Page:Life Movements in Plants.djvu/247

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It is thus seen that the ell‘ect of Indirect stimulus on growth-variation is precisely parallel to that obtained with the response of sensitive plant; that is to say. the effeCt induced by a feeble stimulus applied at a distance from the growing region is a positive variation or acceleration of growth. The etl'ect becomes converted into negative or retardation of growth when the. stimulus is Direct, 1'.e., when applied to the responding region of growth; under intermediate conditions, the growth-variation 1 ﬁnd to be diphasic, a positive acceleration followed by a negative retardation. This is found true not merely in the ease of a particular form of stimulus but of stimuli as dill'erent. as mechanical, thermal, electric, and photie.

I shall in a subsequent paper formulate a generalised Law of Ell'ects of Direct and Indirect Stimulus. From the. experiments already described it is seen that:

Dim'rt stimulus I'nrlu-r'r's ivy/alive variation. of lurgm', (.‘HH/J'ttd/I'HN, full of leaf of Mimosa, wlm‘lrir ch'mgr of {la/Ivummcll'ii' neg/(Mildly, (mil ru/m'du/inn (if Ill/3 rale of growl/a.

Indirect stimulus induces positive warm/inn of turf/or, ermuisinn. (‘rM'l/un. of lmtf of Mimosa, “Irv/rind change of galvmuunetrir pusilim'ly, and mew/era- time of (he rate of grow/Ii.

It is seen that Indirect stimulus giVes rise to dual reac- tions, seen in positive and negative response. ; of these the negative is the more intense. When the intervening distance is reduced, the resulting reSponse becomes negative ; this is due not to the absence of the positive, but to its being masked by the predominant negativtu From the principle of continuity, this Wlll also hold good in the limiting case, where by the reduction of the intumrning distance to zero, the stimulus becomes Direct. In other words, Direct stimulus should also give rise to both positive and nega-