THE SPECIAL IRRITABILITY OF TENDRIL-CLIMBERS 51 



commonly brings the most irritable side against the support. It is rarely 

 the case that a tendril is physiologically perfectly radial, and numerous 

 transitions occur between isotropic and anisotropic tendrils. Contact 

 applied to the convex surface of the hook-tendril of Strychnos causes, 

 for instance, a slight increase of thickening but no coiling 1 . Kohl 2 found 

 tendrils of Pisum sativum to be occasionally irritable on all sides instead 

 of on one only as is usually the case. The branches of the tendril of 

 Bignonia venusta are anisotropic, but the peduncular portion is able to coil 

 towards any side 3 . 



Among leaf-climbers the tip of the leaf of Flagellaria indica is irritable 

 on the upper side, but in all others the under side is the sensitive one 4 . 

 Darwin found the petioles of leaf-climbers to be irritable on all sides, 

 but according -to Derschau 5 not to the same degree. 

 Usually only the concave side of an attaching hook is 

 pronouncedly irritable, the back and sides being less 

 so or almost insensitive to contact. In the case of 

 Artabotrys the median portion of the hook (, Fig. 17) 

 is much more irritable than either the terminal or 

 basal joints 6 . In the case of the tendril of A mpelopsis 

 Veitchii only a particular point at the tip of each 

 branch is irritable, whereas the stems of Cuscnta are 

 physiologically radial to contact stimuli. In most 

 cases anisotropic tendrils are morphologically and 

 anatomically dorsiventral, while isotropic tendrils 

 which undergo secondary growth may become very 

 pronouncedly bilateral as the result of contact stimuli. 

 The same stimulus may cause a flattened tendril to 

 become more or less circular in outline 7 . Anatomical 

 and physiological dorsiventrality are not necessary 

 postulates of each other, and in fact various dorsi- 

 ventral petioles are irritable on all sides. The anatomical structure affords 

 no direct evidence as to the distribution of irritability, and hence requires 

 no discussion 8 . 



Ewart, 1. c., p. 212. a Mohl, 1. c., p. 65. 3 Schenck, 1. c., p. 189 ; Fitting, 1. c. 



Schenck, 1. c., p. 179. The tendril-leaves of Adlumia cirrhosa are irritable on all sides. 

 Cf. Pfeffer, Unters. d. hot. Inst. zu Tubingen, 1885, Bd. I, p. 485. 



L.c., p. 13. 



Ewart, Ann. du Jard. bot. de Buitenzorg, 1898, Vol. XV, pp. 193, 202, 204, 242. 



Ewart, I.e., pp. 218, 222. 



On anatomical relationships cf. Worgitzky, Flora, 1887, p. 2 ; Leclerc du Sablon, Ann. sci. 

 nat., 1887, 7 sen, T. v, p. 5; Miiller, Cohn's Beitr. z. Biol., 1887, Bd. IV, p. 97; Derschau, 

 Einfluss von Contact und Zug auf rankende Blattstiele, 1893 ; Borzi, Rend. Acad. dei Lincei, 1901, 

 5* ser., T. x, p. 395 ; Ewart, Ann. du Jard. bot. de Buitenzorg, 1898, Vol. XV, p. 187 ; Fitting, 1. c., 

 p. 600; Schenck, Beitr. z. Biol. u. Anat. d. Lianen, 1892, I, p. 146 ; Macdougal, Annals of Botany, 

 1896, Vol. x, p. 394; and the literature quoted by these authors. 



E 2 



FIG. 17. a, coiled and thickened 

 hook-tendril of Strychnos lau- 

 n'na \ b, attaching hook of A rta- 

 botrys Blumei. (After Ewart.) 





