592 



SENSORY SYSTEM 



outer walls. The bristle, therefore, bends sharply at the hinge, and the 

 hinge-cells consequently become forcibly extended on the convex, and 

 compressed on the concave side. If the deflection is considerable, the 

 outer wall on the concave side of the hinge actually becomes pushed 

 inwards, so that a transverse furrow is formed (Fig. 242 c). In any 

 case, the protoplasts of the hinge-cells undergo very extensive 

 deformation. The whole structure of the bristle is evidently designed 

 so as to concentrate the mechanical effect of a shock or blow upon 



a limited portion of the organ, the 

 obvious inference being that this 

 portion is responsible for the actual 

 perception of the stimulus. 297 



The tactile bristles of Dionaea 

 muscvpula are much coarser than 

 the corresponding structures of Al- 

 drovandia, but are constructed on 

 the same principle. Either half of 

 the leaf-blade is provided with three 

 bristles, which are placed at the 

 corners of an approximately equi- 

 lateral triangle. The two halves of 

 the lamina will snap together, even 

 if the upper epidermis is briskly 

 rubbed, without any of the bristles 

 being actually touched ; but there is 

 no doubt that the bristles are 

 endowed with a much higher degree 

 of sensitiveness than the rest of the 

 surface, since a very gentle touch 



Fio. 243. 



Basal portion of a tactile bristle of Dionaea 

 museipula in L.S. ; p, pedestal; g, sensitive 

 hinge ; (, zone of tabular cells immediately 

 above the hinge ; e, prosenchymatous cells 

 funning the terminal portion of the bristle. 



suffices to bring about the character- 



istic movement. 



Each bristle is made up of four distinct portions, which may 

 be described in basipetal order as follows : 



1. The stiff, conical, pointed terminal region, which is about 1 mm. 

 in length, acts as a lever, or, in other words, represents the stimulator, 

 or mechanically effective portion of the entire apparatus. It is 

 composed of living cells, which are prosenchymatous, and provided 

 with moderately thick, but unlignified walls (Fig. 243,f). 



2. Next follows a zone of tabular cells, comprising two or three 

 tiers of central, and three or four of epidermal cells. The cells of this 

 zone also invariably contain living protoplasts, in spite of the fact that 

 their walls are more or less suberised. 



3. The sensitive hinge of the bristle was first described by 



