Fig. 72.—Effects of electrical stimulus of increasing in­tens­ities: of 0.25 unit, 1 unit, and 8 units. Short dashes rep­re­sent the mo­ments of ap­pli­ca­tion of stimu­lus.

Effect of Intensity: Experiment 74.—I shall now describe a typical experiment on the effect of intensity of stimulus in retarding the rate of growth. The normal rate of growth of the bud of Crinum was 0.35 µ per second. On the application of electric shock of unit intensity for 5 seconds, the rate became reduced to 0.22 µ per second. When the stimulus was increased to 2 units, the retarded rate of growth was 0.07 µ per second. When the intensity was raised to 4 units, there was a complete arrest of growth. In figure 72 is given records of a different experiment which show the effects of increasing intensity of stimulus in retardation of growth.

Fig. 73.—Effect of continuous electric stimu­la­tion of in­creas­ing in­tens­ity. The last record exhibits the actual shorten­ing of the grow­ing organ under strong stimu­lus.

Effect of continuous stimulation: Experiment 75.—The effect of continuous stimulation of increasing intensity will be seen in the record (Fig. 73), taken on a moving plate. On application of continuous stimulus of increasing intensity an increased flexure was produced in the curve, which denoted greater retardation in the rate of growth. When the intensity of stimulus was raised to 3 units, there was induced an actual contraction.

CONTINUITY BETWEEN INCIPIENT AND ACTUAL CONTRACTION.

It will thus be seen, that external stimulus of electric shock induces a reaction which is of opposite sign to the normal growth elongation or expansion. We may conveniently describe this effect as ‘incipient’ contraction; for under increasing intensity of stimulus, the contractile reaction, opposing growth elongation, becomes more and more pronounced; at an intermediate stage this results in an arrest of growth; at the further stage, it culminates in an actual shortening of the organ. There is no break of continuity in all these stages. I shall, therefore, use the term ‘contraction’ in a wider sense, including the ‘incipient’ which finds expression in a retardation of growth.

In Table XVI is given the results of certain typical experiments on the effect of stimulus of increasing intensity and duration.

TABLE XVI.—EFFECT OF INTENSITY AND DURATION OF ELECTRIC STIMULUS ON GROWTH.