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 particular specimen which exhibited the abnormal positive response had a much feebler rate of growth of 0.30 µ per second. As activity of growth in a plant is an index of its healthy tone, a feeble rate of growth must be indicative of tonicity below par. The fact that plants in sub-tonic condition exhibit abnormal acceleration of growth under stimulus will be seen further demonstrated in the next experiment. In the parallel phenomenon of the response of pulvinated organs we found that under condition of sub-tonicity, the response becomes positive and that this abnormal positive is converted into normal negative in consequence of repeated stimulation. In growth, response likewise the abnormal acceleration of growth under light in the sub-tonic specimen of _Kysoor_ was converted into normal retardation after continuous stimulation for half an hour. From the facts given above, we are justified in drawing the following conclusions: (1) That while light induces a _retardation_ of growth in a tissue whose tonic condition is normal or above par, it brings about an _acceleration_ in a tissue whose condition is below par. (2) That by the action of the stimulus of light itself a sub-tonic tissue is raised to a condition at par, with the concomitant restoration of normal mode of response by retardation of growth. Another important question arises in this connection: Is the restoration of normal response due to light as a form of stimulus, or to its photo-synthetic action? An answer to this is to be found from the results of an inquiry, whether a very different form of stimulus which exerts no photo-synthetic action, such as tetanising electric shocks, also induces a similar acceleration of growth in a sub-tonic tissue. The normal retarding effect of electric stimulus on specimens in active state of growth was demonstrated in record given in Fig. 72, where the normal rate was found greatly reduced after stimulation.