Page:Life Movements in Plants.djvu/247

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INDIRECT STIMULUS ON GROWTH
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It is thus seen that the effect 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 effect becomes converted into negative or retardation of growth when the stimulus is Direct, i.e., when applied to the responding region of growth; under intermediate conditions, the growth-variation I find to be diphasic, a positive acceleration followed by a negative retardation. This is found true not merely in the case of a particular form of stimulus but of stimuli as different as mechanical, thermal, electric, and photic.

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

Direct stimulus induces negative variation of turgor, contraction, fall of leaf of Mimosa, electric change of galvanometric negativity, and retardation of the rate of growth.
Indirect stimulus induces positive variation of turgor, expansion, erection of leaf of Mimosa, electrical change of galvanometric positivity, and acceleration of the rate of growth.

It is seen that Indirect stimulus gives rise to dual reactions, 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 negative. From the principle of continuity, this will also hold good in the limiting case, where by the reduction of the intervening distance to zero, the stimulus becomes Direct. In other words, Direct stimulus should also give rise to both positive and nega-

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