288 BOTANY PAKT i 



in that way are enabled to elevate their assimilating and also their 

 reproductive organs into more favourable situations. In the case of 

 twining plants which possess similar powers of climbing, the process 

 of elevation, as has already been shown, is accomplished by means of 

 the geotropic irritability of the stems themselves. In the case of 

 tendril-climbers, on the other hand, the attachment to the support is 

 effected, not by the main axis of the plant, but by lateral organs of 

 different morphological character. These may either maintain, at 

 the same time, their normal character and functions, or, as is usually 

 the case, become modified and as typical tendrils serve solely as 

 climbing organs. According to FITTING, contact with a solid body 

 quickly 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 quickly this process occurs. 

 Through 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 

 UNKVEN 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 ( 9:! ). 



In the more typically developed tendrils tlic 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 

 tendril 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 is extended between two fixed 

 jxiints, tliis 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 right. POINTS OF 

 UEVKKSAL (x) thus occur in the windings which, in equal numbers to the right 

 and to the left, equalise the torsion (Fig. 232). By the spiral coiling of tin- 

 tendrils the parent-stem is not only drawn closer to the support, but the tendrils 

 themselves acquire greater elasticity and are enabled to withstand the injurious 

 effects of a sudden shock. 



Advantageous changes also take place in the anatomical structure of the tendrils 

 after they are fastened to the supports. The young tendrils, during their rapid 

 elongation, exhibit active nutations, and thus the probability of their finding a 



