[Illustration: FIG. 55.--Positive, diphasic and negative response under successive stimulation.] POSITIVE RESPONSE IN SUB-TONIC SPECIMEN. _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-effect, 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-effect. At the third response the masking is complete and the excitatory negative 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. The modifying influence of tonic condition on response I find to be of universal occurrence. In vigorous specimens the electric response to stimulation is _negative_; but tissues in sub-tonic condition give _positive_ response and after long-continued stimulation the abnormal positive is converted into the normal _negative_. It is very interesting that under condition of sub-tonicity diverse expressions of physiological reaction exhibit similar change of sign of normal response. Thus in my measurement of the velocity of transmission of excitation in the conducting tissue of _Mimosa_, I find that, when the tissue is in an optimum condition, exhibiting high velocity of transmission, excessive stimulus has the effect of diminishing the conducting power. But in a depressed condition of the tissue the effect is precisely the opposite. Thus in a given case the velocity of transmission was low; strong electric stimulation enhanced the rate by 33 per cent. In extreme cases of sub-tonicity, where the conducting power was in abeyance, the excessive stimulus caused by wound not only restored the power of conduction but raised the velocity of transmission to 25 mm. per second (_Expt. 37_). SUMMARY. The excitability of a plant is found to be modified by its tonic condition. A sub-tonic specimen of _Mimosa_, like an excised bloodless muscle, shows a preliminary staircase response. Stimulus induces simultaneously both “A” and “D” effects, with their attendant positive and negative reactions. A tissue in optimum condition exhibits only the resultant negative response, the comparatively feeble positive being masked by the predominant negative. With decline of tone, the “D” effect diminishes and we get “A” effect unmasked. In extreme sub-tonic specimen, we get first only the “A” effect, with its positive response. Successive stimulation converts the pure positive into diphasic and ultimately into normal negative response. PART II. GROWTH AND ITS RESPONSIVE VARIATIONS. X.--THE HIGH MAGNIFICATION CRESCOGRAPH FOR RESEARCHES ON GROWTH[S] _By_