Summary(1) The movements are only expressed in motor cells, regardless of the nature of the stimulation or its point of application. Therefore, these cells have structures capable of traducing the different stimulation‐induced messages which are received in parts incapable of movement.(2) K+, Cl‐and Ca2+are the major ions. Their fluxes have been followed during nyctinastic movements as well as during stimuli‐induced movements. At the moment, the location and the role of these ions are being studied.(3) The movement results from the integrated activity of all (n) motor cells in the pulvini (i.e.n>350 × 103in primary pulvini 3 mm long and 1·9 mm thick).(4) The motor cell is a full‐grown cell whose osmotic activity induces turgor variations allowing foliar movements.(5) The motor cell is a highly differentiated cell, which, up to now, has never been able to dedifferentiate in order to produce callus.(6) The motor cell has original features in its apoplastic compartment (large meatuses, wall foldings, large periplasm with membranes) and in its symplastic compartment (double vacuolar apparatus, morphological polarity given by the tannin vacuole location near the nucleus, abundant mitochondria).(7) Its cytoskeleton includes microtubules, cytoplasmic and vacuolar fibrils (in particular in the tannin vacuole), and a wall with special properties.(8) The motor cell is supposed to contain contractile proteins, whose nature and location are being investigated.(9) The shape change of the motor cell is obvious after pulvinar bending. This change is probably associated with a volume change in several intracellular compartments (vacuole, mitochondria, vesicles).(10) At the cellular and subcellular level the same general features are observed in motor cells of non‐seismonastic and of seismonastic species. Probably, functional differences depend upon differences occurring at the molecular level.(11) The motor cell is an interesting model for the study of the osmoregulation mechanism in plant cells, to test the effect of toxic products, in particular to find their optimal efficiency in the circ