RESPONSE TO STIMULUS OF LIGHT 393 



tropic are induced in different cases by the same 

 stimulus. These effects, moreover, are found to occur in 

 growing as well as in pulvinated organs. This incon- 

 sistency of effects has been a source of great perplexity, 

 inclining observers to the belief that the action of any given 

 plant organ under light is determined, not by some definite 

 reaction, but by its own power to decide what is for its 

 individual advantage. 



I have shown elsewhere, however, 1 that as regards 

 mechanical response, the reaction of plant organs to the 

 stimulus of light is extremely definite. This, like other 

 forms of stimulus, induces negative turgidity variation and 

 contraction, as well as consequent retardation of growth in 

 growing organs. Such excitatory effects, moreover, if the 

 tissue be of fair conducting power, may be transmitted in 

 either a transverse or a longitudinal direction. The intensity 

 of this transmitted excitatory effect is thus dependent, as I 

 have shown, on the intensity and duration of stimulus and 

 on the conductivity of the tissue. If neither the intensity of 

 the stimulus nor the conductivity of the tissue be great, it 

 will be the indirect or hydro-positive effect which will reach 

 the distant point, there to induce a positive turgidity 

 variation and expansion. The foregoing observations relate 

 to tissues in a normal condition of excitability. When the 

 tissue is sub-tonic, however, the absorbed stimulus, as we have 

 seen, increases the internal energy and brings about a re- 

 sponsive expansion. 



I have also shown that the various mechanical move- 

 ments induced by the unilateral action of light, depend (i) 

 upon whether the stimulus remains localised on the proximal 

 side of the organ or is conducted to the distal ; and (2) on 

 the relative excitabilities of proximal and distal. I have 

 shown, moreover, that all the diverse effects induced by light 

 are demonstrably traceable to the action of these various 

 factors in varying combination. And, finally, certain highly 

 excitable tissues, owing to excess of energy derived from 



1 Plant Response, pp. 551 to 685. 



