TACTILE PITS OF TENDRILS 575 



of Drosera, by the foliar or axial tendrils of various Angiosperras, and, 

 finally, by those " irritable " stamens, stigmas, styles, gynostemia or 

 perianth-segments which perform active paratonic movements in con- 

 nection with pollination. 



The perception of mechanical stimuli is often carried out through 

 the mediation of special sense-organs, which are analogous to the 

 tactile sense-organs of animals, and winch indeed often resemble the 

 latter in structure. According to their special mode of construction in 

 different cases, these organs are termed tactile pits, tactile papillae, 

 tactile hairs or tactile bristles. 



It is important to note, that any given mechanical stimulus 

 whether presented as shock, friction or prolonged contact cannot be 

 perceived, and hence cannot induce a responsive movement, unless it 

 produces a distinct deformation of the sensitive ectoplast. Moreover, 

 in order to be effective, the deformation must be sudden, and in certain 

 cases must take place several times in rapid succession. Nor is the 

 actual mode of deformation a matter of indifference ; for the author 

 has shown that the sensitive ectoplast is principally affected by the 

 tangential tensions (compressions or extensions) or deformations that 

 result from shock or contact. In fact, the most characteristic general 

 feature of the anatomical structure of tactile sense-org;ans is the 

 presence of arrangements which facilitate or accentuate the requisite 

 sudden deformation of the sensitive protoplasm, and which more 

 particularly favour the production of tangential tensions within the 

 ectoplast. 



A. TACTILE PITS. 282 



Tactile pits are sharply denned unthickened areas, more or less 

 closely resembling ordinary pits, which occur in the outer walls of 

 superficial sensory cells ; they are occupied by special extensions of the 

 sensitive protoplasm. The thinness of the cell-wall at these points 

 reduces the amount of energy which has to be expended in deforming 

 the insensitive cellulose membrane to a minimum, with the result 

 that the greatest possible proportion of the total energy of the stimulus 

 becomes available for the deformation of the sensitive protoplasm. 



Tactile pits were first discovered by Pfeffer in the tendrils of the 

 Cugurbitaceae (Fig. 232). Most often they are confined to the mor- 

 phologically lower side of the tendril (the concave side after curvature), 

 which, as a rule, is alone sensitive to contact. In Bryonia clioica and 

 B. alba, and probably in several other Cucurbitaceae as well, the 

 upper side (the convex side after curvature) is likewise provided with 

 tactile pits. These structures are, however, never found on the insen- 

 sitive basal region of the tendril. 283 



