THE INFLUENCE OF CONTACT UPON TENDRILS 59 



dorsiventral tendrils, though unable when stimulated to produce a curvature, 

 is nevertheless sufficiently irritable to be able to inhibit a response to 

 contact on the concave side, and this action is awakened by contact with 

 a rough surface but not by contact with smooth moist gelatine. 



Presumably the compensation begins during perception, so that no 

 attempt at curvature is ever awakened. It is also possible that the two 

 excitations might act simultaneously, but that the power of response might 

 be temporarily lost, or that some essential connecting link between percep- 

 tion and response should be suppressed. The former is however impro- 

 bable. Fitting 1 observed that the curvatures produced by a change of 

 temperature or by removing the tip of the tendril are inhibited when the 

 back of a dorsiventral tendril is rubbed, and this fact may when further 

 investigated lead to an explanation of the phenomena mentioned. Since 

 this inhibitory action is largely localized, it is possible to keep a portion of 

 a tendril straight while the remainder is performing a thigmotropic, thermo- 

 nastic or traumotropic curvature. 



Continuous contact causes complete and permanent coiling, the 

 continuation of the coiling involving exactly the same stimulatory reaction 

 as is produced by temporary contact. According to Fitting 2 prolonged 

 contact rapidly induces a complete cessation of growth, so that the 

 acceleration of growth on the concave side which produces straightening 

 after temporary contact no longer occurs. Evidently, therefore, the reactions 

 leading to this secondary response are inhibited by continued contact. 

 This applies only when permanent contact is assured, and in fact even 

 sensitive tendrils only partially raise themselves from the support during 

 coiling, partly as the result of accommodation, of orthotropism or of irregular- 

 ities in the support. Since the free portions usually again come into contact 

 with the support, continue to coil and show an acceleration of growth, 

 they must retain the power of growth for some time. In this way aided by 

 the tendency to curvature of the uncoiled basal portion, the tendril is often 

 able to creep over the surface of a support and increase the number of coils, 

 as was first observed by Darwin 3 . 



Sachs concluded that changes in the rate of growth on the different sides of the 

 tendril were responsible for its curvature, and this has been confirmed by Fitting. The 

 curvature is therefore not due, as certain authors have assumed, to an active con- 

 traction of the concave side 4 . The measurements made by de Vries 5 , although not 

 extremely accurate, pointed against this conclusion, but since they were taken after 



1 L. c., p. 562. 2 L. c., p. 609. 



3 Climbing Plants, 1875. 



* Id., 1875, p. 180; Macdougal, Ber. d. hot. Ges., 1896, p. 151; Annals of Botany, 1896, 

 Vol. X, p. 399; Torrey Botanical Club, 1898, Vol. XXV, p. 69. Cf. Fitting, I.e., p. 565; Sachs, 

 Textbook of Botany, 1875, p. 779. 



5 De Vries, Arb. d. bot. Inst. in Wurzburg, 1873, Bd. I, p. 309. 



