Page:Life Movements in Plants.djvu/70

40 increased turgor travelled was thus 2 mm. per second. The leaf exhibited increasing erection with absorption of water.

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.)

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.

As regards stimulation of vegetable tissues, there are various agencies besides electric shock, which induce excita­tory contraction; these agencies I shall designate as stimuli. Excitation is detected in Mimosa by the downward movement of the leaf. It will be found that such excitatory movement is caused by a mechanical blow, by a prick or a cut, by the application of certain chemical agents, by the action of electric current and by the action of strong light. The study of the action of these stimuli will be given in greater detail in subsequent Papers.