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How does auxin cause a stem to bend toward light?
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plant development
The life cycle of the fern. (1) Clusters (sori) of sporangia (spore cases) grow on the undersurface of mature fern leaves. (2) Released from its spore case, the haploid spore is carried to the ground, where it germinates into a tiny, usually heart-shaped, gametophyte (gamete-producing structure), anchored to the ground by rhizoids (rootlike projections). (3) Under moist conditions, mature sperm are released from the antheridia and swim to the egg-producing archegonia that have formed on the gametophyte's lower surface. (4) When fertilization occurs, a zygote forms and develops into an embryo within the archegonium. (5) The embryo eventually grows larger than the gametophyte and becomes a sporophyte.
In the process of phototropism, auxin—specifically indolylacetic acid—regulates stem growth by stimulating cell elongation.12
When a stem is exposed to light, auxin is unevenly distributed within the plant. Greater quantities of the hormone accumulate on the shaded side of the stem compared to the side facing the light. Because auxin promotes the elongation of cells by acting on the chemical bonds of plant cell walls, the cells on the shaded side grow more rapidly than those on the illuminated side. This differential growth rate causes the stem to curve or bend toward the light source.12