the fall of Mimosa leaf the factors of expansive force of the upper half of the pulvinus and the weight of the leaf are negligible compared to the active force of contraction exerted by the lower half of the pulvinus (p. 87).
With regard to the fall of turgor, it is not definitely known whether excitation causes a sadden diminution in the osmotic strength of the cell-sap or an increase in the permeability of the ectoplast to the osmotic constituents of the cell. Pfeffer favours the former view, while others support the theory of variation of permeability.[1]
RESPONSE OF PULVINUS OF MIMOSA TO VARIATION OF TURGOR.
Whatever difference of opinion there may be in regard to the theories of osmotic and permeability variations, we have the indubitable fact of diminution of turgor and contractile fall of the pulvinus of Mimosa under excitation. The restoration of the original turgor brings about recovery and erection of the leaf. In connection with this the following experiments on responsive movements of the leaf under artificial variation of turgor will be found of interest:—
- ↑ With reference to the fall of Mimosa leaf Jost says: "When the pressure of the cell decreases we naturally assume this to be due to a decreasing osmotic pressure due to alterations in the permeability of the plasma, and an excretion of materials from the cell. It is a remarkable fact that plasmolytic research (Hilburg 1881) affords no evidence of any decrease in osmotic pressure. No complete insight into the mechanism of the stimulus movement in Mimosa has yet been obtained, although one thing is certain, that there is a decrease in the expansive power on the under side of the articulation."—Jost, "Plant Physiology"—English Translation, p. 515. Clarendon Press (1907). Blackman and Paine think that the loss of turgor on excitation "is probably due to the disappearance or inactivation of a considerable portion of the osmotic substances of the cells."—Annals of Botany, Vol. XXXII, No. CXXXV, Jan. 1918.
