270 Mr W. Gardiner, On the continuity [Nov. 13, 



parts of the organ. The water was the cause of the propagation 

 of the stimulus. This explanation is, however, unsatisfactory. 

 That a stimulus is attended by an escape of water into the inter- 

 cellular spaces is certain, for on stimulation the under side of the 

 pulvinus is seen to assume a darker green colour due to the fact 

 that the escaping water displaces the air in the intercellular spaces. 

 But Pfeffer, who described the phenomenon, says himself, that 

 when he touched a point of the irritable side he saw the darker 

 colour spread instantaneously from the point of contact. We can- 

 not imagine that the mere passage of water from cell to cell can 

 explain this. We should have to assume such a great velocity and 

 such a high pressure that rupture of the tissue must occur. Some 

 other means of propagation of the stimulus must take place. The 

 water must be the effect, not the cause. From a consideration of 

 this and similar phenomena, Professor Sachs suggested to me that 

 I should investigate in as complete a manner as possible the micro- 

 scopical structure of pulvini, and endeavour to ascertain among 

 other things how this propagation of the stimulus occurred. My 

 results in the main confirmed those of other observers. One im- 

 portant additional fact which struck me was the uniform occurrence 

 of very numerous pits on the walls of the parenchymatous cells, 

 and it seemed not improbable that, through these pits and by 

 means of fine protoplasmic filaments, continuity of the protoplasm 

 from cell to cell was established. That this is the case I have 

 succeeded in demonstrating. 



My method of investigation depends on the fact that cellulose 

 but not protoplasm is dissolved by strong sulphuric acid. This 

 fact had been made use of by Sachs to demonstrate the continuity 

 of protoplasm in sieve tubes. Mine is a modification of his 

 method. The usual plan was to treat thin sections with Iodine 

 by which means the protoplasm was stained. They were then 

 mounted in strong Sulphuric acid — the cell Avail dissolves, the 

 stained protoplasm is left. The method is however open to two 

 objections — First, the protoplasm is not sufficiently deeply stained; 

 Secondly, one cannot regulate the action of the acid which in 

 a short time will begin to act on the protoplasm also. I divided 

 the process into two parts. The sections were first cautiously 

 treated with Sulphuric acid and well washed with water to get rid 

 of the reagent. Thus its action could be stopped at any desired 

 point. Then the section was stained in Aniline blue and mounted. 

 The protoplasm was contracted, the cell wall partially dissolved, 

 and although the middle lamella still remained it was easy to 

 make out that the processes from one cell to another were opti- 

 cally continuous. I have dwelt on the pulvinus of Mimosa because 

 it affords the best text for such a sermon. I cannot however doubt 

 that the same means of communication exist in organs other 



