UNIVERSALITY OF SENSITIVENESS IN PLANTS 41 



tion of response in one case— such as fatigue, temperature 

 changes, stimulating or depressing chemical reagents — act 

 in the same way in the other. 



The capability of responding, so long regarded as the 

 peculiar characteristic of the organic, is also found in the 

 inorganic, and seems to depend in all cases, both qualitatively 

 and quantitatively, on the condition of molecular mobility. 

 In the course of the present work, then, the term ' physio- 

 logical ' is to be understood as a convenient expression 

 for the phenomena of plant or animal tissues under in- 

 vestigation, and not as in any sense opposed to the word 

 ' physical.' 



Summary 



Stimulus causes molecular derangement in matter. The 

 conditions of molecular upset and return to the state of equi- 

 librium correspond to the state of excitation and recovery 

 from that state. 



The molecular disturbance is attended by various physico- 

 chemical changes in the properties of the substance, among 

 the most important of which in a living tissue may be men- 

 tioned (i) contraction of the excited cell, and expulsion 

 of water ; (2) electromotive variation ; (3) conductivity- 

 variation. 



The true excitatory change causes negative variation of 

 turgidity, with depression or negative mechanical response 

 of the leaf, and galvanometric negativity. The intensity of 

 these effects varies with the physiological condition, being 

 totally abolished by those molecular changes which are con- 

 comitant with the death of the tissue. 



By means of electrical response it is found that, not 

 sensitive plants alone, but every plant and every organ of the 

 plant is excitable. 



Positive turgidity-variation, by whatever means produced, 

 causes positive mechanical response — or, in the case of 

 Mimosa, erection of the leaf — and galvanometric positivity. 



