312 



PHYSIOLOGY OF GROWTH AND CONFIGURATION 



If a twining plant is placed upon a clinostat and slowly rotated about a 

 horizontal axis, the twining movement ceases and growth proceeds in a direc- 

 tion parallel to the axis of rotation, while the younger turns of the previously- 

 formed spiral become straightened out. Such experiments indicate that a 

 geotropic response is necessary for twining. 



§2. Non-twining Climbers. 1 — The long stems of non-twining climbers are 



Fig. 153. — Twining stem of Humulus lupulus, in Fig. 154. — A. Pharbitis, shoot 



successive stages of movement. (After Pfeffer.) showing counter-clockwise twining. 



B. Myrsiphyllum asparagoides, shoot 

 showing clockwise twining. (After 

 Bonnier.) 



unable to twine, but they climb by means of hairs, thorns, aerial roots, tendrils, 

 etc. Tendrils are the most frequent of these special structures. These are 

 morphologically different in different plants; in some forms (Vitis, Ampelopsis, 

 the Cucurbitaceae) they correspond to twigs, while in others they are leaves; 

 thus the upper part of the pea leaf is a tendril while the pinnately arranged leaf- 

 ets of the lower part are quite like those of ordinary leaves. 



1 Darwin, Charles, 1875. [See note 1, p. 311.] Vries, Hugo de, Längenwachsthum der Ober-und Un- 

 terseite sich krümmender Ranken. Arbeit. Bot. Inst. Würzburg, 1 : 302-316. 1874- Schenck, Heinrich, 

 Beiträge zur Biologie und Anatomie der Lianen im besonderen der in Brasilien einheimischen Arten. 2 v. 

 Jena, 1802-1893. [Lengerkin, August von, Die Bildung der Haftballen an den Ranken einiger Arten der 

 Gattung Ampelopsis. Bot. Zeitg. 43 : 337-346, 353-361, 360-379, 385-303, 401-411- 1885. MacDougal, 

 D. Т., Mechanism of the curvature of tendrils. Ann. bot. 10: 373-402. 1896. Idem, 1901. (See note 

 i, p. 311.) Pringsheim, 1912. (See note 1, p. 253-)] 



