38 PLANT RESPONSE 



helpful in the future if we uniformly distinguish (i) the true 

 excitatory or normal as the direct, and (2) this positive or 

 abnormal as the indirect effects of stimulation. 



Discrimination of differences of excitability by electric 

 test.— Not only does the electrical response enable us to detect 

 the state of excitation, but I have been able further to devise 

 an electrical test by which differences in the natural excita- 

 bility of two points might be distinguished. The demonstra- 

 tion of the existence of two electrical responses, one of which 

 alone, namely the negative, constitutes the true excitatory 

 effect, is of much theoretical interest. For the hydraulic, or 

 positive electrical effect, has been mistaken for the true 

 excitatory response in plants. As the excitatory effect in 

 animal tissues, moreover, was known to be negative, this fact 

 was supposed to indicate a difference between the proto- 

 plasmic reactions of animal and vegetable. But the experi- 

 ments which I have just described conclusively prove that 

 such a difference does not exist, the sign of response in 

 animal and vegetable being the same. They offer us an 

 explanation, further, of the source of error. A more detailed 

 account of this subject will be found in my forthcoming work 

 on the Electro-Physiology of Plants. 



Many of the motile phenomena of mechanical response 

 which we shall have to study in the course of the present 

 work are modified by differences of excitability at different 

 parts of a tissue. In the case of the primary pulvinus of 

 Mimosa for example, we can see how the responsive fall is 

 brought about by the evidently greater excitability of the 

 lower half of the pulvinus. But in organs which apparently 

 exhibit little motility, it is impossible from inspection to 

 know whether all parts of the tissue are equally excitable, 

 and, if not, which parts exhibit the greater excitability. 

 Such variation of excitability is often due to invisible molecular 

 differentiation, and eludes visual scrutiny. Fortunately, as 

 already said, I have been successful in devising a mode 

 of electrical investigation by which this differentiation is 

 detected with the greatest certainty. This method will be 



