594 



SENSORY SYSTEM 



greatest amount of deformation, when a Dionaea bristle is bent ; 

 there can, therefore, be no doubt that the actual perception takes 

 place within these cells. One can easily gain a clear idea regarding 

 the distribution of deformation, if one bends a bristle under the 

 microscope by pressing a needle against its stiff terminal portion. 

 Under these conditions, one cannot fail to observe that curvature chiefly 

 takes place in the region of the hinge, the transverse furrow being 



Fin. 246. 



Semi-diagrammatic drawings of a flower of Pterostylis (type of P. curta). A, before ; 

 B, after stimulation of the labellum ; s, column ; an, anther ; /, wing-like appendages 

 of the column ; st, stigma ; I, limb of the labellum ; n, its claw (the motor-organ) ; 

 a, appendage of the labellum ('? sense-organ). 



partially flattened out on the convex surface, and becoming narrower 

 and deeper on the opposite side (Fig. 245). In a particular case 

 observed by the author, the vertical extent of the hinge-area was found 

 to be equal to nineteen divisions of the eye-piece micrometer, when the 

 bristle was straight. On bending the bristle, the length of the convex 

 side of the hinge increased to twenty- three divisions, an elongation of 

 2 1 per cent. The large tabular marginal cells may be seen to undergo 

 great deformation, the obvious inference being that these elements 

 represent the actual sensory organs.' 21 ' s 



Various species of Pterostylis, a genus of Orchids inhabiting 

 Australia, N". Zealand, and N. Caledonia, possess a sensitive labellum, 

 furnished with a peculiar brush-like appendage ; as the latter probably 

 represents a tactile organ, it may conveniently be discussed at this 

 point. The labellum is made up of two parts namely, a narrow 



