Page:Life Movements in Plants.djvu/197

Rh 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.

Experiment 56. — As a further example of the capabi- lity of the Crescograph, I shall give the record of a single pulse of growth obtained with the peduncle of Zephyran- thes Sulphured (Fig. 62). The magnification employed was

Fig. 62.-Record of a single growth-pulse resultant growth in each of Zephyanthes. (Magnification 10,000 . ' times.) *

10,000 times, the successive dots being at intervals of one second. It will be seen that the growth pulse com- mences with a sudden elongation, the maximum rate being 0*4 /a 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 palse is 2^ seconds. The pulse IS therefore the dirler- ence between elongation and recovery. Had a very highly magnifying arrangement not been us.^d, 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.