Page:Researches on Irritability of Plants.djvu/73

50 we calibrate other positions of the scale in terms of this unit. In this manner the scale is marked so as to indicate intensities of ·1, ·5, 1, 2, 3, 4, 5, and so on. The calibration is carried out by means of a ballistic galvanometer. In subsequent chapters we shall employ these practical units, which will thus have a definite significance.

Having shared the prevailing belief that the sensitiveness of the plant was very feeble compared with that of the animal, I was considerably surprised to find that the intensity of induction-shock which is barely sufficient to induce sensation in man is quite enough to cause excitatory fall in a Mimosa of moderate sensitiveness. Indeed, I found that in the case of a highly excitable specimen an intensity only one-tenth of this was sufficient to excite it. In other words, under this particular test Mimosa may prove ten times as sensitive as a human subject! Later on I shall give details of measurements which will show that, as far as electric mode of stimulation is concerned, the plant is in no way inferior to the animal in sensitiveness.

The extent of responsive fall in Mimosa increases with increasing intensity of stimulus. The rate of movement is also greater under stronger stimulus.

The rate of responsive movement becomes slower under fatigue. In a given case the normal maximum rate of movement of 50 mm. per second was reduced to 8 mm. under fatigue.

Temperature enhances the rate of movement. A rate of 10 mm. per second at a temperature of 22° C. was found enhanced to 105 mm. per second when the temperature was raised to 28° C.

In a typical case of Mimosa, in summer, the latent period was found to be one-tenth of a second. The maximum contraction was attained in 3 seconds and the recovery completed in 15 minutes. The rate of recovery was relatively