TACTILE BRISTLES OF CARNIVOROUS PLANTS 593 



Fig. 244. 



Part of a longitudinal section 

 through the hinge of a tactile 

 bristle of Bionaea muscipula, 



Oudemans ; its position may be detected from the outside by the 



presence of a furrow which completely encircles the bristle at this 



level (Figs. 243, g, 244). It consists of a central core of tissue 



surrounded by a ring of vertically elongated, 



tabular or wedge-shaped sensory cells, repre- 

 senting modified epidermal elements. These 



sensory cells contain a massive protoplast 



with a centrally situated nucleus (Fig. 244); 



their outer walls are thick, except for the 



portion immediately underlying the transverse 



furrow, and are covered by a well developed 



cuticle, which is furnished with numerous 



minute nodules and serrations upon its inner 



surface. The radial, longitudinal and inner 



walls of the sensory cells, which are also 



fairly thick, are traversed by delicate proto- 

 plasmic fibrils, so that the protoplasts of the 



sensory cells communicate not only with one 



another, but also with the living cells of the 



central core. This core usually consists of 



two superimposed tiers of cells ; in a median 



vertical section each tier is seen to be three *SSm ycells ' 



cells wide. The w T alls of these central cells 



are thick and highly refractive ; their middle lamellae enclose numerous 



microscopic granules and rods of a cutin-like material (Fig. 244). 



In all probability, the principal 

 function of the central core consists 

 in the transmission of the excitation 

 produced by deformation of the sen- 

 sory elements to the pedestal of the 

 bristle, and thence to the tissues of 

 the lamina. 



4. The lowermost portion of the 

 bristle takes the form of a cylindrical 

 pedestal composed entirely of paren- 

 chymatous cells. It is important to 

 note that the pediment is widest 

 above, and contracts somewhat to- 

 Consequently, when 

 the bristle is violently deflected it 



bends to a certain extent at the base, so that the delicate hinge is 



preserved from excessive deformation. 



As Goebel first remarked, it is the hinge-cells which suffer the 



2r 



Fig. 24.-.. 



Diagram showing the effects of bending a 

 tactile bristle of Dionnea mutcipula. The 

 furrow encircling the hinge is partially flat- 

 tened out on the convex side (a), but is made wtitIg ire Kaoo 

 deeper on the concave side (6). 



