How do plants know which way is up?

Plants detect gravity using statocytes, specialized cells containing amyloplasts, which are dense starch-filled organelles that sink to the bottom of the cell under gravity. Their position triggers redistribution of the hormone auxin, which then accumulates unevenly and causes differential cell elongation. In shoots, auxin accumulating on the lower side makes those cells elongate more, bending the shoot upward. In roots, high auxin concentration has the opposite effect and inhibits elongation, so the root bends downward. The result is that a seed planted in any orientation sends its root down and its shoot up in the dark, before any light is available as a cue.

This is separate from the light response. Phototropism, growth toward light, works through a different mechanism involving blue-light receptors called phototropins, also mediated by auxin redistribution. The two systems operate together, which is why a plant on a windowsill leans toward the window while still growing generally upward, and why a houseplant turned regularly grows straighter.

Experiments in orbit have confirmed the mechanism by removing the variable. Plants grown on the International Space Station in microgravity lose reliable directional root and shoot growth and rely more heavily on light and moisture cues, and studying them has been a longstanding research thread partly because growing food is a requirement for long-duration spaceflight. Related tropisms round out the picture: hydrotropism directs roots toward moisture, thigmotropism produces the coiling of tendrils on contact, and some plants show measurable responses to touch and vibration that are still being characterized.