Traditional carton box

(1) a-ray bending growth; Vertical growth; C direction light bending growth; D will not grow.

(2)2; 3. The top of coleoptile; The part below the top of the coleoptile;

Note: the tip will produce auxin to promote plant growth. The tip will sense the light, so that auxin will be distributed to the backlight side, making the backlight side grow faster than the sunny side, causing plants to bend to the sunny side. If the top is cut off and there is no auxin, the plant will not regenerate and bend. If the top is covered, the auxin is evenly distributed, so the plant grows upright.