22 COMPARATIVE ELECTRO-PHYSIOLOGY 



varies so much that its motile response would seem at times 

 almost to have disappeared. I have already pointed out 

 that it is by the unequal excitabilities of the upper and 

 lower halves of the pulvinus that that differential contrac- 

 tion is induced which brings about the lateral response of 

 the Mimosa leaf. Now it is clear from this that if the 

 differential excitability should be reduced or abolished, by 

 any means whatsoever, there will then be a corresponding 

 diminution or abolition of response. We shall see later that 

 the excitability of a tissue depends upon its state of turgor, 

 and in Mimosa, from internal causes, a periodic variation is 

 induced in the relative turgescence of the two halves of the 

 pulvinus. We might then expect, in consequence of this, to 

 find a periodic variation of motile sensibility. And certainly, 

 whatever may be the cause, a long course of observation 

 will convince the inquirer of the occurrence of great varia- 

 tions in the sensibility of Mimosa at different times of the 

 day. Thus, I had six specimens of this plant growing in 

 pots in the open, and I found, watching them in the month of 

 August, that at eight o'clock in the morning the pulvini of 

 the leaves of all these plants were sensitive in the highest 

 degree. Half an hour later, however, this sensitiveness had 

 so far waned that they would give hardly any motile indi- 

 cation. It is, perhaps, worth while to remark, in connection 

 with this, that a constant observer is able to judge, by a 

 peculiar, though indescribable, attitude of the leaves, whether 

 or not this condition of insensitiveness has supervened. Thus 

 the mechanical movements of the belauded sensitive plants, 

 such as Mimosa, on which depended the arbitrary assumption 

 that ' ordinary ' plants were insensitive, rest on a basis which 

 is itself extremely unreliable. For by this standard one 

 identical plant ought to be classed as belonging to both the 

 sensitive and insensitive groups, according to the time of day 

 at which any particular observation is made. 



The fact that when the mechanical response of the leaf of 

 Mimosa is physically restrained, excitatory electrical response 

 takes place unimpeded, shows that we have a criterion by 



