As regards the excitatory fall of the leaf of _Mimosa pudica_, Pfeffer and Haberlandt are of opinion that this is due to the sudden diminution of turgor in the excited lower half of the pulvinus. The weight of the leaf, no longer supported by the distended lower cells, causes it to fall. This is accentuated by the expansion of the upper half of the pulvinus which is normally in a state of compression. According to this view the excitatory fall of the leaf is a passive, rather than an active, movement. I have, however, found that in determining the rapidity of 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 sudden 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.[F] [F] 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. 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:--