CHAPTER III 



ON THE UNIVERSALITY OF SENSITIVENESS IN PLANTS 

 AS DEMONSTRATED BY MEANS OF ELECTRICAL RESPONSE 



Arbitrary classification of plants into sensitive and ordinary — Method of electro- 

 motive variation for detecting state of excitation — Hydraulic model — Excita- 

 tion of vegetable tissue, like that of animal tissue, induces galvanometric 

 negativity — Methods of direct and transmitted excitation — Electrical and 

 mechanical response alike record molecular derangement and recovery — 

 Similarities in simultaneous record of mechanical and electrical response — 

 True excitation has a concomitant negative turgidity-variation, negative 

 mechanical response or fall, and galvanometric negativity — These are true 

 physiological responses, and are abolished at death — Abnormal positive 

 mechanical and electrical responses brought about by positive turgidity- 

 variation — Direct and indirect effects of stimulation— Discrimination of differ- 

 ences of excitability by electric test — Excitability of plant-tissues in general — 

 Responsive power characteristic of matter. 



We have seen that when stimulus is applied to a sensitive 

 organ like the pulvinus of Mimosa there is a fall of the leaf, 

 which fall is due to the excitatory contraction of the more 

 excitable lower half of the pulvinus. 



Ordinary plants are said to give no motile indications, 

 hence they are usually regarded as insensitive. 1 It is difficult, 

 however, to conceive that while the protoplasm of certain 

 plants reacts to stimulus, that of others should not do so. On 

 the other hand, it may be that the absence of mechanical 

 response in these ordinary plants is not due to any want of 

 excitability, but rather to the fact that conditions favourable 

 to the conspicuous exhibition of motile effects do not in 



1 Vines has already drawn attention to the possibility of error here : We 

 must be careful not to assume that irritability is restricted to growing and to 

 motile organs. For all we know to the contrary, it is possessed by the proto- 

 plasm of all plant organs, and if in any case the action of a stimulus is not 

 followed by a responsive movement, we must, before we assume the absence of 

 irritability, assure ourselves that the structure of the organ is such that a move- 

 ment is a mechanical possibility.' — Vines, Physiology of Plants, 1886, p. 372. 



