578 SENSORY SYSTEM 



experiment has shown that radial compression does not constitute 



a stimulus. When, on the contrary, very small stimulating surfaces, 



such as are provided by the innumerable small irregularities that 



are present on the surface of every solid body, come into contact 



with the sensory epithelium of a tendril, the protruding outer walls of 



the sensory cells become locally bent or depressed and the underlying 



ectoplast accordingly suffers the tangential compressions and extensions 



which constitute effective stimuli. It is obvious that the tangential 



tensions caused by contact with solid bodies, will be most likely to 



exceed the value corresponding to the lower limit of sensitiveness, if 



the cells of the sensory epithelium are small, and are furnished with 



thin and protuberant outer walls. Pfeffer's experiments have shown 



that these tangential tensions must be produced suddenly, and often 



also several times in quick succession if they are to be effective ; 



hence, the presence of longitudinal cuticular ridges, which is a very 



conspicuous feature of many tendrils, will increase the chances of 



effective stimulation, by transforming the smooth gliding movement of 



the tendril over any object which it encounters in the course of its 



circumnutation, into a rapid succession of jerks. Each of the shocks 



produced in this way must be infinitesimally small ; on the other 



hand, many tendrils are endowed with an extraordinary degree of 



sensitiveness. 



With regard, finally, to the tactile pits of Cucuebitaceae, it can 

 readily be demonstrated, that the disc-like protoplasmic processes which 

 extend into the pit-cavities, will be subjected to even greater tangential 

 stresses than the rest of the ectoplast associated with the outer wall, 

 when the tendril rubs against a solid object. Tendrils which are 

 furnished with tactile pits, must accordingly be held to have attained a 

 higher level of specialisation than those which are devoid of this 

 complication. 



The tentacles of Drosera respond both to mechanical and to 

 chemical stimulation. Pfeffer has shown that the mechanical sensi- 

 tiveness of these organs is closely similar to that of tendrils. Two 

 conditions must be fulfilled, if stimulation is to be effective : In 

 the first place, the outer walls of the glandular cells of the " head " 

 must come into contact with a solid body ; secondly, several " shocks " 

 must occur in rapid succession. The response consists in a curvature 

 of the stalk of the tentacle ; the ecological aim of the whole perform- 

 ance is the capture, asphyxiation and digestion of insects. 2S5 



The sensory elements of the Drosera tentacle are represented by 

 the superficial glandular cells of the head, which thus combine several 

 quite distinct functions. They secrete a large amount of mucilage and 

 smaller quantities of a digestive enzyme ; they are responsible for the 



