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- 
7toides, But the most perfect adaptation is shown in the tendrils of the Virginian 
Creeper, Bignonia capreolaia, 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. j 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.] 
