Page:Life Movements in Plants.djvu/165



leaf of Mimosa pudica undergoes an almost instantaneous fall when the stimulus is applied directly on the pulvinus which is the responding organ. The latent period, i.e., the interval between the application of stimulus and the resulting response is about 0.1 second. Indirect stimulus, i.e., application of stimulus at a distance from the pulvinus, also causes a fall of the leaf; but a longer interval will elapse between the incidence of stimulus and the response; for it will take a definite time for the excitation to be conducted through the intervening tissue. I have already shown that this conduction of excitation in plant is analogous to the transmission of nervous impulse in animal.

The power of conduction varies widely in different plants. In the petiole of Mimosa pudica the velocity may be as high as 30 mm. per second. In the stem the velocity is considerably less, i.e., about 6 mm. per second in the longitudinal direction; but conduction across the stem is a very much slower process. In the petiole of