Page:Life Movements in Plants.djvu/133

Rh amplitude of response seen in the more erect curve in the upper record.

Different specimens of Mimosa are found to exhibit differences in physiological vigour. Some are in an optimum condition, others in an unfavourable or sub-tonic condition. I shall now describe certain characteristic differences of conductivity exhibited by tissues in different conditions.

Effect of intensity of stimulus on velocity of transmission.—In a specimen at optimum condition, the velocity remains constant under varying intensities of stimulus. Thus the velocity of transmission in a specimen was determined under a stimulus intensity of 0.5 unit; the next determination was made with a stimulus of four times the previous intensity, i.e., 2 units. In both these cases the velocity remained constant. But when the specimen is in a sub-tonic condition, the velocity is found to increase with the intensity of the stimulus. Thus the velocity of conduction of a specimen of Mimosa in a sub-tonic condition was found to be 5.9 mm. per second under a stimulus of 0.5 unit; with the intensity raised to 2.5 units, the velocity was enhanced to 8.3 mm. per second.

After-effect of stimulus.—In experimenting with a particular specimen of Mimosa I found that on account of its sub-tonic condition, the conducting power of the petiole was practically absent. Previous stimulation was, however, found to confer the power of conduction as an after-effect. It is thus seen that stimulus canalises a path for conduction.

The effect of excessive stimulus in a specimen in an optimum condition is to induce a temporary depression of conductivity; the effect of strong stimulus on a sub-tonic