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Rh will be seen in Fig. 78(1)) side by side with the record of effect of continuous electric stimulation (Fig. 78a) on growth. In both the cases the effect. of continuous stimula- tion is seen to be the same, namely, a growing retardation, which in the, given instances culminated in arrest of growth. This is true of stimulus of moderate intensity. Under a more intense stimulation the incipient contraction does not end in a mere arrest of growth, but the responding organ undergoes an actual shortening.

FIG. 73.——l*‘.ll‘oets of continuous (a) electric and (h) photie stimulation nit llllNlCl'ﬂLI‘ intensity. taken on a. moving plate.

Ditl'erent observers have found‘ that it is the more refran- gihle rays which exercise the greatest influence upon growth and tropic curvature. The relatiVo effects of. dill'erent lights will, however, become more precise from the curves of re- sponse to the action of dill'erent rays. For this purpose, I ﬁrst employed monochromatic lights from different parts of the spectrum, producod by prism of high dispersion. In practice, the usual colour filters were found very con- venient, as they allowed the application of more intense light. A thick stratum of hichromate of potash solution


 * PEBPr‘HIh—l’hysiology of Plants—Vol IL, p. 101 (English Translation)