MECHANICAL RESPONSE IN ORDINARY LEAVES 5/ 



Excitatory reaction in older tissues.— One very impor- 

 tant and hitherto undecided question, which is answered 

 by this experiment, is as to whether all tissues, even the 

 old, give contractile response by expulsion of water, or 

 by negative turgidity-variation. In the pulvinus such an 

 effect is made evident by the differential contractile move- 

 ment produced. In young tissues it will be shown that 

 we have effects exactly similar. But in older tissues no 

 such movement is observable. Of this fact, two alternative 

 explanations are possible: (i) there may be no excitatory 

 expulsion of water in old tissue ; (2) such expulsion may 

 occur, while movement is at the same time rendered 

 impossible, by the inflexible condition of the tissue. We 

 have already seen how, whenever there is negative turgidity- 

 variation due to excitation, there is also a simultaneous 

 galvanometric negativity. Conversely, induced galvanometric 

 negativity may be taken as an indication of the excitatory 

 expulsion of water. And from this indication the con- 

 clusion is arrived at that, even in relatively old tissues, this 

 excitatory expulsion occurs, since these tissues on excitation 

 exhibit galvanometric negativity. 



I was desirous, however, to obtain additional proof ot 

 this excitatory expulsion of water in the older tissues, and 

 from the preliminary positive twitch seen in the mechanical 

 response (fig. 32) the existence of such action is clearly 

 proved. For the preliminary erectile twitch observed in the 

 leaf, when the rigid part of the petiole — or of the stem at 

 a distance beyond the petiolar junction — is excited, can 

 only be explained by a pulse of increased pressure towards 

 the motile area, initiated by water expelled from the 

 stimulated point. Independent experiments, carried out by 

 other methods, also tend to show that, though the power of 

 excitatory contraction undergoes diminution with age, it 

 does not, generally speaking, disappear entirely. 



We saw in Biophytujii that when stimulus was applied 

 at a distance, the hydrostatic disturbance produced the 

 preliminary erectile twitch. But no such effect was observed 



