Page:Life Movements in Plants.djvu/249

The normal response of a growing organ to Direct stimulus is negative, that is to say, a retardation of the rate of growth. This is the case under forms of stimuli as diverse as those of mechanical and electric shocks, and of the stimulus of light.

After my investigations on the normal retarding effect of light on growth, I was considerably surprised to find the responses occasionally becoming positive, an acceleration instead of retardation of growth. I shall first give accounts of such positive responses and then explain the cause of the abnormality.

Abnormal acceleration under stimulus of light: Experiment 87.—A rather weak specimen of Kysoor was exposed to the action of light of 5 minutes' duration. This induced an abnormal acceleration in the rate of growth from 0.30 μ to 0.40 μ per second. But continuous exposure to light for half an hour brought about the normal effect of retardation. In trying to account for this abnormality in response I found that while specimens of Kysoor in a vigorous state of growth of about 0.8 μ per second exhibit normal retardation of growth under light, the