CHAPTER XIII 



THE POSITIVE RESPONSE 



Two opposite kinds of responses, negative and positive — Excitatory 

 contraction, negative turgidity variation, fall of leaf, and concomitant 

 negative electric variation — Positive electric response — Positive or 

 erectile mechanical response — Dual impulses under different forms 

 of stimuli — Exhibition of positive and negative impulses by different 

 plants — Conditions for obtaining positive response — Characteristics 

 of positive impulse — Masking and unmasking of positive effect — Laws 

 of Direct and Indirect effects of stimulus. 



When the leaf of Mimosa is excited, the lower half of the 

 pulvinus undergoes relatively greater contraction ; in conse- 

 quence of this differential action the leaf falls down. There 

 is a concomitant expulsion of water from the excited cells, 

 with diminution of turgor. I have shown elsewhere i that 

 the excitatory reaction of plant tissue may also be detected 

 by a method altogether different — namely, by means of 

 electric variation. As regards the sign of electrical change, 

 the excited point is found to become galvanometrically 

 negative ; a similar electrical change is also known to take 

 place in the excited animal tissue. The excitatory electro- 

 motive change of galvanometric negativity is therefore the 

 same in the plant and the animal. 



Excitation in a plant tissue is thus characterised by 

 concomitant effects of contraction, diminution of turgor or 

 negative turgidity variation, mechanical fall of the leaf, 

 and by the electrical response of galvanometric negativity. 

 For the sake of clearness we shall designate this normal 

 excitatory effect as the negative response. 



In recording electrical responses of plants to indirect 



* Bose : Comparative Electro-physiology (Longmans, Green & Co.)* 



176 



