THE COILING OF TENDRILS. 



663 



is thus, in the same sense as the twining round the support itself, an effect of 

 irritability, and it is only the mechanical impossibility of its twining round the 

 support which impels the part of the tendril between the support and the base 

 to coil up in this peculiar way. Like the curvature of a long piece of tendril 

 in consequence of contact at a single point, this spiral incoiling also is a proof 

 that the local stimulus is propagated along the tendril for a considerable distance, 

 down to the rigid basal portion. The after- 

 eflect of the stimulus is not ended even 

 with these processes, however ; for tendrils 

 fixed to a support subsequently grow in 

 thickness, often even very strongly, they 

 lignify, become solid, and are endowed with 

 longer vitality than those tendrils which have 

 failed in their purpose. This long continued 

 after-effect is very striking in the case of the 

 petioles which act as tendrils in Solanmn jas- 

 viinoides ; the petiole, when it has twined round 

 the support, swells up to 3-4 times its former 

 thickness. This effect is less marked in the 

 case of the tendrils of the Vine, though it 

 happens not seldom that some weeks after 

 the fixing of the tendrils the part closely 

 applied to the support becomes considerably 

 thicker than the parts not in contact. The 

 most conspicuous example of such cases, 

 however, is afforded by the tendrils of the 

 so-called Wild Vine (Virginian Creeper); if 

 these fail to attain their object they dry 

 up completely and fall off, whereas the 

 fixed tendrils become thicker, lignify, and 

 after they have perished serve for years to 

 fix the shoots to their supports. In many 

 other tendrils, it is true, there is very little of 

 this after-effect to note, but it must be added 

 that the activity of the tendrils promotes 

 the growth and well - being, and therefore 

 affects the whole of the plant. On cultivat- 

 ing tendril-plants in the open for many years 

 one cannot fail to perceive that those speci- 

 mens which are afforded the opportunity of fixing themselves to bushes, cords, 

 wires, &c., by many tendrils, grow and thrive much more vigorously than such as 

 have to clamber up unsuitable supports and are fixed by few tendrils only 



The coiling up of the fixed tendrils differs again by another peculiarity from 

 that of those which are coiled up spontaneously; for in the latter all the coils 

 of the spiral run evenly in one direction, whereas in the turns of the spiral of a tendril 

 fixed to a support there are points where the direction is reversed (Fig. 337 w, iv'). 



Fig. 377.— White Bryony (Bryonia dioica). S a portion 

 of tlie shoot-axis from which, close to the petiole * and the 

 bud k, the tendril mu arises. The lower piece of the ten- 

 dril u is rigid (not tendril-like) ; its upper portion x has 

 become coiled round a branch. The long middle portion 

 of the tendril between the rigid base ii and the fixed point 

 X has become coiled up into a spiral, thus raising the stem 

 />. 7v and w' points where the direction of the coils is 

 reversed. 



