Studies in the Permeability of the Pulvinus of Mimosa pudica

Abstract

AS Pfeffer a result (1) of and the Jost work (2)), of numerous it may be investigators considered as (see established the summaries that the by Pfeffer (1) and Jost (2)), it m y be con ider as stablished that e movement of the pulvinus of Mimosa pudica under the stimulus of shock (the so-called seismonastic movement) is due to a change in the turgor of the cells. The change in the shape of the pulvinus is mainly brought about by a loss of turgor in the cells of the lower half of the pulvinus ; the loss of turgor allowing of the elastic recoil of the stretched cell wall, assisted under normal conditions by the weight of the leaf and the tension of the upper half of the pulvinus. The loss of turgor appears to be associated, as is to be expected, with a loss of water from the cell. But although the seismonastic movement of Mimosa is associated with a turgor change yet the mechanism of that change remains uncertain. Pfeffer has pointed out that the reduction of turgor is most likely to be due either to a reduction in the quantity of osmotically active substances in the cell, thus allowing water to escape from the cell, or to an increase in the permeability of the cell, thus allowing of the escape of some of these osmotic substances together with some of the water of the cell-sap. The second hypothesis, that of the increase of protoplasmic permeability to such a degree as to bring about a serious reduction in the cell turgor, is difficult of acceptance as a working hypothesis since there is no obvious mechanism by which the osmotic substances which thus escape in solution can be drawn back so as to allow the cell to regain its turgor. Lepeschkin (3) has shown that the pulvinus of Mimosa and of Phaseolus is sensitive to light, the permeability of the cells being increased when illuminated ; he associates the nyctinastic movements of the leaves