438 PLANT RESPONSE 



of the organ was, in some cases, brought about by that in- 

 creased suctional activity by which energy was transmitted, 

 by means of water forced into the responding organ. This 

 increased suctional activity was due in its turn to external 

 stimulation of the roots. Or a similar transmission of energy 

 to the responding organ might be the result of stimulation 

 applied at a distance on the stem, in consequence of which a 

 wave of positive turgidity-variation would travel towards the 

 responding organ from the excited point. For the exhibition 

 of this latter effect, however, it is necessary that the direct 

 excitatory effect of stimulation should not be conducted to 

 the organ. This condition is met if the point of stimulation 

 be at a sufficient distance, or when the intervening tissue is 

 not a good conductor of excitation. Again, the plant as a 

 whole, it must be remembered, has its internal energy raised, 

 as the after-effect of the absorption of stimulus from its sur- 

 roundings. 



Direct and indirect effects of stimulus and laws of 

 growth.- — In the phenomenon of growth-response we have 

 a case which is exactly parallel. Direct stimulation of a 

 growing organ always induces a negative turgidity-variation, 

 with a concomitant responsive retardation of growth. Any- 

 thing, on the other hand, which increases the internal energy, 

 brings about the opposite effect, of positive turgidity-variation, 

 with concomitant responsive increase in the rate of growth. 

 This may be induced by a favourable rise of temperature, 

 or by stimulating the root, and so increasing the ascent of 

 sap. It may also be brought about by stimulating a distant 

 point, and so causing a wave of positive turgidity-variation 

 to be transmitted to the responding organ, the stimulated 

 point being at a sufficient distance to prevent the direct effect 

 of stimulus from reaching it. And, finally, the energy ab- 

 sorbed from external stimulus may, as an after-effect, increase 

 the internal energy of the plant. The increase of internal 

 energy, under all these different conditions, we shall for the 

 sake of convenience designate as the INDIRECT EFFECT OF 

 STIMULUS. 



