ppiNE Journal of Science, 
Vol. VII, No. 1, April, 1912. 
THE MECHANISM OF CURVATURE IN THE PULVINI OF 
MIMOSA PUDICA. 
By William H. Brown. 
(From the Botanical Section of the Biological Laboratory, Bureau of 
Science, Manila, P. /.) 
According to Pf eff er ^ the curvature resulting from the stimu- 
lation of the pulvini of the petioles of Mimosa pudica L., is due 
to a fall in the turgor of the cells of the lower or reacting half 
of the pulvini, which cells are compressed by the expansion of 
those of the upper turgid half. The decrease in turgor is shown 
by the stimulated pulvini becoming more flaccid and less rigid 
than the unstimulated ones, and by the passage of water from 
the cells, of the lower half, to the intercellular spaces. Pfeffer 
states that from the load required to prevent curvature it 
can be calculated that the energy necessary for movement is 
from two to five atmospheres and that hence the curvature can 
not be due to an active contraction of the protoplasm as was 
claimed by Vines = and by Gardener.' 
A reduction in turgor might be produced by a change in the 
permeability of the plasma membranes, as claimed by Lepesch- 
kin,* although as pointed out by Pfeffer this would neces- 
sitate a passage of dissolved substances from the cells. It 
seems more likely, as is thought by Pfeffer, that the reduction 
in the turgor of the cells of the pulvini is due to a decrease in 
the osmotic pressure of the cell-sap, which would allow an 
exosmosis of water without the dissolved substaijces. In either 
case, however, there should be a decrease in the number of 
osmotically active molecules in the cells, and if this were the 
cause of the curvature in the living pulvini, it seemed likely 
' Pfeflfer, W., Physiology of Plants, translated by A. J- Ewart, Oxford, 
1906. 
•Vines, Arb. Bot Inst. Wurzburg 2 (1878) 146. 
■Gardener, Annals of Botany 1 (1887-8) 366. 
•LepescWcin, W. W. Ber. Deutsch. Bot. Gesell. 26 (1908) 724. 
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