TACTILE BRISTLES OF CARNIVOROUS PLANTS f.fll 



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muscipula, possess very highly specialised tactile bristles. The sensory 

 function of these structures was first recognised by Sydenham 

 Edwards (1804) in the case of Dionaea, and by F. Colin (1861) in 

 that of Aldrovandia. 



In Aldrovandia vesiculosa, which is a submerged aquatic plant, the 

 two approximately semicircular halves of the leaf-blade close together 

 instantaneously, when one of the tactile 

 bristles situated on its upper surface is 

 touched. About eighteen or twenty 

 of these bristles occur on either side 

 of the mid-rib, a smaller number being 

 located on the concave surface of the 

 lamina, especially towards its margin. 

 Each bristle (Fig. 242) is made up of 

 from five to seven superimposed tiers 

 of cells ; in the lower part of the 

 bristle each tier is composed of four 

 cells, but towards the apex the number 

 of cells per tier is reduced to two. 

 The lowermost tier represents the base 

 or foot of the hair, and is made up 

 of short cells. It is succeeded by 

 from one to three tiers of elongated 

 elements, with fairly thick outer, but 

 quite thin inner walls. Next follows 

 a tier of very short cells (they are 

 only about twice as long as their 

 width), with thin external walls. This 

 hinge-like zone constitutes the sensi- 

 tive region of the whole bristle. The 

 remaining tiers again consist of elon- 

 gated cells with thick outer walls, 

 although this latter feature is less 



... Structure of the tactile bristles of Atdro- 



prOllOlinced here than it IS m the vandia vesiculosa. A. An entire bristle, 



highly magnified; /, base or foot h u h, A 3 , 



iOWer part 01 the hail'. ihe tWO th e tiers of elongated cells with thick outer 



... . walls; g, the perceptive region or hinge. 



terminal Cells generally diverge a little B. The hinge, and adjoining tiers, more 



. . . . highly magnified. C. Diagram Bhowingthe 



at their tipS. Every Cell 111 the deformation which the hinge suffers, when 



.. . . . the bristle is bent. 



bristle is furnished with a well 



developed peripheral layer of protoplasm, and a fairly large nucleus, 

 which is usually spindle-shaped with pointed ends. When one of 

 these bristles is touched near its upper end, it does not simply 

 bend or move to one side ; for the regions above and below the hinge 

 are stiff and inflexible, owing to the thickened condition of their 



