Page:Life Movements in Plants.djvu/177

Rh 3 minutes. Figure 54 shows how the functional activity of the sub-tonic specimen is enhanced by stimulus, the successive responses thus exhibiting the staircase effect.

Fig. 51. Fig. 55.

Fig. 54. — Staircase response in sub-tonic Mimosa. Fig. 55. — Positive, diphasic and negative response under successive stimulation.

Experiment 50. — A still lower degree of sub-tonicity was ensured by keeping the specimen in an isolated condition for 12 hours. Stimulus of light for 20 seconds' duration was applied at intervals of 2 minutes. In the record (Fig. 55) the first two responses, not shown, were purely positive. The third exhibited a positive A-eft'ect, followed by the negative response D-effect. The A-effect is thus seen fully unmasked. In subsequent responses the A-effect became more and more overshadowed by the D-etiect. At the third response the masking is complete and the excitatory nega- tive response is at its maximum. The record of staircase effect (Fig. 54) also exhibits a preliminary positive twitch at the beginning of the series, which disappeared after the second response.