RELATION OF MORPHOLOGICAL NATURE OF ORGANS TO ADAPTATION, 839 



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, Bigjionia capreolata, and some other plants. As in the grape-vine, the 

 tendrils are here branched axial structures, and are to a much greater extent nega- 

 tively 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 swelHng 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 of attachment 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 lolbs. 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, even the upright shoot is 

 scarcely positively heliotropic, since this property would cause it and its tendrils to 

 move further away from the supports ; while the young shoots which exhibit such 

 very slight heliotropism become erect under the influence of gravitation ; otherwise 

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



If looked at merely from the outside, the mode in which the Virginian creeper 

 climbs up rocks, walls, and thick trees, presents a certain resemblance to the climbing 

 of the ivy ; but in fact the adaptations of the two are altogether difl"erent. It has 

 already been shown how negative heliotropism causes the leafy branches of the ivy 

 to become closely pressed to the support, and how the summit of the branch at 

 first exhibits slight positive heliotropism, so that it is attached to the support 

 with a slight convexity. At this point of pressure rows of aerial roots afterwards 

 arise (not in consequence of pressure, for they make their appearance also on 

 branches which hang free) which apply themselves to the inequalities of the bark 

 of the tree or the rock which serves as a support, and thus fix the ivy-stem to it. 

 Other weak-stemmed plants attain the same object (that of elevating their assimilating 

 and flowering shoots) by apparently much simpler means, as the bramble, rose, 



[Movements and Habits of Climbing Plants ; Journ. Linn. Soc. vol. IX, 1865, p. 87.] 



