_Experiment 55._--In the determination of time-relations of responsive change in growth under external stimulus, I shall take the typical case of the effect of electric shock from a secondary coil of one second’s duration. Two electrodes were applied, one above and the other below the growing region of a bud of _Crinum_. The record was taken on a moving plate, the magnification employed being two thousand times, and successive dots made at intervals of two seconds. It was a matter of surprise to me to find that the growth of the plant was affected by an intensity of stimulus far below the limit of our own perception. As regards the relative sensitiveness of plant and animal, some of my experiments show that the leaf of _Mimosa pudica_ in a favourable condition responds to an electric stimulus which is one-tenth the minimum intensity that causes perception in a human being. For convenience I shall designate the intensity of electric shock that is barely perceptible to us as the unit shock. When an intensity of 0.25 unit was applied to the growing organ, it responded to it by a retardation of growth. Inspection of Fig. 61 shows that there is a flexure induced in the curve in response to stimulus, the flattening of the curve denoting retardation of growth. The latent period, in this case, is 6 seconds. The normal rate was restored after 5 minutes. The intensity of shock was next raised from 0.25 unit to one unit. The second record shows that the latent period is reduced to 4 seconds, and a relatively greater retardation of growth was induced by the action of the stronger stimulus. The recovery of the normal rate was effected after the longer period of 10 minutes. I took one more record, the intensity being three units. The latent period was now reduced to 1 second, and the induced retardation was so great as to effect a temporary arrest of growth. TABLE X.--TIME-RELATIONS OF RESPONSIVE GROWTH-VARIATION UNDER ELECTRIC SHOCK (_Crinum_). +------------+--------------+---------------+----------------+ |Intensity of|Latent period.| Normal rate. | Retarded rate. | |stimulus. | | | | +------------+--------------+---------------+----------------+ | 0.25 unit. | 6 seconds. |0.62 µ per sec.|0.49 µ per sec. | | 1 " | 4 " |0.62 " |0.25 " | | 3 " | 1 " |0.62 " |Temporary arrest| | | | |of growth. | +------------+--------------+---------------+----------------+