42 MOVEMENTS OF CURVATURE 



the stimulus is applied so as to aid the autonomic movement, and to slightly retard 

 the latter when acting against it. Baranetzsky found that during the symmetric 

 nutation of Ipomoea purpurea the half of the orbit towards the light was performed 

 in 45 minutes and that away from it in 55 minutes. Similar differences were 

 observed in Ipomoea sibirica and Polygonum Convolvulus. The heliotropic action is 

 weakened during symmetric nutation by the fact that the anterior side is continually 

 changing, whereas since the latter takes place to a less degree during asymmetric 

 circumnutation, the heliotropic action would naturally be somewhat stronger, being 

 more prolonged on the respective sides. 



Baranetzsky observed that an asymmetrically nutating apex of Ipomoea sibirica 

 performed the half of the orbit towards the light and downwards in 35 minutes, 

 that away from the light in 75 to 85 minutes. 



The positive heliotropism of the apex is unfavourable to twining rather than an 

 aid to it, but it is of some advantage that the coiled parts should become negatively 

 heliotropic, for this causes them to curve towards the shaded side and hence towards 

 the support. Baranetzsky * found this negative heliotropism to be very pronounced 

 in the shoots of Ipomoea purpurea, Polygonum Convolvulus, and Dioscorea smuata, 

 whereas it was weaker in the stems of Dioscorea Batatas, and was not developed at 

 all in the stems of Boussingaultia baselloidcs and Menispermum dahuricum 2 . 



SECTION u. Tendril-climbers. 



Tendril-climbers show much more varied special adaptations than 

 twiners, and they may be classed in different groups according to the 

 type of irritable attaching organ they possess 3 . 



Under tendril-climbers we include all such plants as Bryonia and 

 other Cucurbitaceae, Passiflora, Pisum, Latkyrus, Cobaea scandens, Bignonia, 

 Eccremocarpus, Vitis, Cardiospermum Halicacabum which possess filamentous 

 coiling attaching organs which are irritable to contact (Fig. 9). The 

 tendrils are continually produced at the growing apex, and radiate and 

 attach themselves in all directions. The spiral coiling of the portion 

 between the stem and the support acts like a spring against the tearing 

 effect of violent shocks of wind or rain, and also draws the plant nearer 

 to the support and by the antagonistic action of different tendrils affords 

 more rigid support. The same applies when the tendril itself is branched, 



1 Baranetzsky, 1. c., p. 21. Cf. also Wiesner, Die heliotropischen Erscheinungen, 1880, II, p. 38 ; 

 Voss, Bot. Ztg., 1902, Orig., p. 238. 



3 Mohl, 1. c., p. 1 20, observed that certain twiners succeed in coiling around a support if they 

 meet it while growing away from the light, but not if they are growing towards it. It is, however, 

 not certain whether this is due to negative heliotropism. 



9 For details see Darwin, Climbing Plants, 1875 ; Schenck, Beitrage z. Biol. u. Anat. d. Lianen, 

 1892, i, p. 135; Ludwig, Lehrb. d. Biol. d. Pflanzen, 1895, p. 126; Goebel, Organography, 1900. 

 The physiologist has to deal with the tendril as a functional organ without regard to its morphological 

 origin. Hence Schenck's classification has no value here. 



