244 PLANT PHYSIOLOGY 



the opposite side may grow less rapidly than when the 

 tendril is free. Presumably either the contraction of the cells 

 on the side touched, or the expansion ( fixed by growth ) of 

 the cells of the opposite side, would develop the mechanical 

 force needed to accomplish the bending, for the resistance 

 to be overcome is slight. Indeed, until they have attached 

 themselves to a support and have developed strengthening 

 tissues as a result of strain (see pp. 187-8), tendrils are 

 mechanically weak as well as irritable. 



The irritability of tendrils varies greatly, the tendrils 

 of grape-vine requiring prolonged contact with a compara- 

 tively rough object, those of passion-vine (PassiSora) 

 responding to extremely slight and transient stimuli in 25 

 to 30 seconds. The size of the solid object must be pro- 

 portioned to the size, structure, and sensitiveness of the 

 tendril if it is to be successfully clasped. The tendrils of 

 grape, for example, cannot twine about objects of small 

 size, while tendrils of Echinocystis will coil about a spider's 

 thread.* A piece of thread weighing 0.00025 milligram, 

 placed as a rider on a tendril of passion-vine, causes no 

 stimulus if motionless, but induces bending if swinging 

 on the tendril, f All solids except moist gelatine irritate ; no 

 harmless liquid, free from solid particles, not even mercury, 

 stimulates the most sensitive tendril to bend. From this 

 we may infer that unless there be a certain amount of 

 adhesion between the tendril and the object by which it is 

 touched, there will be no irritation. This adhesion changes 

 the pressure in the cells of the part touched, increasing 

 or decreasing the pressure according to circumstances. 



The petioles of several well-known plants are more or 

 less sensitive to contact, e. g: Lophospermum scandens, 

 Solanum jasminoides, Tropoeolum majus, Clematis. The 

 stems of dodder (Cuscuta) are also periodically sensitive 

 to contact. J This parasite twines about its host, forming 



* MacDougal, loc. cit., p. 376. 



•f Pfeffer, W. Zur Kenntniss der Contaktreize. Untersuch. a. d. Bot. 

 Inst. Tubingen, Bd. I, 1885. 



J-^Peirce, G. J. Contribution to the physiology of the genus Cuscuta. 

 Annals of Botany, VIII., 1894. 



