RELATION OF MORPHOLOGICAL NATURE OF ORGANS TO ADAPTATION. 937 



supports are, so to speak, sought out in the most industrious manner ; when one is 

 touched by a tendril, the tendril bends in consequence of the stimulation and 

 twines firmly round it; and when several tendrils do the same in different direc- 

 tion from the stem, it hangs suspended between the points of support. If this 

 were all, the attachment would be a very weak one, and the elevation of the stem 

 would only take place slowly ; but the whole contrivance is perfected in the most 

 ingenious way. When the tendrils have fixed themselves by their extremities, they 

 draw the stem towards the support by twisting themselves spirally. When several 

 tendrils do this in different directions, the stem which is suspended between them 

 is tightly stretched, and the tenacity of the tendrils is at the same time con- 

 siderably increased by the twisting. Many tendrils, while very tender at the time 

 when they are sensitive, become afterwards hard and woody, and some become 

 much thicker ; this is strikingly the case in Clematis glandulosa and Solanum jasmi" 

 noides. But the most perfect adaptation is shown in the tendrils of the Virginian 

 Creeper, Bignonia capreolata, and some other plants : it is most perfect in Ampelopsis 

 hederacea. As in the Grape -Vine, the tendrils are here branched axial structures, 

 and are to a much greater extent negatively heliotropic ; their power of twining 

 round slender supports is but slightly developed, but when, in consequence of their 

 negative heliotropism, they come into contact with a wall, or in the wild state with a 

 rock, trunk of a tree, &c., there is formed in the course of a few days on each branch 

 of the tendril which touches the support with its curved and hooked apex, a cushion- 

 like swelling which afterwards expands into a red flat disc, and becomes firmly 

 attached by its surface to the support. The adhesion of this organ of attachment is 

 probably at first occasioned by an exudation of viscid sap ; but the attachment to 

 the support is caused mainly by this organ forcing itself into all the depressions in 

 the surface of the support and growing over the slight elevations. After this has 

 taken place the whole tendril becomes thicker; it contracts spirally, the stem to 

 which it belongs being thus drawn towards the wall, rock, &c. ; then it becomes 

 woody, and the firmness of its tissue and the power of retention of the disc are so 

 considerable that, according to Darwin ^ a tendril ten years old and furnished with 

 five of these discs can support a weight of 10 lbs. without giving way and without 

 the disc becoming detached from the wall. Since a shoot which is growing upwards 

 forms a number of tendrils, this attachment to the flat support is a very effectual one, 

 and enables the plant to endure the annually increasing weight of the stem which is 

 gradually becoming thicker and more woody; and in this way it climbs over the 

 walls and roofs of buildings more than 100 feet high. The fact is very interesting 

 that those tendrils of the Virginian Creeper which do not come into contact with the 

 wall or rock die after some time, and wither up into slender threads which then fall 

 off, no adhesive disc having been formed on them. But in order that these peculiar 

 tendrils may more readily come into contact with the support, the upright shoot 

 is scarcely at all positively heliotropic, since this property would cause it and its 

 tendrils to move further away from the supports ; while the young shoots which ex- 

 hibit very slight heliotropism become erect under the influence of gravitation; other- 

 wise the whole of the contrivances connected with the tendrils would be purposeless. 



^ [Movements and Habits of Climbing Plants, 1875.] 



