On the relation between permeability variation and plant movements

Abstract

One of the most conspicuous examples of plant movement is the sudden fall of the stimulated leaf of Mimosa pudica . Several contradictory views had been held about the pulvinular mechanism by which this sudden movement is brought about. Bose (1905-13), by automatically recording the electrical and mechanical responses of the leaf of Mimosa pudica , has proved that both the upper and lower halves of the pulvinus are irritable—the fall of the leaf being due to the predominant contraction of the lower half. That there is a diminution of turgor of the stimulated pulvinus has been agreed by all observers. But as regards the cellular mechanism by which this diminution of turgor is brought about, contradictory views are held. We may take a mature plant cell to consist of a cellulose envelope saturated with water, and lined internally with a protoplasmic membrane enclosing solutions of various colloids and crystalloids. Owing to the difference of concentration of solutions on the two sides of the protoplasmic membrane, which is semi-permeable, osmotic pressure is set up within the cell. It is to the maintenance of this difference of concentration that the turgidity of plant cells is due. Any change which increases this difference of concentration on the two sides of the protoplasmic membrane will increase the turgor of the cells, and decrease it when the difference is lessened. Apart from any direct variation of the osmotically active substances on the two sides of the membrane, the difference of concentration will be modified by any variation of the permeability of the membrane; an increased permeability will diminish the difference, and therefore the turgor, and vice versâ . Thus a diminution of turgor may be induced by any one of these following factors:— (i) The introduction of osmotically active substances outside the cell. (ii) A diminution of the existing osmotically active substances inside the cell. (iii) An increased permeability of the protoplasmic membrane.