Tropism and Auxins
Does it matter in what position a seed is placed when it is planted? No, it does not. Regardless of its position, the roots of the new plant grow down and the stem grows up. Gravity is the stimulus for this behavior. The response of a plant to a stimulus is called a tropism. Response toward a stimulus is a positive tropism. Response away from a stimulus is a negative tropism. Growth of roots down into the earth is an example of a positive geotropism. Upward growth of a stem away from the earth is negative geotropism. The response of a plants growth to light is phototropism. Would plants growing toward light show a positive phototropism?
Tropisms are controlled by chemical compounds in plants called hormones. An auxin is one kind of plant hormone. Auxins produced in one part of a plant can cause changes in another part of a plant. One kind of auxin stimulates the growth of cells. If a plant has light on one side, the auxin moves away from the lighted side of the stem. The amount of auxin increases on the shaded side. It causes the cells to grow faster on the shaded side than on the lighted side. The unequal growth rates cause the stem to bend toward the light.
แหล่งข้อมูล: BIOLOGY living system. Page 462.
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