

POSITIVE AND NEGATIVE TURGIDITY-VARIATIONS 59 



and of true excitation, induce, as we have now seen, opposite 

 responsive reactions. But of these, that due to true excita- 

 tion is, generally speaking, greatly predominant. Hence, 

 when these two waves reach the responding organ in close 

 succession, as is the case when the point of stimulation is 

 very near, the excitatory effect masks the hydrostatic. In 

 order, then, to separate them, we may employ various 

 methods. First, in the case of highly conducting tissues, the 

 stimulus must be applied at a sufficient distance to make the 

 slow excitatory wave lag adequately behind the quickly 

 travelling hydrostatic wave. Or we may choose a direction 

 of transmission of excitation which will be relatively slow. 

 Thus I have found that transmission across a stem, for 

 example, is very much slower than along its length. Hence, 

 on applying moderate stimulus at S / (fig. 42) at a point on the 

 stem diametrically opposite the pulvinus, of the given leaf, it 

 is found that the excitation reaches the pulvinus only after a 

 measurable interval, the hydrostatic effect inducing erectile 

 response much earlier. Thus in a given experiment, whose 

 record was taken on a fast-moving drum (fig. 45), the erectile 

 response took place '6 second after the application of stimu- 

 lus, whereas the true excitatory fall did not occur till 3*45 

 seconds had elapsed that is to say, 2*85 seconds later. It is 

 to be borne in mind that a certain interval of time passes, 

 even after the arrival of the respective waves, before the tur- 

 gidity-variation is able to give rise to the motile indication. 



Let us next examine the results at the responding tissue 

 of the indirect effect of stimulus. The distant receptive point 

 contracts on stimulation, and sends to the responding organ 

 a wave of increased hydrostatic tension. This, as we have 

 seen, forces water in, and expands the tissue. Work is thus 

 done on the tissue which increases its store of energy. In 

 this the indirect is unlike the typical direct effect of stimulus. 

 For the latter causes the impulsive fall of the leaf, which 

 represents work done by the tissue, and an expenditure of 

 energy. We must, therefore, recognise two distinct respon- 

 sive effects, according as the work done is positive done on 



