266 



BOTANY 



Fig. 208. — Portion of a stem of Sicyos angulatus with 

 tendril ; x, point of reversal. 



points, it cannot be exerted in one direction only, the spiral, for purely 

 mechanical reasons, coils partly to the left and partly to the right. 



Points of reversal (x) 

 thus occur in the windings 

 which, in equal numbers to 

 the right and to the left, 

 equalise the torsion (Fig. 

 208). Through the spiral 

 coiling of the tendrils the 

 parent- stem is not only 

 drawn closer to the support, 

 but the tendrils themselves, 

 through their consequent 

 elasticity, are enabled to 

 withstand the injurious 

 effects of a sudden shock. 



Advantageous changes 

 also take place in the ana- 

 tomical structure of the ten- 

 drils after they are fastened 

 to the supports. The young 

 tendrils, after their elongation, exhibit active nutations, and thus the 

 probability of their finding a support is enhanced. During this time 

 they remain soft and flexible, while the turgor rigidity of their apices 

 is maintained only by collenchyma. In this condition they are easily 

 ruptured, and have but little sustaining capacity. As soon, however, 

 as a support is grasped, the coiled-up portion of the tendrils thickens 

 and hardens, while the other part lignifies, and becomes so strengthened 

 by sclerenchymatous formations that the tendrils can finally sustain 

 a strain of many pounds. When the tendrils do not find a support 

 they usually dry up and fall off, but in some cases they first coil them- 

 selves into a spiral. 



The tendrils of many plants ( Oobaea, Cissus) are irritable on all sides ; others, 

 on the contrary, on only the lower side (tendrils of Cucurbitaceae and others with 

 hooked tips) ; while others possess extremely sensitive shoots (Mutisia). In some 

 cases the tendrils quickly coil themselves to the support, but others coil more 

 slowly (Passiflora, Sicyos, Bryonia) ; while in other tendrils the supports are 

 very slowly grasped (Smilax, Vitis). 



According to Pfbffer's investigations, it is of great importance to 

 the tendrils in the performance of their functions that they are not 

 induced to coil by every touch, but only through CONTACT with the 

 uneven surface of solid bodies (as thus adjacent cells become 

 unequally affected). Eain-drops consequently never act as a contact 

 stimulus ; and even the shock of a continued fall of mercury produces no 

 stimulation. Tendril-climbers are not, like twining plants, restricted 



