584 LECTURE XXI. 



connected by an open tube filled with water, the disturbance 

 of the hydrostatic equilibrium due to the forcing of water into 

 the tube in consequence of the stimulation of one pulvinus 

 might be propagated to the other pulvinus and might act as a 

 stimulus upon it, a conception which might be realised by 

 means of the vessels of the wood. But Dutrochet clearly 

 shewed, and in this he is confirmed by Pfeffer, that the vessels 

 of the wood are, as a matter of fact, not concerned in the 

 transmission of the stimulus. We have to believe, then, that 

 a disturbance of the hydrostatic equilibrium can be propagated 

 for a considerable distance in the plant through a series 

 of closed cells. It is a question whether or not such a propa- 

 gation is possible ; but even assuming that it is, there is the 

 question of fact to be decided, whether there is any ground 

 for assuming that the forcing of water out of the parenchyma- 

 tous cells of the pulvinus on stimulation necessarily affects the 

 fluid-pressure in the fibrovascular tissue. It is admitted on 

 all hands that the water which is expelled from the irritated 

 cells of a pulvinus is driven into the intercellular spaces, 

 so that we cannot assume that more than a portion, probably 

 a very small portion, of it enters the fibrovascular tissue. 

 There is, in fact, no direct proof that any water is forced into 

 the fibrovascular tissue of the pulvinus. 



It appears from these considerations that the theory which 

 regards the transmission of a stimulus as a mere propagation 

 of a disturbance of hydrostatic equilibrium is not satisfactorily 

 established. This theory is altogether too purely mechanical 

 to account for so remarkable a phenomenon in a living 

 organism, and we must endeavour to establish for it some 

 other explanation which, while borne out at least as fully 

 by the facts, will be more in harmony with our conceptions of 

 the organisation of living beings. 



We learned in one of the earlier lectures (p. 23), that the 

 protoplasm of adjacent cells is, in many cases and perhaps 

 universally, continuous through the cell-walls. This impor- 

 tant fact, for which we are indebted mainly to Gardiner's 

 researches, may help us in our endeavour to ascertain the 

 means by which the transmission of a stimulus may be 



