196 COMPARATIVE ELECTRO-PHYSIOLOGY 



on reaching the death-point, the contained positive com- 

 ponent in response was unmasked by the abolition of the true 

 excitatory effect. But this positive response disappears also 

 after a short time. It will thus be seen that by this method 

 the death-point is capable of determination within very narrow 

 limits, having been, in the present case, near 60 C. When 

 the tissue is thin, this temperature soon proves fatal. But 

 should it be thick, a very much longer exposure to it 

 is necessary, if the interior of the tissue is to be killed 

 effectively. 



I have also discovered another method of obtaining the 

 death-point with precision, in the symptoms afforded by 

 mechanical responses. For I found that a death-spasm occurs 

 at a certain critical moment in a plant, which is analogous 

 to the death-throe of the animal. The experimental plant 

 Mimosa> for instance was placed in a bath of water, whose 

 temperature was being raised gradually, at a uniform rate of, 

 say, i C. per one minute and a half, until the death-point was 

 reached. During all this time there was no responsive fall 

 of the leaf, for, we have seen, it is a sudden variation, and not 

 a gradual rise of temperature, which acts as an excitatory 

 stimulus. This gradual rise, on the other hand, increases the 

 internal energy of the plant, by which the turgidity of the 

 pulvinus is continuously augmented. In this process the 

 increase of turgidity is more energetic in the more excitable 

 lower half than in the upper. The greater expansion of the 

 lower side of the pulvinus thus raises the leaf continuously. 

 But immediately on reaching the death-point, there is a 

 reversal of this movement, and an abrupt fall of the leaf. 

 This spasmodic movement is sudden and well-defined. In 

 a vigorous Mimosa the death-spasm is found to occur at 

 ,or very near 60 C. This contraction of death is followed 

 jafter some time by a post-mortem relaxation. 



That the death-response is an excitatory phenomenon 

 is seen from the fact that any circumstance which lowers 

 physiological activity lowers the death-point also. Thus, 

 after a spell of cold weather, I found that the death-point 



