IRRITABILITY. 559 



Cynareae is to cause shortening of the filament. The move- 

 ment of the filament on stimulation bears a superficial re- 

 semblance to the contraction of a muscular fibre, but it is 

 in reality an altogether different process. The contraction 

 of a muscular fibre is accompanied, as mentioned above, by 

 a thickening, so that the total bulk of the fibre is not per- 

 ceptibly altered ; but the shortening of the staminal filament 

 is, as Pfeffer has clearly shewn, unaccompanied by any ap- 

 preciable thickening, so that the bulk of the filament is con- 

 siderably diminished, and is not due, as Unger thought, 

 merely to a change in form of the cells. Pfeffer has ascer- 

 tained that the diminution in bulk of the filament as a whole 

 is the result of the diminution in volume of its constituent 

 parenchymatous cells. Each cell, on stimulation, parts with 

 a portion of the water which it contains, so that from being 

 turgid it becomes flaccid ; and, inasmuch as the cells are 

 scarcely at all expanded in the tangential direction, the effect 

 of the loss of water is to cause them to shorten. The escape 

 of water, on shortening, from the parenchymatous cells into 

 the intercellular spaces, is not infrequently manifested by 

 the appearance of a drop of water at the cut surface of a 

 filament, an effect which is always produced when the inter- 

 cellular spaces of the filament have been previously injected 

 with water. The mechanism of the movement is then this : 

 when the filament is at rest the cells are turgid, and their 

 elastic cell-walls are on the stretch; on stimulation the escape 

 of water from the cells is rendered possible, and the cells 

 consequently become relatively flaccid and shorten. 



In the other cases of movement induced by mechanical 

 or electrical stimulation, one side only of the organ is af- 

 fected ; the result is that, instead of a shortening of the whole 

 organ, an up or down movement is performed ; but the 

 mechanism is essentially the same. Thus, in the case of the 

 stamens of Berberis, it is the upper or inner surface of the 

 filament which alone is irritable, and it is the corresponding 

 half of it which loses its turgidity on stimulation. The 

 mechanism of the movement is briefly this : the turgidity 

 of the cells of the inner longitudinal half of the filament 



