314 



PHYSIOLOGY OF GROWTH AND CONFIGURATION 



if the diameter of the support is as great as 23 cm. Many tropical twiners 

 can twine about thick supports. 



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. 



a ' b c 



Fig. 153. — Twining stem of Hamulus lupulus, in 

 successive stages of movement. (After Pfeffer.) 



A 3 



Fig. 154. — A. Pharbitis, shoot, 

 showing counter-clockwise twining. 

 B. Myrsiphyllum asparagoides, shoot 

 showing clockwise twining. {After 

 Bonnier.) 



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

 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; 



1 Darwin, Charles, 1875. [See note 1, p. 312.] Vries, Hugo de, Langenwachsthum der Ober-und Un- 

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

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

 Jena, 1 892-1 893. [Lengerkin, August von, Die Bildung der Haftballen an den Ranken einiger Arten der 

 Gattung Ampelopsis. Bot. Zeitg. 43 : 337-346, 353-361, 369~379, 385-393, 4° I -4U- 1885. MacDougal, 

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

 1, P- 313.) Pringsheim, 1912. (See note 1, p. 253.)] 



