Page:Darwin - On the movements and habits of climbing plants.djvu/95

 elapsed from the first observation, that is until 17 days after the tendril was fully grown.

The best proof of the intimate connexion between the spiral contraction of a tendril and the previous act of clasping a support, is afforded by those tendrils which, when caught, invariably contract into a spire, whilst as long as they remain unattached they continue straight, though dependent, and thus wither and drop off. The tendrils of Bignonia, which are modified leaves, thus behave, as do the tendrils of the three genera of Vitaceæ, and these are modified flower-peduncles. The tendrils, however, of Eccremocarpus, which is allied to Bignonia, contract spirally even when they have caught nothing. The uncaught tendrils of the Cardiospermum, and to a certain extent those of the Mutisia, roll themselves up not into a spire, but into a helix.

The spiral contraction which ensues after a tendril has caught a support is of high service to all tendril-bearing plants; hence its almost universal occurrence with plants of widely different orders. When a shoot is inclined and its tendril has caught an object above, the spiral contraction drags up the shoot. When the shoot is upright, the growth of the internodes, subsequently to the tendrils having seized some object above, would slacken the stem were it not for the spiral contraction, which draws up the internodes as they increase in length. Thus there is no waste of growth, and the stretched stem ascends by the shortest course. We have seen in the Cobæa, when a terminal branchlet of the tendril has caught a stick, how well the spiral contraction of its branches successively brings them one after the other into contact with the stick, until the whole tendril has grasped it in an inextricable knot. When a tendril has caught a yielding object, this is sometimes enveloped and still further secured by the spiral folds, as I have seen with Passiflora quadrangularis; but this action is of little importance.

A far more important service rendered by the spiral contraction is that the tendrils are thus made highly elastic. As was previously remarked under Ampelopsis, the strain is thus equally distributed to the several attached branches of a branched tendril; and this must render the whole tendril far stronger, as branch after branch cannot separately break. It is this elasticity which saves both branched and simple tendrils from being torn away during stormy weather. I have more than once gone on purpose during a gale to watch a Bryony growing in an exposed hedge, with its tendrils attached to the surrounding bushes; and as the