The gravitropic response of plants to a change in the gravity vector may be divided. in the phase of induction and expression. During the induction phase the amyloplasts, due to their greater density than the cytoplasmic density, shift their position in less than a minute. During this shift there is an interaction with the endoplasmic reticulum, although a role of actin-like proteins of the cytoskeleton may also be involved in this process. The endoplasmatic reticulum maintains a store of sequestered calcium through the action of an ATP-dependent calcium uptake mediated by the Ca2+, Mg(2+)-ATPase system present in the membrane of this organelle. The interaction of the amyloplast with the endoplasmic reticulum leads to the release of free calcium ions from the endoplasmic store. The increased free Ca2+ level in the cytoplasm may modify the activities of certain enzymes and receptor proteins. The gravitropic induction phase is completed when the lateral polarization of the tissues has taken place. These tissues contain information about changes in direction of the IAA transport system and in competition of the IAA-receptor system for the phytohormone. This information is fixed in "memory" and its expression is achieved when the lateral gradient of IAA concentration and of the activity of the IAA-receptor protein complexes is formed in the horizontally oriented plant organ. Flows of IAA and calcium ions in opposite directions may lead to the expression of laterally differentiated growth.