CONDUCTIVITY AND EXCITABILITY 21/ 



second, the differential excitability, on which responsive 

 curvature depends, was also slight. It is only at the pul- 

 vinated organ that the state of excitation is conspicuously 

 exhibited by motile response. 



In the case of the plant, therefore, we have to study the 

 excitable property of the receptive area, the conducting 

 property of the transmitting tissue, and that property of the 

 responding organ by which the excitatory effect is outwardly 

 manifested. It will be convenient to distinguish the excit- 

 ability at the point of application of stimulus from that of 

 the motor region, by using a specific term for the former. I 

 shall therefore designate it as receptive excitability, or merely 

 as receptivity, whereas the excitability of the motor region 

 will be described simply as excitability. At the point ot 

 application, the stimulus comes from outside, and produces 

 internal changes. In the motile region, the internal excita- 

 tory disturbances are manifested outwards. In the physio- 

 logical study of excitation, some confusion is apt to arise 

 from the failure to discriminate between these three factors. 

 And this confusion becomes greater in those cases in which 

 the area of motile excitability coincides with that of 

 receptivity. 



Molecular model. — The state of excitation being ulti- 

 mately due to molecular upset from the position of equi- 

 librium, we can understand that such a disturbance is 

 propagated from molecule to molecule, till it reaches the 

 responding organ. We may, perhaps, be enabled to 

 visualise this better by means of a mechanical model. The 

 individual molecules in our model should hold a position of 

 stable equilibrium. When disturbed from this stable poise, 

 they should return automatically to the equilibrium position ; 

 and further, the derangement of one molecule should cause a 

 subsequent disturbance of the next, and this disturbance 

 should be transmitted from point to point. 



These conditions are realised in the case of the following 

 model, which consists of a row of small suspended spheres of 

 cork, within each of which is placed a magnetic needle. Each 



