CLIMBING PLANTS. 687 



will have the best grip, as it is stifFer and its obliquely-running strands admit of a 

 better hold. So it is with the climbing stem. By the torsion of its own axis it gets 

 a better hold. The longitudinal ridges on its surface — due to its bundles — corre- 

 spond to the strands of the rope. When these, by torsion, run obliquely, more 

 purchase on the support is obtained. 



Not infrequently the attachment of the twining stem is also strengthened by 

 stiff, backwardly-directed bristles, and by barbs which are developed on the ridges, 

 as is the case, for example, in the twining Polygonum, and in bean-plants. These 

 reversed prickles are comparatively large in Ipomcea mwricata, a species of bind- 

 weed. Hops also possess prickles of a remarkable form. In the Hop, as may be 

 seen in fig. 160, they have the shape of an anvil; that is to say, a cell which is much 

 extended in the longitudinal direction, and tapers to a point at either end, is 

 developed on a peg-shaped or conical base. Its wall is silicified and very hard, and 

 the points hook into softer tissue like claws. Such climbing hooks are found in 

 regular rows on the six ridges of the twining hop stem, and are a great assistance 

 in attaching it to the entwined support. 



In Hoya carnosa, known for its waxen flowers, and often cultivated in green- 

 houses, the young twining stems are thickly beset with reversed hairs which under 

 certain circumstances contribute materially to the adhesion to rugged substrata. 

 Moreover, the stems of this plant, as soon as they have ceased to nutate, develop 

 light-avoiding, climbing roots which nestle to the substratum and unite with it, 

 thus adding to the security of the stem. The stems of Hoya, like those of Cassytha 

 and Cuscuta, described on p. 171, are thus, in a way, intermediate between those 

 of twining plants, in the strict sense, and climbing plants provided with clinging 

 roots, which latter will be discussed presently. 



When the nutating end of a twining stem has found no erect support in its 

 neighbourhood, the older portions of its stem which no longer revolve take on, even 

 without a support, a spiral twisting and a torsion of the axis. Just as a rope 

 becomes more rigid when twisted, so the stiffness of these twisted stems, though 

 they have no support, is increased in comparison with untwisted stems. Such a 

 twisted stem may even rise a little above the ground, and in many instances the 

 still nutating free end is enabled to reach some bough of a neighbouring tree or 

 bush, and winding round it, to attain to the tree-crown. Many twining plants, as, 

 for example, hops, frequently send up above the ground from their subterranean 

 perennial portions several shoots. If these find no support in the ordinary way, 

 they wind round one another, and a regular coil or cable is produced (cf. p. 364). 

 These cables often rise without any foreign support to a considerable height above 

 the ground, and thus single nutating apices are afforded the possibility of grasping 

 a support which otherwise might have been denied them. 



Should all these methods prove of no avail the twisted stem takes up its 

 position on the ground; its growth is retarded, and it has the appearance of a 

 stunted, sickly plant. This fact is in so far interesting because it seems to indicate 

 that the pressure experienced by a twining stem adhering to a supporting prop has 



