586 LECTURE XXI. 



traversed the chloroformed portion of the leaf. He assumes 

 that since, as we know with regard to the pulvinus, the irrita- 

 bility of protoplasm is abolished by chloroform, the conducti- 

 vity of protoplasm is similarly affected, an assumption for 

 which there is no justification. This experiment, then, fails to 

 overthrow the theory that the stimulus is transmitted from 

 cell to cell by the protoplasm. 



The theory which we substitute for what we may term the 

 water-theory of the transmission of stimuli in Mimosa is 

 briefly this : that stimuli are transmitted by means of the pro- 

 toplasm from cell to cell in some portion of the fibrovascular 

 tissue. It is possible to define the portion of the fibrovascular 

 tissue in which the stimuli travel. Dutrochet found that a 

 stimulus was transmitted from one leaf to another when the 

 intermediate portion of the stem was cut away at some point 

 so as to leave only the outer parts of the fibrovascular bundles 

 intact, and he came to the conclusion that the stimulus travels 

 in these remaining parts of the fibrovascular bundles through 

 certain structures which he terms " tubes corpusculiferes ". 

 Now the researches of Russow have shewn that the continuity 

 of the protoplasm of adjacent cells is especially well-marked 

 in the bast-parenchyma. It seems probable, therefore, that 

 Dutrochet's " tubes corpusculiferes " are the elongated bast- 

 parenchyma cells of Mimosa. 



But it will be objected that if the stimulus travels from 

 cell to cell by means of the protoplasm, it ought to be trans- 

 mitted equally well by the cortical or medullary parenchyma- 

 tous tissues, for the protoplasm of these cells is probably also 

 continuous. Gardiner has, in fact, shewn, that the protoplasm 

 of the cortical parenchymatous cells of the pulvinus is con- 

 tinuous. But according to Dutrochet's observations, these 

 tissues do not transmit a stimulus. In reply to this objection 

 it may be pointed out that the conduction of a stimulus of a 

 particular kind from one cell to another is not a necessary 

 consequence of the continuity of the protoplasm. The con- 

 clusion from these facts is simply this, that the bast-paren- 

 chyma in the plant constitutes a system of tissue which 

 possesses the property of conductivity in a high degree. 



