1882.] of protoplasm in the motile organs of leaves. 2G9 



Now as to the cause of these movements. If a spirogyra 

 filament be examined it will be seen that the transverse wall 

 between any two cells will be perfectly horizontal and at right 

 angles to the long axis of the whole filament. If, however, one 

 of the cells happens to be unhealthy from the attacks of para- 

 sites or otherwise, then the walls of the healthy cells on either 

 side will no longer be horizontal but will project convexly in- 

 wards into the injured cell. The cubical contents of the healthy 

 cells will in fact become greater — that of the unhealthy less. Now 

 this is due to the fact that the layer of protoplasm surrounding 

 the cell wall — the primordial utricle as it is called — possesses the 

 power of absorbing into its interior large quantities of water, and 

 thus becoming turgescent. In healthy cells the hydrostatic pres- 

 sure of the one is resisted by the equal pressure of its neighbour. 

 The wall between them will remain horizontal. But if the proto- 

 plasm be unable to maintain its turgidity, then its resistance to 

 pressure will be less, and the common wall will be forced inwards 

 by the increased pressure caused by the additional quantity of 

 water which the neighbouring cell immediately takes up. And 

 this hydrostatic pressure not only keeps the primordial utricle close 

 to the cell wall, but actually expands the elastic cell membrane, as 

 can easily be proved by treating such a cell with any substance 

 which will cause a loss of turgidity, e.g. a solution of nitre. Thus 

 any stimulus which will cause an increase or decrease of turgidity 

 will also cause a contraction or expansion of the elastic cell wall. 

 And this is what happens in Mimosa. We may regard the ex- 

 panded position as the resultant of two equal and, for the time, 

 opposite forces ; the weight of the leaf and the pressure exerted 

 by the cells of the upper half of the pulvinus tend to force the 

 leaf down, while the turgescence of the cells of the lower half 

 exactly counteract this downward pressure ; hut by an appro- 

 priate stimulus, once cause the lower cells to lose their turgidity, 

 and the equilibrium is destrojed : the downward pressure now 

 exerts its force : the leaf falls. These statements are supported 

 by facts. A stimulus causes a loss of turgescence and an escape of 

 water. 



Now as to the propagation of the stimulus. When the main 

 pulvinus or the terminal leaflets or any part of the leaf is stimu- 

 lated there is a contraction of the whole leaf. But to limit our 

 inquiries we may ask, How is the stimulus propagated from cell to 

 cell in any one pulvinus, The usually received explanation is that 

 as a result of a stimulus to any given point of the pulvinus the cell 

 or cells affected suffer a disturbance of their coll equilibrium, 

 there is a diminution of turgidity and water escapes. This water 

 as it passes from cell to cell in turn upsets the equilibrium of each, 

 causing a disturbance which is gradually propagated to the other 

 VoL. IV. I'T. V. 20 



