How do plant roots and shoots know which way is up and down?
The phenomenon known as gravitropism enables plants to sense and respond to gravity. How the shoots respond to gravity is not entirely clear at this point, but scientists do have a plausible explanation for the roots. When a seed germinates, or sprouts, cells at the tips of the root called statocytes release specialized starch containing plastids, or statoliths. The aggregation of the statoliths towards the end of the root causes a redistribution of calcium and the release of a plant hormone called auxin. Calcium and auxin settle at the lower side of the root and prevent cell elongation. The cells on the upper side of the root continue to elongate, and, as a result, the root bends downward as it grows.
