Plant shoots and roots steer their own growth without moving a muscle. Both responses rely on the plant hormone auxin (indole-3-acetic acid), which is actively pumped by PIN-protein transporters toward the shaded or lower side of a growing organ. Because auxin makes shoot cells elongate faster, the shaded flank of a stem grows longer than the lit flank, bending the tip toward the light — this is phototropism. In roots the same hormone has the opposite effect, slowing cell elongation, so the lower flank grows less and the root curves downward — gravitropism.
Charles Darwin and his son Francis first demonstrated in 1880 (The Power of Movement in Plants) that covering only the very tip of a grass coleoptile blocks phototropic bending — proof that the light signal is perceived at the tip but the bending happens lower down, exactly where auxin later turned out to accumulate.
An interactive 3D seedling that steers its own growth: reposition the light source and tilt the pot to watch auxin-driven differential elongation bend the stem toward light and the root back down toward gravity.
The shaded flank of the stem elongates faster than the lit flank (phototropism), while the lower flank of the root elongates slower than the upper flank (gravitropism) — both driven by lateral auxin redistribution, traced here by the glowing amber strand.
Drag the light azimuth and elevation sliders to move the light around the plant, tilt the pot to test gravitropic correction, and adjust auxin sensitivity to see stronger or weaker bending. Toggle the auxin flow strand on or off.
Darwin's 1880 coleoptile-tip experiments first showed that light is sensed at the shoot tip even though the bending happens further down the stem — exactly where auxin later turned out to accumulate.