Auxin is a plant hormone, which promotes growth at low concentration and inhibits growth at high concentration. The growth of roots and stems have different responses to auxin concentration: low auxin concentration promotes root growth, while high auxin concentration inhibits root growth, but it promotes stem growth, while higher auxin concentration inhibits stem growth.
When the plant is laid flat, the auxin moves to the lower side due to the action of gravity, and the auxin concentration on the lower side of the root and lotus of the stem plant is high, which grows faster than that on the upper side, making the stem tip bend upward; The concentration of auxin on the lower side of the root is high enough to inhibit the growth, and the growth is slower than that on the upper side, which makes the root tip bend downward.
Extended data:
1880 C.R. Darwin and his son stated in the last published book "The Skill of Plant Movement" that the coleoptile of Gramineae canary grass lost its phototropic response when the top was cut off.
His explanation is that when the seedlings are exposed to light from the side, the influence from the top will be transmitted downwards, resulting in different growth rates on both sides of the light and the backlight, which will cause bending to the light-receiving side, so after cutting off the top, there will be no phototropic response.
1928 f.w. winter proved by experiments that there is a growth-promoting substance at the tip of coleoptile, which is called auxin. It can diffuse into agar cubes, and the obtained cubes can be put back to the side where the top of the coleoptile is cut off, which can cause the coleoptile to bend to the other side.
Moreover, the curvature is roughly proportional to the amount of growth-promoting substances contained. This experiment not only proved the existence of growth-promoting substances, but also created the famous "oat test method" for determining auxin.
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