Text-book of botany : morphological and physiological / by Julius Sachs ; translated and annotated by Alfred W. Bennett ; assisted by W.T. Thiselton Dyer.

Date:
1875
Catalogue details

Licence: Public Domain Mark

Credit: Text-book of botany : morphological and physiological / by Julius Sachs ; translated and annotated by Alfred W. Bennett ; assisted by W.T. Thiselton Dyer. Source: Wellcome Collection.

Provider: This material has been provided by the Royal College of Physicians of Edinburgh. The original may be consulted at the Royal College of Physicians of Edinburgh.

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    much thicker; this is strikingly the case in Clematis glandulosa and Solatium jasmi- noides. But the most perfect adaptation is shown in the tendrils of the Virginian creeper, Bignotiia 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 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 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 Darwin1, a tendril ten years old and furnished with five of these discs can support a weight of iolbs. 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 different. 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, 1 [Movements and Habits of Climbing Plants ; Journ. Linn. Soc. vol. IX, t86j, p. 87.]