POSITIVE AND NEGATIVE TURGIDITY- VARIATIONS 57 



at a greater distance s /y , the hydrostatic wave causing the 

 positive turgidity-variation brings about an erectile twitch. 

 This is followed by a responsive fall, when the true excitation 

 reaches the organ (fig. 43, &). 



It has thus been shown that by separating the responding 

 point from the point stimulated, or the receptive point, it is 

 possible to discriminate two 

 different effects which are both 

 brought about by stimulus. It 

 is most important, moreover, to 

 distinguish between these two 

 factors : namely, the direct effect 

 of stimulus causing contraction, 

 and its indirect effect, causing 

 expansion. We have seen that 

 direct excitation and transmitted 

 excitation both induce contrac- 

 tion, negative turgidity-variation 

 and fall of the leaf. Unfor- 

 tunately, in animal physiology, 



it has been customary to apply 

 the term indirect to that form 

 of stimulation which is applied 

 at a distance. And it has not 

 been noticed that such stimulus 

 is capable of inducing diametri- 

 cally opposite results, according 

 as the true excitatory effect 

 reaches, or does not reach, the 

 responding organ. When the 



FIG. 43. Mechanical Responses 

 of Leaf of Mimosa 



(a) record of responsive fall when 

 stimulus applied near the re- 

 sponding organ ; (b) response 

 when stimulus is applied on 

 same side, but at greater dis- 

 tance, s /y . A preliminary erectile 

 response is here followed by 

 the true excitatory depression. 

 This is due to the indirect effect 

 first transmitted being succeeded 

 by the direct. Had the stimulus 

 applied been feebler, or more 

 distant, there would have been 

 only the first, or indirect erectile 

 effect. 





intervening tissue is highly 



conducting, the transmitted effect induces exactly the same 

 result as if stimulus were applied directly. But we shall 

 see that when the intervening tissue is non-conducting or 

 feebly conducting, true excitation is not transmitted, and the 

 effect which makes its appearance at the responding point 

 is then due to increase of hydrostatic tension, causing positive 



