46 MOVEMENTS OF CURVATURE 



or less marked secondary thickening as the result of contact. In the case 

 of Uncaria and Artabotrys no coiling occurs, but a slight one in Roucheria 

 and Ancistrocladus, while the hook-like tendrils of StrycJinos and Bauhinia 

 are able to form one or more complete coils around a suitable support 1 . 

 These latter forms show a transition to the watch-spring tendrils whose 

 coiled apices grasp supports and then twine around them. 



A feeble contact irritability is shown by various organs. Thus the 

 aerial roots of Vanilla and of a few other plants are irritable enough to 

 function as root-tendrils, from three days to a week being, however, required 

 to produce a complete coil, and the coiling often not being completed 

 until after the lapse of three weeks 2 . Both terrestrial and aquatic roots, 

 as well as the rhizoids of Catharina, have been observed to coil around 

 foreign bodies, but it is not certain whether this is or is not the result 

 of contact stimulation. Presumably, however, certain special branches of 

 CystocloniumpurpurascenS) of Hypnea musciformis, of Nitophyllum uncinatum, 

 and of a few other marine algae, are able to coil like tendrils 3 , which power 

 is also possessed according to Zopf 4 , by the curved hyphae of Arthrobotrys 

 oligospora, and by the hyphae of a few other fungi 5 . In addition Wort- 

 mann 6 observed a strong sporophore of PJiycomyccs nitcns coiling around 

 a weaker one. 



The stimulus of contact not only hastens the coiling of a tendril but 

 also causes its strength to increase. In some cases, as in Bauhinia tomentosa 

 and Amphilobium mutisii the tendril undergoes a secondary increase in 

 thickness, such as is shown to a marked degree in the tendril-hooks of 

 Strychnos, Roucheria, and in the non-coiling hooks of Uncaria and Arta- 

 botrys" 1 . Similarly the petioles of leaf-climbers may double in thickness 

 at the point of contact (cf. Fig. 13, p. 45), while an attached twining 

 pulvinus of Dalbergialinga may attain double the diameter of an unattached 

 one, owing mainly to the very rapid growth of the wood-cylinder and 

 partly to the enlargement of the cortical cells 8 . Similarly the secondary 

 growth of the wood in the petiole of Solatium jasminoides leads to the 



1 Cf. Ewart, 1. c., p. 239. 



2 Ewart, Ann. du Jard. hot. de Bnitenzorg, 1898, Vol. xv, p. 233. The attaching roots were 

 noticed by Mohl, Ranken- und Schlingpflanzen, 1827, p. 24; and Darwin, 1. c., p. 144. These and 

 other aerial roots apply themselves closely to the trunk and walls as the result of their negative 

 heliotropism, and become attached by their root-hairs. A root of Vanilla attached in this way to 

 the smooth surface of another leaf was able to support a weight of 250 grams (Ewart, I.e., p. 234). 

 On the roots of the potato developed in moist air, see Sachs, Flora, 1893, p. 8. 



8 Wille, Bot. Jahrb. f. System, u. Pflanzengeographie, 1886, Bd. vn, p. 21 ; Nordhausen, Jahrb. 

 f. wiss. Bot., 1899, Bd. xxxiv, p. 236. 



1 Zopf, Nova Acta d. Leopold. Carolin. Akad., 1888, Bd. LIT, p. 325. 



s Boudier, Bull, de la Soc. bot. de France, 1894, p. 371 ; Ludwig, Bot. Central bl., 1899, 

 Bd. xxxvn, p. 359. 



* Wortmann, Bot. Ztg., 1887, p. 806. 



7 Ewart, 1. c., pp. 189, 208, 218, 222. 8 Ibid., p. 228. 



