Like animals, plants also produce chemical hormones that affect the growth when present in even minute amounts. One such chemical regulator is called the auxins. Auxins occur in all of the plant's growth tissues, their production controlled by light and other factors. They move throughout the plant in the phloem or sap, just as animal hormones do in the blood.
Growers use hormones to nurture their crops. A plant's growth movements are controlled by the internal chemical substances. When one side of the plant is shaded, auxin moves to the darker side of the plant and causes that side to grow much faster. As a result, the stem becomes bent toward the light.
The reaction of some plants to light, changes as they mature. The flower stalk of the peanut plant, for example, at first reacts positively to light (grows toward the sun). But after the flower is pollinated, the reaction reverses. The flower stalk then grows several inches into the soil, and the peanut develops underground.
Searching for water, roots of plants grow into the darkness of the soil, a movement that is both a negative reaction to light and a positive reaction to gravity. In growing downward, roots are reacting to the same plant hormone that causes stems to bend toward the light. The hormone collects on the underside of the root tip, but there it inhibits growth. The upper side grows faster, and the root tip bends down.
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