SECT. II PHYSIOLOGY 317 



(d) Mechanical Stimuli 



Thigmotropism (^^^'). A curvature inwards on one-sided contact 

 is found especially in climl:>ing 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 morpho- 

 logical chai^acter. These may either maintain, 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. According to Fitting, 

 contact with a solid bod}'' C|uickly induces an increase in the growth 

 of the opposite side of the organ, and this, without any retardation of 

 growth on the touched side, leads to a sharp curvature of the tendril 

 which coils it about the support. The more slender the tendrils and 

 the stronger their growth, the more easily and cjuickly this process 

 occurs. Through the tendency of the ciu'vature 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 0"00025 mgr. is sufficient to stimulate the tendril. Probably 

 the arrangements described on p. 74 (Figs. 74, 75) favour the reception 

 of such weak stimuli. 



In the more typically developed tendrils the curvature does not remain restricted 

 to the portions directly subjected to the action of the contact stimulus. Apart 

 from the fact that, in the act of coiling, new portions of the tendrils are being 

 continually brought into contact with the support and so acted upon by the 

 stimulus, the stimulation to curvature is also transferred to the portions of the 

 temlril not in contact with the support. Through the action of the propagated 

 stimulus, not only is the free apex of the tendril turned more quickly around the 

 support, but a tendency to curvature is imparted to the portion of the tendril 

 between the support and the parent shoot. As this intervening part is extended 

 between two fixed points, this tendency causes it to coil spirally, like a corkscrew. 

 With the spiral coiling, a torsion is produced, and since, on account of the fixed 

 position of the two end points, it cannot be exerted in one direction only, the 

 spiral, for purely mechanical reasons, coils partly to the left and partly to the 

 riglit. Points of reversal {x) thus occur in the windings which, in equal 

 numl)ers to the right and to the left, equalise the torsion (Fig. 235). By the 

 spiral coiling of the tendrils the parent-stem is not only drawn closer to the 



