DIV. II 



PHYSIOLOGY 



353 



4. HAPTOTROPISM (THIGMOTROPISM) ( ni ) 



A curvature inwards on one-sided contact is found especially in 

 climbing plants which seek by such grasping movements to .encircle 

 the touching body and utilise it as a support. The arrangement 

 thus resembles what was seen in the case of twining plants, but the 

 movements are not in any sense geotropic. In the case of tendril- 

 climbers, the attachment to the support is effected, not by the main 

 axis of the plant, but by lateral organs of various morphological 

 character (cf. p. 182). These may either retain, at the same time, their 

 normal character and functions (as foliage leaves, shoots, or inflores- 

 cences), or, as is usually the case, become modified and as typical 



tendrils serve solely as climbing 

 organs. Contact with a solid body 

 quickly induces an increase in the 

 growth of the opposite side of the 

 organ, and this, without any retarda- 

 tion of growth on the touched side, 

 leads to a sharp curvature of the 

 tendril which coils it about the 



FIG. 285. Surface views of cells from the 

 sensitive side of the tendril of CucurUta 

 Pepo, showing tactile pits, s. (x 540. 

 After STRASBURGFR.) 



FIG. 286. Transverse section through similar 

 cells to those in Fig. 68 ; a small crystal of 

 calcium oxalate (s) is present in the tactile 

 pit. (x 450. After STRASBURGER.) 



support. The more slender the tendrils and the stronger their 

 growth, the more easily and quickly this process occurs. Owing to the 

 tendency of the curvature to press the tendrils more and more firmly 

 against the support, deep impressions are often made by them upon 

 yielding bodies, soft stems, etc. 



According to PFEFFER'S investigations, it is of great importance to 

 the tendrils in the performance of their functions that they are not 

 induced to coil by every touch, but only through CONTACT WITH THE 

 UNEVEN SURFACE OF SOLID BODIES. Rain-drops consequently never 

 act as a contact stimulus; and even the shock of a continued fall 

 of mercury produces no stimulation, while a fibre of cotton-wool 

 weighing '000 25 mgr. is sufficient to stimulate the tendril. Probably 

 the so-called tactile pits (Figs. 285, 286) favour the reception of such 

 weak stimuli. These are pits in the outer epidermal walls which 



2 A 



