Fig. 61.—Time-relations of re­sponse of grow­ing organ to elec­tric stimu­lus of increas­ing in­tens­ities applied at the short hori­zon­tal lines. Suc­ces­sive dots at in­ter­vals of 2 seconds.

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
stimulus.		Latent period.Normal rate.Retarded rate.
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.

It is thus found that growth in plants is affected by an intensity of stimulus which is below human perception; that with increasing stimulus the latent period is diminished and the period of recovery increased; and that the induced retardation of growth increases continuously with the stimulus till at a critical intensity there is a temporary arrest of growth. I shall speak later of the effect induced by stimulus above this critical point.

Fig. 62.—Record of a single growth-​pulse of Zephyr­anthes (Mag­ni­fi­ca­tion 10,000 times.)

Experiment 56.—As a further example of the capability of the Crescograph, I shall give the record of a single pulse of growth obtained with the peduncle of Zephyranthes Sulphurea (Fig. 62). The mag­ni­fi­ca­tion employed was 10,000 times, the successive dots being at intervals of one second. It will be seen that the growth pulse commences with a sudden elongation, the maximum rate being 0.4 µ per sec. The pulse exhausts itself in 15 seconds, after which there is a partial recovery in the course of 13 seconds. The period of the complete pulse is 28 seconds. The resultant growth in each pulse is therefore the difference between elongation and recovery. Had a very highly magnifying arrangement not been used, the resulting rate would have appeared continuous. In other specimens, owing probably to greater frequency of pulsation and co-operation of numerous elements in growth, the rate appears to be practically uniform.