Gravity sensing by plants

A brief description of the effects of gravity on plants, particularly their orientation with reference to its direction, is followed by a presentation of evidence for the operation of specific "gravity sensors", statoliths, subcellular bodies which move or reorient themselves upon reorientation of a plant organ with respect to the direction of the force of gravity. In the singly reacting apical cells of rhizoids of Chara, the statoliths locally regulate the growth of the cell wall simply by blocking or permitting the access of carriers of cell wall material. In multicellular structures (root and stem tips etc.) the statoliths, most likely amyloplasts (starch-containing plastids), must act by creating in the organ a transversal polarity which can be transmitted to the growing region, often several millimetres away. In addition, gravity produces what may be called tonic effects, which quantitatively modify the reaction of the organ to the gravitational stimulation.