Cell biophysics and plant gravitropism.

Biological systems respond to environmental stimuli with the following sequence of events: (1) the signal is received; (2) membrane depolarization occurs; (3) messenger molecules are released and the biological response occurs. Zea mays (corn) seedlings respond to the gravity stimulus with the same sequence elaborated into a working hypothesis: (1) the gravitational vector is perceived by a statolith or by an unknown sensor system; (2) ion and hormone gating channels connecting vascular to surrounding tissues are depolarized; (3) the asymmetrically released ions and hormones initiate a cascade of reactions resulting in asymmetric growth. Mechanisms for amplification of the weak gravity signal are discussed. An experiment is proposed for distinguishing between statolith detection of the gravity vector and possible alternative detectors.