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:—
Effect of Increased Turgor: Experiment 12.—A young Mimosa plant was carefully transplanted and the root embedded in soil placed in a linen bag. This was held securely by a clamp, and one of the leaves of the plant attached to the recorder. Withholding of water for a day caused a general loss of turgor of the plant. A vessel full of water was now raised from below so that the linen bag containing the roots was now in water. The effect of increased turgor by suction of water by the roots became apparent by the upward movement of the leaf. The distance between the immersed portion of the plant and the leaf was 2 cm. and the up-movement of the leaf was indicated within 10 seconds of application of water (Fig. 13). The velocity with which the effect of increased turgor travelled was thus 2 mm. per second. The leaf exhibited increasing erection with absorption of water.
Fig. 13. Response of Mimosa pulvinus to variation of turgor. Increased turgor by application of water at point marked with vertical arrow induced erectile movement. Diminution of turgor by application of KNO3 solution at the point marked with the horizontal arrow, brought about the fall of the leaf within 80 seconds. Successive dots at intervals of 5 seconds (The down curve represents up-movement and vice versâ.)
Effect of Diminution of Turgor: Experiment 13.—While the leaf in the above experiment was in process of erection, a quick change was made by substituting KNO3 solution for the water of the vessel in which the roots were immersed. The plasmolytic withdrawal of water at the roots gave rise to a wave of diminished turgor, the effect of which became perceptible within 40 seconds by the movement of fall of the leaf. (Fig. 13.)
DIFFERENT MODES OF STIMULATION.
In Mimosa excitation is manifested by the contraction of the pulvinus and the consequent movement of the leaf. But in most plants, excitatory movement cannot be realized on account of the rigidity of the plant structure, the thickness of the cell-wall and the want of facility for escape of water from the excited cells. I shall show later how excitation may be detected in the absence of mechanical movement.