MOTOR ORGAN (Lat. motor, one who moves. from morcrr, to move). A structure which when stimula fed MOW'S' the at ta cited. Motor organs are found at the base of leaves and leaflets in various plaints, particularly members. of the bean family (Leguminosp). They are formed by the modification of the structure of the main leafstalk and of the of the leaflets (Fig. 1 ). \\ ben the latter are short the motor organ may constitute the whole stalk. The motor organ is a cushion (pulvinus) of thin walled tissue (parenchyma) ) whose centre is oc cupied by a strand of tibro-vascular and mechan ical tissue ( Fig. 2). This is in sharp contrast with the structure of the stalk elsewhere. There the out er parenchyma forms the usual cortex. the vascu lar and mechanical strands are distinct and often numerous, and the centre is occupied by a pith (Parenchyma I of considerable volume. The central position of the strands in the motor other plants variations in the intensity of light cause similar but slow curvatures in the motor organs. Exactly how the exudation of water from the cells of the motor organ is brought organ obviously permits freer bending. The cells
of the parenchyma are normally turgid, and by mutual pressure the organ is kept rigid, support ing the leaf or the leaflet. The action of an appropriate stimulus on the plant result- in the exudation of water from the cells on one side of the pnlvinus. This water passes into the inter cellular spaces of the organ and the cells of course lose their turgor. Owing to the now un opposed pressure of the still turgid cells on one side of the organ, it bends more or less. the flaccid side becoming coma cc. and the leaf-blade is car ried through a considerable arc. In the sensi• Live plants (Mimosa) of the tropics, contact is the usual stimulus. The whole reaction takes place very rapidly, the time from shock to re sponse being only a fraction of a second. In about is not known. It is probable that the stimulus in some way alters the physical struc ture of the protoplasm within the affected cells so that the contained solution is allowed to filter out. See MOVEMENT.