354 RESEARCHES ON IRRITABILITY OF PLANTS 



also be arrested by the action of electrotonic block. This 

 arrest persists during the continuation of the blocking 

 current, the conductivity being restored on its cessation 

 (p. 167). 



Block of conduction by action of poison. — Finally, the 

 conductivity of a selected portion of a petiole may be 

 abolished by the local application of poison. The abolition 

 of conducting power proceeds slowly under the action of 

 copper sulphate solution and quickly under potassium 

 cyanide (p. 173). 



These results prove conclusively that the transmission 

 of excitation in plant is a process fundamentally similar 

 to that which takes place in the animal, being in the one 

 case as in the other a propagation of protoplasmic change. 



Direct and Indirfxt Effects of Stimulus 



When stimulus is applied directly on the responding 

 organ there is induced an excitatory fall of the leaf, con- 

 comitant with contraction and negative turgidity-variation. 

 This particular reaction is designated as the Direct Effect 

 of Stimulus. When, on the other hand, a feeble stimulus 

 is applied at a distance there occurs only a positive or 

 erectile response of leaf with concomitant expansion and 

 positive turgidity-variation. This particular reaction is 

 designated as the Indirect Effect of Stimulus. If the 

 intervening tissue be highly conducting and the stimulus 

 sufficiently strong, then the excitatory negative effect 

 masks the positive. In such a case the response to indirect 

 application of stimulus is negative — that is to say, the same 

 as caused by direct stimulation. But, if the intervening 

 tissue be semi-conducting, or if the stimulus be feeble or 

 applied at too great a distance, then there is induced the 

 positive or Indirect Effect (p. 196). 



These two opposite reactions are found to take place 

 in various plants under definite conditions and under diverse 

 forms of stimuli. The two opposite effects are demonstrated 





