250 THE ACTION OF THE LIVING CELL 



such as a stick or plant stock. This of course sug- 

 gested frictional irritation as the factor responsible 

 for the onset of a more or less circuitous growth path. 

 Experiments, notably those of Pfeffer, disclosed that 

 contact with any rigid body suffices to induce ten- 

 dril-coiling, but that contact with moist gelatin, water, 

 oil, or mercury did not lead to a similar thigmotropic 

 response. Further experiments of Peirce (1894) 

 showed that, while contact with glass rods led to thig- 

 motropy, such was not the case if the rods were cov- 

 ered with a layer of moist gelatin. On the other hand 

 if sand is incorporated into the gelatin, then a normal 

 coiling response ensues. Peirce found further that 

 dry gelatin stimulated the coiling of tendrils in much 

 the same fashion as did any other solid body; there- 

 fore the failure of the gelatin coated rods to excite 

 such coiling cannot be attributed to any specific chem- 

 ical character of the gelatin. 



In consequence it must be concluded that the sur- 

 face of the gel covered rods was not possessed of suffi- 

 cient rigidity to permit the development of the degree 

 of friction necessary to excite the usual thigmotropic 

 response. 



It seems reasonable to assume that such frictional 

 force must attain a certain minimum value in order 

 that the surface cells may become sufficiently irritated 

 to discharge their cytost. In this connection it is of 

 interest to recall the experiments of Sir Thomas Lewis 

 and his collaborators on the production of a skin tache 

 and subsequent wheals by stroking the forearm with 

 a blunt instrument. A considerable amount of such 

 stroking is necessary in order to elicit capillary dila- 

 tation and subsequent wheal formation. Nor is this 



