Page:Collected Physical Papers.djvu/384

 XXVIII

THE RESONANT RECORDER

The exact determination of extremely short intervals of time is an important problem in various investigations. This is specially so in the measurement of time relations of different phases of response of living tissues.

When the sensitive pulvinus of Mimosa pudica is directly stimulated, say by an electric shock, a responsive contraction and fall of the leaf is initiated after the lapse of a short interval. After the completion of the fall of the leaf the contracted pulvinus slowly recovers as seen in the re-erection of the leaf. The lost time between the incidence of stimulus and the beginning of responsive movement is designated as the Latent Period. If instead of direct stimulation of the pulvinus, stimulus be applied on the petiole at a certain distance from the motile organ, then an excitatory impulse is transmitted through the intervening distance. The time-interval between stimulus and response will now be longer than under direct stimulation; the latent period of the pulvinus and the length of transmission afford sufficient data for the determination of the velocity of excitatory impulse in the plant, which I have shown elsewhere, is analogous to the nervous impulse in the animal.

The response of Mimosa is recorded by means of a writing lever suitably attached to the leaf (fig. 115).