2. A membrane-bound group of starch grains (see amyloplast) that according to the starch statolith hypothesis acts as a sensor to gravity. Starch statoliths are found in specialized cells (statocytes) in the root cap at the growing tip, and in the tissues close to the vascular bundles in shoots. Under the influence of gravity the statoliths sink to the bottom of the cell and influence the release of the plant hormone auxin (see geotropism). Their mechanism of action in triggering the transport of auxin across the plasma membrane is not understood. One theory is that the statoliths exert pressure on membrane systems inside the cell (e.g. the endoplasmic reticulum), which in turn potentially affects the opening or closing of ions channels and ultimately leads to differential growth. Alternatively, the weight of the statoliths may mechanically ‘stretch’ the upper side and compress the lower side of the root cells, causing the root tip to grow downwards.