IRRITABILITY. 581 



We are led to conclude that the difference in turgidity 

 upon which curvature depends is brought about by changes 

 in the protoplasm of the cells of the two sides. In accord- 

 ance with what has been said with reference to other modifi- 

 cations of turgidity, we attribute the diminution of the 

 turgidity of the cells of the concave side to an increased 

 permeability of the protoplasm in consequence of which 

 water is forced out of them. With regard to the expansion 

 beyond the normal which the cells of the convex side 

 undergo, this appears to be due to an increased motility of 

 the protoplasm in consequence of which it yields more readily 

 to the pressure of the cell- sap. We will defer the full dis- 

 cussion of this very important point for a few minutes. 



Now as to the curvatures of unicellular organs. Inasmuch 

 as there is no ground for the assumption that the rigidity of 

 the cell-wall is increased on the concave and diminished on 

 the convex side, and inasmuch as the pressure of the cell-sap 

 must be the same at all points, it is clear that curvature must 

 be due to changes in the properties of the protoplasm of the 

 two opposite halves of the cell. Without pretending to say 

 precisely what these changes may be, we may suggest that 

 they consist in a modification of the motility of the protoplasm 

 of one or both sides of the cell ; the motility of the protoplasm 

 of the concave side may be diminished, or that of the con- 

 vex side increased, or, as is more probable, in view of the 

 curvature of multicellular organs, both these effects may be 

 simultaneously produced. In any case the result is that the 

 protoplasm of one side yields less readily to the pressure of 

 the cell-sap than the other. 



The changes which the protoplasm of a unicellular organ 

 undergoes in connexion with curvature are not such as affect 

 the permeability of the protoplasm, for in this case the mecha- 

 nism of curvature is in no wise dependent upon an escape of 

 water. The consideration of these phenomena again raises 

 the question as to whether or not a diminution in the turgidity 

 of cells is not in all cases accompanied by a molecular change 

 of the nature of a contraction in the protoplasm, and an 

 increase in turgidity by an active expansion of the protoplasm. 



