Studies in the Permeability of the Pulvinus of 
Mimosa pudica. 
BY 
V. H. BLACKMAN 
AND 
S. G. PAINE, 
From the Department of Plant Physiology and Pathology , Imperial College of Science and 
Technology , London . 
With five Figures in the Text. 
A S a result of the work of numerous investigators (see the summaries by 
Pfeffer ( 1 ) and Jost ( 2 )), it may be considered as established that the 
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 
[Annals of Botany, Vol. XXXII. No. CXXV. January, 1918. ] 
