Movements are either autonomous, or are induced by an identifiable extern signal. Its recognition requires the existence of suitable receptors within the cell. As has been mentioned before is light one of the most important factors in a green plant’s life. A number of light receptors exist that do not only differ in their restriction to certain cell types and their position within the cell, but especially in their spectral sensitivity. In the case of induced movements instructs the genome of a cell the production of a receptor that is activated upon receiving a certain signal. It then elicits a number of activities within the cell finally resulting in the conversion of the signal into a certain movement.
The exogenous signal does not apply with autonomous movements. One or more gene products start a metabolic activity that causes observable movements. The genome may, too, join in the development of an organ and thus in its capacity for movements by the very complex means of a differential program. A co-ordination of the developments or a fixing of the time of a bending can be controlled by growth regulators.
Everything referred to above as metabolic activity could also in a cybernetic sense be regarded as a black box, about the content of which we do not know anything. We do therefore miss essential parts of the causal chain in order to understand induced (and autonomous) movements. Since most of them belong to the turgor movements can at least part of the reaction kinetics be identified: strictly regarding phenomenons can the increase in turgor be described as an uptake of water. The accumulation of osmotically effective substances within the cell (in the cytosol, or, even more likely, in the vacuole) is a necessary precondition. Both inorganic ions (potassium, chloride) a nd metabolic intermediates (like malate) are possible. Accumulation is based on the active and thus energy-consuming transport of the molecules (ions) through the membrane (plasmalemma (?), tonoplast) requiring specific, selective membrane-bound pumps (also called carriers or transferases). An increase of the cell’s osmotic pressure indicates always an increase in the activity of the involved transferases. Our problem is therefore confined to the part stimulus (signal) > activation of the transferases.
The understanding is complicated by the fact that extern stimuli are often perceived by the cells of a certain part of the tissue, tough the movement itself occurs in other cells. Consequently has an intercellular signal chain to exist. An example: C. DARWIN could prove that the tips of growing coleoptiles perceive light stimuli and that the stimulus is forwarded to the subapical tissues that are thereby arranged for bending towards the light (see also phototropism). Meanwhile do we know that the synthesis, and transport of auxin and the building of an auxin gradient are responsible for this reaction. We do not know, though, how the perception of the light stimulates auxin synthesis, how transport occurs, and what causes the auxin within the target cells to induce elongation. At least several hints for the process mentioned last exist.
A number of defined plant movements and its causes will be explained below by several selected examples for illustration.