TACTILE PITS OF TENDRILS 577 



(viz. that of the Sapindaceous genus Urvillea), the author has described 

 an arrangement which evidently must be interpreted in the same sense. 

 Here, the more or less distinctly elongated epidermal (presumably 

 sensory) cells on the lower side of the tendril contain abundant pro- 

 toplasm, and are provided with remarkably thick outer walls. On 

 either side of the longitudinally directed radial walls there is often 

 present a narrow unthickened strip on the outer wall ; the fissures 

 or grooves produced in this way are filled by thin, flange-like 

 projections of the protoplast. The whole structure, in fact, strongly 

 resembles a very much elongated tactile pit. The transversely directed 

 radial walls are usually flanked by several slit-like pits, while an 

 irregular longitudinal series of similar pits is generally developed along 

 the middle line of the outer wall. 



Where the sensitive epidermis (sensory epithelium) of a tendril is 

 devoid of tactile pits or analogous organs, it is still possible to point 

 out various histological features that are most probably connected 

 with the perception of mechanical stimuli. Thus, the small size and 

 abundant protoplasmic contents of the epithelial cells, the thin and 

 protuberant outer walls and the often very conspicuous longitudinal 

 corrugations of the cuticle, are all features which directly or indirectly 

 facilitate the process of perception. 



Charles Darwin was the first to notice that tendrils can only be 

 stimulated by contact with, or friction against solid objects, and not, 

 for example, by the impact of drops of water ; subsequently Pfeffer, 

 with the aid of very elaborate experiments, arrived at the following 

 more precise definition of the conditions under which tendrils perceive 

 a mechanical stimulus. " In order that the stimulus may be perceived, 

 it is necessary that separate circumscribed areas situated in the sensi- 

 tive region of the tendril should be subjected to a shock or a tension 

 of sufficient intensity, either simultaneously or in rapid succession. 

 On the other hand, a tendril does not respond to a shock which affects 

 every point within a considerable area with approximately equal 

 intensity," a condition which is fulfilled, for example, in the case of 

 contact with a liquid, or with a soft flexible rod of gelatine. 284 



The author, in his turn, has attempted to define, as exactly as 

 possible, the difference in the character of the deformation which 

 the outer walls of the sensory epidermal cells and the ectoplast asso- 

 ciated with them suffer through contact with solid and liquid bodies 

 respectively. A large, plastic, stimulating surface, which moulds 

 itself to the shape of the tendril, produces a purely radial pres- 

 sure ; hence the impact of water, mercury, or recently solidified 

 gelatine only brings about a radial compression of the ectoplast 

 which is in contact with the outer wall of each sensory cell. But 



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