Page:Life Movements in Plants.djvu/254

224 become circumscribed, and the intensity and duration of stimulus necessary to convert the positive to negative will he fechlcr and shorter. It will he very seldom that a plant is likely to he found at the optimum. Hence plants in general may he expected to give a feeble positive reSpmme under sub-minimal stimulus.

'l‘hvse considerations led me to look for the positive respmme under sub-minimal stimulation; the tracings which I have obtained with my highly sensitive Cresco- graph and other recorders show that my anticipations haVe been justilied.

Positive res/mm- under sub-minimal stimulus: Erperi- mrenl. elm—In normal specimens, lightof strong intensity in- duces a retardation of growth. When the source of light is placed at a distance, the intensity of light. undergoes great diminution. Under the action of such l‘eehle stimulus I obtained an :wceleration of growth even in specimens which may he regarded as moderately vigorous (lt‘lg. *3). Similar acceleration of growth was also

Fin. S.i-~\i'i'i'li-ratinn of growth under sub—minimal llg'nt stimulus ltci‘nrll on moving plate: stimulus applied at 5th dot. and sulM-qurut erection of cuer exhibits acceleration of growth. l.a.~t part of curve shows recovery of normal growth on ccsmtiun of SLlUlltlllr.