It has been shown that while favourable tonic condition has the effect of raising the excitability and enhancing the negative response with the associated D-change, a condition of sub-tonicity, on the other hand, induces depression of excitability, a diminution of negative response and of the attendant D-change. In this condition the positive element in the response with the A-change will come into greater prominence. These considerations led me to experiment with specimens exhibiting increasing sub-tonicity, with a view of unmasking the positive element in the response, _i.e._, the A-change. In the last experiment a specimen was found which happened to be in a sub-tonic condition on account of the unfavourable condition of its surroundings. I was next desirous of securing specimens in which I could induce increasing sub-tonicity at will. I have shown (_Expt. 23_) that a detached branch of _Mimosa_ can be kept alive for several days with the cut end immersed in water. In this condition the pulvinus retains its sensitiveness for more than two days. The excitability undergoes a continuous decline and is abolished about the fiftieth hour. Isolation from the parent organism thus causes a continuous depression of the tonic condition of the specimen. The case is somewhat analogous to the depression of excitability in an excised bloodless muscle. It is thus possible to secure specimens of varying degrees of sub-tonicity. A specimen that has been detached for six hours will exhibit a slight amount of depression, while a different specimen isolated for twenty-four hours will occupy a very much lower position in the scale of tonicity. _Experiment 49._--The staircase response of _Mimosa_ given in figure 53 was obtained with the stimulus of induction shock. In order to establish a wider generalisation I now used the stimulus of light given by an arc lamp. There may be a difficulty on account of the diurnal movement of _Mimosa_; the leaf, generally speaking, has a movement in a downward direction from morning till noon, after which there is a comparative state of rest. It is better to choose the time of noon for experiment. In any case the response to stimulus is very abrupt and in strong contrast with the slow diurnal movement. A horizontal pencil of light was thrown upwards by means of a small mirror and made to fall on the lower half of a pulvinus of the _Mimosa_ leaf. The excitatory down movement is followed by recovery on the cessation of light. The intensity of stimulus can be modified by varying the intensity of light. I took for my first series of experiments a specimen that had been isolated for six hours. Stimulation was caused by successive applications of light for 25 seconds at intervals of 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. [Illustration: FIG. 54.--Staircase response in sub-tonic _Mimosa_.]