38 MOVEMENTS OF CURVATURE 



have no special contact irritability, it remains possible that the pressure against the 

 support, or the curvature which this maintains, ma} 7 play a certain part in twining, 

 which like circumnutation appears to result from the co-operation of various 

 stimuli. 



SECTION 10. Twining Plants (continued). 



In certain climbers special long shoots are adapted for twining 1 , 

 while some of the less active climbers may only develop the twining 

 habit in moist shady situations where long weak stems are produced. 

 This applies to such occasional twiners as Solanum Dulcamara and 

 Cynanchum vincetoxictim 2 , but the causes of the non-twining of Polygonum 

 convolvulus during certain seasons are uncertain 3 . Darwin 4 mentions that 

 Ipomoea argyroides and two species of Ceropegia develop in England as 

 twiners, but not in the dry South African regions to which they are 

 indigenous. Furthermore various cultivated varieties of Phaseolus multi- 

 florus have lost the power of twining in correspondence with their dwarfed 

 habit. Twining is shown more especially by the aerial stems of flowering 

 plants, but in Lygodium scandens and Blechnum volubile we have instances 

 of twining leaves 5 . It is, however, uncertain whether the twining occa- 

 sionally shown by rhizomes and by roots in air, water, and even soil, 

 is produced by the aid of circumnutation or by contact stimulation 6 . 

 The same applies to the filaments of Algae which sometimes twine around 

 supports 7 , and to the rhizoids of Catharinea undulata which may coil 

 around each other 8 . 



In all the cases hitherto observed circumnutation and twining take 

 place in the same direction. Usually the direction is against that of the 

 hands of a watch, as for instance in the stems of Convolvulus (fig. 6, p. 34), 

 Phaseolus, Ipomoea purpurea, Menispermum canadense, Aristolochia sipho, 

 Periploca graeca. On the other hand the stems of Humulus Lupulus 

 (Fig. 7, p. 34), Polygonum convolvulus, Lonicera caprifolium, Testudinaria 

 elephantipes twine to the right in the opposite direction. Usually the 

 direction of twining is constant, but in the cases of Polygonum complexum, 

 Testudinaria sylvatica, and Solanum Dulcamara it sometimes happens that 



1 For .details see Schenck, Beitr. z. Biol. u. Anat. der Lianen, 1892, p. 115 ; Goebel, Organo- 

 graphy, 1902 ; Darwin, Climbing Plants, 1875, p. 41 ; Voss, Bot. Ztg., 1902, p. 249 (Celastra- 

 ceae). 



Darwin, 1. c. ; Schenck, 1. c., p. 128. 



Palm, Ueber d. Winden d. Pflanzen, 1827, pp. 43, 94. 



Darwin, 1. c. 



Cf. Schenck, 1. c., p. 113. 



Hochreutiner, Rev. gen. de Bot., 1896, T. viil, p. 92. 



Palm, I.e., p. 44. 



Schimper, Rech. s. 1. mousses, 1848, Plate iv, Figs. 15, 16. Groups of filaments of Spirogyra 

 projecting into moist air may show coiling. Hofmeister, Jahreshefte d. Vereins f. vaterland. Naturk. 

 in Wiirttemberg, 1847, Jahrg. 30, p. 226; Winkler, Krummungsbewegungen von Spirogyra, 1902. 



