POSITIVE AND NEGATIVE TURGIDITY-VARIATIONS 65 



TYPICAL CASES OF RESULTANT RESPONSE 





We thus see how, under the physiological modifications 

 induced by various agents, the normal negative response of 

 a tissue may undergo diminution, or even reversal. We 

 have, for the sake of simplicity, assumed that the two 

 antagonistic effects act on the specimen simultaneously. 

 But, as a matter of fact, under certain circumstances, their 

 time-relations may be subjected to change. Hence various 

 effects of interference may be seen, giving rise to diphasic 

 responses, such as positivity followed by negativity, or the 

 reverse. Or, instead of diphasic, there may even be multi- 

 phasic responses, since it will be shown that very strong 

 stimulus may cause not a single but repeated responses. 

 Some of these effects will be described in detail in a sub- 

 sequent chapter. 



I shall here meanwhile describe another method, in which, 

 by means of a selective physiological block, we can unmask 

 the contained hydrostatic effect of positivity, from the 

 ordinary response of galvanometric negativity. The oc- 

 currence of the excitatory contraction, in Mimosa for 

 example, as shown by the fall of the leaf, depends on a 

 favourable excitatory condition of the tissue. If this motile 

 excitability should be in any way depressed or abolished as, 

 say, by application of ice-cold water on the pulvinus, the 

 true excitatory response could not take place. But if, under 

 these circumstances, we applied stimulus at the diametrically 

 opposite point s, (fig. 42), the hydrostatic wave alone would 

 be found to reach the organ and, m et armis, to produce 

 expansion there, with a consequent erection of the leaf. 

 It will thus be seen that the hydrostatic transmission of the 

 indirect effect of stimulus is not to any great extent affected 



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