how do seeds know which way is up?

Question

askNib · Accepted Answer

Drop a seed in soil any which way, tumbled and sideways, yet its root always digs down and its shoot always climbs up. How does it know? Inside every root tip sit special cells called statocytes, packed with tiny dense grains called statoliths — heavier than the rest of the cell. Gravity pulls those grains straight down inside the cell, wherever the seed is turned — they settle to the true low side, like sand in a jar. That pile of grains is a signal. It triggers a plant hormone called auxin to build up on the lower side of the root. Extra auxin actually slows cell growth on that lower side, so the upper side grows faster and the root curves downward — and the shoot does the reverse, curving up. So a seed doesn't see up or down — it feels it, as tiny grains falling into place inside its own cells.