196 RESEARCHES ON IRRITABILITY OF PLANTS 



of erection of leaf with concomitant expansion and positive 

 turgidity variation. For simplicity we shall designate 

 this particular reaction 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 effect of 

 indirect stimulation is the same as that caused by direct 

 stimulation. But if the intervening tissue be semi-conduct- 

 ing, or if the stimulus be feeble or applied at too great a 

 distance, then we obtain the positive or Indirect Effect. 



Two diametrically opposite effects may thus be induced 

 by an identical stimulus, depending on direct or indirect 

 application. The existence of the positive or indirect 

 effect of stimulus has hitherto been unsuspected. It 

 must be taken into full account in unravelling the com- 

 plexities of reaction in a responding organ. 



The laws of Direct and Indirect Effects of stimulus 

 may thus be enunciated : — 



The effect at the responding region of a strong excitation 

 transmitted through a short distance, or through a good con- 

 ducting channel, is negative, being the same as the effect under 

 direct stimulation. The response is by negative turgidity vari- 

 ation, contraction, fall of leaf, and electrical change of galvano- 

 metric negativity. This is the Direct Effect of Stimulus. 



The effect of feeble stimulus transmitted through a great 

 distance, or through a semi-conducting channel, is positive. 

 The responsive reaction is by positive turgidity variation, expan- 

 sion, erection of leaf, and electrical change of galvanometric 

 positivity. This is the Indirect Effect of Stimulus. 



Summary 



A single stimulus gives rise in the plant to two impulses, 

 positive and negative. The positive travels at a faster rate 

 and induces positive or erectile response of the responding 

 leaf. The negative or excitatory impulse travels at a slower 

 rate and induces in the responding leaf a contractile fall. 

 The laws of Direct and Indirect Effects of stimulus are — 



