354 



BOTANY 



PART 



widen outwards and are filled with protoplasm. They are found, for 

 instance, in the Cucurbitaceae, but may be wanting from some very 

 irritable tendrils (e.g. in Passiflora). 



The tendrils of some plants (Cobaea, Eccremocarpus, Cissus) are 

 irritable and capable of curving on all sides ; others (tendrils of 

 Cucurbitaceae and others with hooked tips) are, according to FITTING, 

 sensitive on all sides but only curve when the under side is touched ; 

 if the upper surface is at the same time stimulated, curvature is 

 arrested. Some tendrils, only sensitive on one side, have the tactile 



pits confined to this. In 

 some cases the tendrils 

 quickly grasp the support 

 (Passijiora, Sicyos, Bryonia) ; 

 while in other tendrils the 

 supports are very slowly 

 grasped (Smilax, Fit-is). 



In the more typically de- 

 veloped tendrils the curvature 

 does not remain restricted to the 

 portions directly subjected to 

 the action of the contact stimu- 

 lus. Apart from the fact that, 

 in the act of coiling, new portions 

 of the tendril are being continu- 

 ally brought into contact with 

 the support and so acted upon 

 by the stimulus, the stimulation 

 FIG. 287. Portion of a stem of Sicyos angulatus, one of the to curvature is also transferred 

 Cucurbitaceae, with tendril. The branch-tendril has to the portions of the tendril 

 grasped the upright support on the right and the not in contact with the sup p or t. 

 free portion has become spirally wound, x, Point of 



reversal in the coiling of the tendril. (After NOLL.) Through the action of the propa- 

 gated stimulus, not only is the 



free apex of the tendril twined more quickly around the support, but a tendency 

 to curvature is imparted to the portion of the tendril between the support and 

 the parent shoot. As this intervening part is extended between two fixed points, 

 this tendency causes it to coil spirally, like a corkscrew. With the spiral coiling 

 a torsion is produced, and since, on account of the fixed position of the two end 

 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. 287). By the spiral coiling of the tendrils the 

 parent-stem is not only drawn closer to the support, but the tendrils themselves 

 acquire greater elasticity and are enabled to withstand the injurious effects of a 

 sudden shock. 



Advantageous changes also take place in the anatomical structure of the tendrils 

 after they are fastened to the supports. The young tendrils, during their rapid 

 elongation, which under favourable conditions may amount to 90 per cent of their 

 length, 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 



