CONDUCTIVITY AND EXCITABILITY 229 



is found to be conducted through the anaesthetised area, and 

 to produce responsive depression, not only of the leaflets 

 beyond, but also of those of other leaves. 



This experiment is important in its relation to the theory 

 of the mode of transmission of excitation. I have already 

 adduced conclusive proofs that the conduction of stimulus is 

 dependent, not on the mere mechanical transmission of 

 hydrostatic disturbance, but on the propagation of proto- 

 plasmic changes. Strong support has been lent to the 

 hydro-mechanical theory by a classical experiment in which 

 the pulvinus of a leaf of Mimosa was chloroformed. On 

 then strongly exciting the leaflets of this leaf, the ex- 

 citation was found to be conducted across the anaesthetised 

 pulvinus and to produce depression of leaves beyond. At first 

 sight it was natural to suppose that, as the motile excitability 

 of the pulvinus was abolished by chloroform, the conductivity 

 must also have been abolished. It was therefore inferred 

 that, unlike the conduction of stimulus in animal tissues, 

 where such transmission takes place by the propagation of 

 protoplasmic changes, the conduction of excitation in the 

 plant was purely mechanical. It will be seen, however, that 

 the assumption on which this conclusion is based — that con- 

 duction must necessarily be abolished, with the abolition of 

 motor excitability — has been invalidated by the experiments 

 which I have just described. 



In the present chapter, then, it has been shown that those 

 agencies which, like cold, anaesthetics, and fatigue, diminish 

 molecular mobility, also diminish the excitability and conduc- 

 tivity of the plant-tissue. I shall in the next chapter describe 

 a series of experiments on the profound excitatory changes, 

 of opposite character, which are induced in the experimental 

 tissue, by the passage of an electrical current, the nature of 

 such changes being dependent on the question whether the 

 current enters or leaves the tissue at a given point. It must 

 be added that this series of observations will be found to 

 offer a further disproof of the hydro-mechanical theory of 

 conduction of stimulus. 



