5 o MOVEMENTS OF CURVATURE 



heliotropic *. It may also be remarked that the tendrils of Bryonia remain irritable 

 even when developed in darkness 2 , and that the absence of light exerts no perceptible 

 influence upon the development of the haustoria of Cuscuta 3 . 



SECTION 12. The Special Irritability of Tendril-climbers. 



The contact of a tendril against a support induces a greater activity 

 of growth on the free side, and hence produces coiling. Since the 

 irritability is only attained at a certain stage of development, and 

 gradually disappears as the tendril grows old and loses the power of 

 growth, the effect produced by a stimulus depends not only upon its 

 intensity but also upon the age of the tendril. Ordinary tendrils only 

 become sensitive to contact when fully unfolded, and either at or after 

 the commencement of circumnutation. The irritability usually persists 

 until growth has entirely ceased, which occurs after the circumnutation 

 has become imperceptible 4 . 



All parts of the tendril are not equally irritable, the basal portion 

 in many cases responding feebly or not at all to contact. Usually only 

 one side responds to contact, but the tendrils of Cobaea scandens, Cissus 

 discolor, Smilax aspera, Actinostemma paniculatum 5 , and the pulvinar 

 tendrils of Dalbergia linga* are able to coil around an object touching 

 any side. The tendril of the last-named plant has the proximal surface 

 concave when young, but when older one of the other surfaces becomes 

 convex, and the slightly greater irritability of the original concave side 

 is transferred to the new one 7 . A physiologically dorsiventral tendril 

 remains unstimulated, and moves away from a support which touches one 

 of its non-irritable flanks. In such tendrils the irritability usually appears 

 to decrease from the irritable flank towards the sides, which are however 

 usually sufficiently irritable to commence coiling, and then a slight twist 



1 Peirce, I.e., pp. 87, 116. 2 Sachs, Bot. Ztg., 1863, Beilage, p. 12. 



3 Peirce, 1. c., p. 88. 



* For facts see Darwin, Climbing Plants, 1875; Wortmann, Bot. Ztg., 1887, p. 53; Schenck, 

 Beitr. z. Biol. u. Anat. d. Lianen, 1892, I, pp. 141, 154; Fitting, Jahrb. f. wiss. Bot., 1903, 

 Bd. xxxvin, p. 554. On leaf-tendrils and hooks cf. Schenck, 1. c. ; Derschau, Einfluss von Contact 

 und Zug auf rankende Blattstiele, 1893, p. 12 ; Ewart, Ann. du Jard. hot. de Buitenzorg, 1898, 

 Vol. XV, p. 1 88. On the distribution of growth in developing tendrils cf. Macdougal, Annals of 

 Botany, 1896, Vol. X, p. 379 ; Fitting, 1. c., p. 547. Mohl (Ranken- u. Schlingpflanzen , 1827, p. 65) 

 incorrectly supposed that the irritability only appeared when growth in length had ceased. In many 

 cases growth may be re-awakened and a curvature be produced after the tendril has ceased to 

 elongate. Cf. Fitting, Jahrb. f. wiss. Bot., I.e., p. 554. 



5 Darwin, I.e.; Schenck, I.e., p. 141; Derschau, I.e., p. 13; Fitting, 1. c , p. 551. The 

 decision is made according to the presence or absence of a curvature after contact on each flank. 

 Even in dorsiventral tendrils, the side on which contact produces no response is actually sensitive in 

 a special way, for contact on this side may prevent simultaneous contact on the irritable side from 

 producing any response. 



6 Buitenzorg garden name, not given in Kew Index. 7 Ewart, 1. c., p. 229 



