the roundworms away. Rosebushes which grow near marigolds are healthy. Rosebushes also grow better when garlic plants are near. The garlic keeps beetles away from the roses. Asparagus plants keep worms away from tomatoes. Onion plants keep carrot flies away from carrots.