It’s the most commonly observed tropic responses in plants where the shoot grows towards light and hence they’re positively phototropism whereas in case of roots,they show negative phototropism. This phototropic bending is caused due to the migration of auxin (phytohormone) molecules away from lighted side and their concentration on dark side increases.This stimulates growth on the dark side of the plant organ relative to lighted side causing it to bend towards light source. A well known and often repeated experiments with oat seedlings (Avena coleoptile) have shown that the auxin IAA which causes elongation of cells,migrates to the shaded side of coleoptiles when irradiated unidirectionally.
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