How Do Plants Know Which Way Is Up?

Plants detect gravity through gravitropism, using a mechanism that acts like a biological spirit level. In root caps and stem endodermis, specialised cells (statocytes) contain starch-filled organelles called amyloplasts (statoliths) that are denser than the surrounding cytoplasm. When a plant is tilted, these statoliths settle to the lowest side of the cell within minutes. This physical displacement triggers a signalling cascade that redistributes the plant hormone auxin: in roots, auxin accumulates on the lower side, inhibiting growth there (so the upper side grows faster, bending the root downward). In shoots, auxin on the lower side stimulates growth (so the lower side elongates faster, bending the shoot upward). This mechanism works even in total darkness and was first described by Charles and Francis Darwin in "The Power of Movement in Plants" (1880). In microgravity (space stations), plants grow in random directions, confirming that gravity sensing is essential for normal growth orientation.