IRRITABILITY. 585 



effected in a plant. It is a well-known fact in physiology 

 that living protoplasm is capable of transmitting a stimulus ; 



a 



^T_i^4 a 



Fig. 64 (after Gardiner). Continuity of the protoplasm of adjacent cells of the en- 

 dosperm of a Palm-seed (Bentinckia) : a, contracted protoplasm of a cell; b, a 

 group of delicate protoplasmic filaments, passing through a pit in the cell- wall. 



in fact, the nerve-fibres of animals are simply protoplasm 

 which has been specially modified for the performance of 

 this very function. We may suggest, then, that a stimulus 

 travels through a Mimosa-stem from cell to cell by means 

 of the delicate filaments which place the protoplasm of the 

 cells in direct communication. 



The possibility of a vital, as distinguished from a mechani- 

 cal, transmission of a stimulus from cell to cell had occurred 

 to Pfeffer, but he believed that he had disproved it by 

 the following experiment. He placed a leaf of a plant of 

 Mimosa so that the middle portion of it was enclosed in 

 a vessel containing chloroform-vapour in sufficient proportion 

 to abolish the irritability of the pulvinus. Under these cir- 

 cumstances he found that a stimulation of the leaflets pro- 

 jecting beyond the vessel caused a movement in another leaf 

 borne on the stem. The stimulus obviously travelled through 

 that portion of the leaf which was exposed to the action of the 

 chloroform From this he infers that the transmission of the 

 stimulus is not vital, for, had it been so, it could not have 



