^6 LECTURE V. 



climb up foreign bodies, usually other plants. In the Ivy, for example, and Ficus 

 scandens, this takes place simply by the shoot-axes attaching themselves firmly to 

 tree trunks or walls, and fastening themselves by means of clinging roots. Much 

 more perfect is the arrangement for climbing, however, in the foliage shoots of 

 twining plants, where, in the long drawn shoot-axes, a tendency to spiral curvature is 

 combined with the irritability for the geotropic influence of gravitation ^ by which 

 shoot-axes of this kind are caused to wind themselves spirally close around upright 

 poles, stems, or branches, and so push their leaf-forming buds continually higher. 



Well-known examples of such twining 

 plants are the Hop, Bindweed {Convol- 

 vulus), Bean {Phaseolus), and Dioscorea. ' 

 We find'the most perfect mechanism of 

 climbing shoots, however, in the tendril- 

 plants; the leafy shoot-axes of which are 

 likewise so thin and flexible, that they are 

 not able to support the weight of leaves, 

 flowers, and fruits. Such plants, therefore, 

 just as true twining plants, must climb 

 upon shrubs and similar supports, since; 

 if they find no opportunity of doing this, 

 they become arrested. The climbing 

 organs, now, are simply the tendrils. 

 These are long, thin, filiform organs, 

 which, by the continual contact of a thin 

 solid body, are induced, to wind fast around 

 it, so that at the same time the whole 

 leaf shoot is caused to elevate its apex 

 higher and higher. Such sensitive tendrils 

 may be developed in very different ways: 

 sometimes it is the long thin stalks d 

 the proper foliage leaves themselves, which 

 wind around their supports, as in the 

 Spanish cress (Tropaolum), Clematis, Maur 

 randia, Fumaria, Solanum jasminoides, &c. In other cases, it is the elongated mid- 

 rib of the foliage leaf, with or without ramifications, as in Nepenthes, Cobea scandens, 

 Pisum sativum (Pea), and species of Vetch ( Vicid). Finally, also, metamorphosed 

 shoot-axes, with entirely arrested leaflets, may occur as irritable tendril filaments ; 

 thus, the tendrils of the Vine and the Virginian Creeper {Ampelopsis) are meta- 

 morphosed lateral shoots on the nqrmal leaf shoots of these plants. They may in 

 fact be considered as flower shoots degenerated to climbing organs. To the same 

 category belong, probably, the extraordinarily sensitive tendrils of the Gourd-like 

 plants and the Passion-flowers (Passiflored) ; while in the genus Smilax (Monocoty- 

 ledons) the tendrils are outgrowths of the leaf stalks. I shall return later to the highly 



Fig. 50. — Apex of a shoot of Akebia giiinata, growing 

 out beyond the supporting rod; four free spiral turns have 

 been produced. 



^ That the theory of the twining of climbing plants proposed by Darwin is wrong, has been 

 shown by H. de Vries. Compare Arb. des Bot, Instit. I. p. 317, and II. p. 718. 



