IRRITABILITY. 565 



stimulation, and who described it as follows : " the cells, 

 instead of being filled with homogeneous purple fluid, now 

 contain variously shaped masses of purple matter, suspended 

 in a colourless or almost colourless fluid." Darwin came to 

 the conclusion that the " masses of purple matter " consist of 

 protoplasm, a view which, though rejected by Cohn, who 

 considered that these bodies consisted of cell-sap, was re- 

 asserted by F. Darwin. Schimper's observations, and those 

 of Gardiner, place it beyond doubt that Cohn's view is the 

 correct one. From Gardiner's observations it appears that 

 the process of aggregation takes place in the following man- 

 ner : the colourless protoplasm swells up very considerably 

 by taking up water from the cell-sap, and, in consequence of 

 this removal of a portion of its water, the tint of the cell-sap 

 becomes deeper : then the rate of rotation of the protoplasm 

 becomes much more rapid, with wave-like elevations making 

 their appearance on the internal free surface of the endoplasm ; 

 this leads to a churning up, as it were, of the cell-sap, which 

 becomes distributed in drops of various form throughout the 

 protoplasm, these drops subsequently becoming rounded as 

 the rate of rotation diminishes. This change in the condition 

 of the protoplasm of its cells is accompanied, as Darwin 

 ascertained, by flaccidity of the tentacle, so that probably 

 water escapes from the cells in the process of aggregation. 



Gardiner has observed in the stalk-cells of the tentacle of Drosera 

 and in the mesophyll-cells of Dionaea a spindle-shaped or acicular body 

 which he terms the plastoid or rhabdoid. It appears to be protoplasmic, 

 and, in the resting state of the cells, it stretches diagonally across the 

 cell, its two ends being embedded in the protoplasm. When the tentacle 

 is stimulated, this body tends to become spherical. This change in form 

 of the rhabdoid does not take place when the cell is plasmolysed. It is 

 not clear what the significance of this body is, but it clearly affords an 

 indication of the state of the protoplasm. From the behaviour of the 

 rhabdoid it appears that stimulation induces a change in the protoplasm 

 which leads to a diminished turgidity ; but that this change is not induced 

 when the turgidity of the cell is diminished by plasmolysis, for then the 

 rhabdoid does not change its form. 



The tentacles of Drosera afford us, then, an instance of a 

 loss of turgidity which is brought about by changes in the 

 protoplasm of the cells. But we have yet to determine 



