Page:Life Movements in Plants.djvu/249



The norinal response of a growing organ to Direct stimulus is negative, that is to say, a retardation of she 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 reaponscs occasionally becoming,r 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 (lore/gratin”. under slinmlus of light: [ctr/wri- menl 87.— rather weak specimen of Kysmu' was exposed ‘to the action of light of 5 lninutes’ luration. 'l‘his induced an abnormal acceleration in the rate of growth from ‘30 a to 0'40 ,1. per second. But continuous exposure to light for half an hour brought about the normal ell'cct of retar- dation. In trying' to account for this abnormality in reslionsc I found that while Specimens of Kysmn' in a vigorous state of growth of about 0'8 a per second exhibit normal retardation of growth under light, the