The spiral contraction of tendrils is quite independent of their power of spontaneously revolving, for it occurs in tendrils, such as those of Lathyrus grandiflorus and Ampelopsis hederacea, which do not revolve.It is not necessarily related to the curling of the tips round a support, as we see with the Ampelopsis and Bignonia capreolata, in which the development of adherent discs suffices to cause spiral contraction.Yet in some cases this contraction seems connected with the curling or clasping movement, due to contact with a support; for not only does it soon follow this act, but the contraction generally begins close to the curled extremity, and travels downwards to the base.If, however, a tendril be very slack, the whole length almost simultaneously becomes at first flexuous and then spiral.Again, the tendrils of some few plants never contract spirally unless they have first seized hold of some object; if they catch nothing they hang down, remaining straight, until they wither and drop off: this is the case with the tendrils of Bignonia, which consist of modified leaves, and with those of three genera of the Vitaceae, which are modified flower-peduncles.But in the great majority of cases, tendrils which have never come in contact with any object, after a time contract spirally.All these facts taken together, show that the act of clasping a support and the spiral contraction of the whole length of the tendril, are phenomena not necessarily connected.
The spiral contraction which ensues after a tendril has caught a support is of high service to the plant; hence its almost universal occurrence with species belonging to 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 stem, after the tendrils have seized some object above, would leave it slack, were it not for the spiral contraction which draws up the stem as it increases in length.Thus there is no waste of growth, and the stretched stem ascends by the shortest course.When a terminal branchlet of the tendril of Cobaea catches a stick, we have seen how well the spiral contraction successively brings the other branchlets, one after the other, into contact with the stick, until the whole tendril grasps 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 of the tendrils is that they are thus made highly elastic.As before remarked under Ampelopsis, the strain is thus distributed equally between the several attached branches; and this renders the whole far stronger than it otherwise would be, as the branches cannot break separately.It is this elasticity which protects both branched and simple tendrils from being torn away from their supports 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 thick and thin branches were tossed to and fro by the wind, the tendrils, had they not been excessively elastic, would instantly have been torn off and the plant thrown prostrate.But as it was, the Bryony safely rode out the gale, like a ship with two anchors down, and with a long range of cable ahead to serve as a spring as she surges to the storm.
When an unattached tendril contracts spirally, the spire always runs in the same direction from tip to base.A tendril, on the other hand, which has caught a support by its extremity, although the same side is concave from end to end, invariably becomes twisted in one part in one direction, and in another part in the opposite direction;the oppositely turned spires being separated by a short straight portion.This curious and symmetrical structure has been noticed by several botanists, but has not been sufficiently explained. It occurs without exception with all tendrils which after catching an object contract spirally, but is of course most conspicuous in the longer tendrils.It never occurs with uncaught tendrils; and when this appears to have occurred, it will be found that the tendril had originally seized some object and had afterwards been torn free.
Commonly, all the spires at one end of an attached tendril run in one direction, and all those at the other end in the opposite direction, with a single short straight portion in the middle; but I have seen a tendril with the spires alternately turning five times in opposite directions, with straight pieces between them; and M.Leon has seen seven or eight such alternations.Whether the spires turn once or more than once in opposite directions, there are as many turns in the one direction as in the other.For instance, I gathered ten attached tendrils of the Bryony, the longest with 33, and the shortest with only 8 spiral turns; and the number of turns in the one direction was in every case the same (within one) as in the opposite direction.
The explanation of this curious little fact is not difficult.I will not attempt any geometrical reasoning, but will give only a practical illustration.In doing this, I shall first have to allude to a point which was almost passed over when treating of Twining-plants.If we hold in our left hand a bundle of parallel strings, we can with our right hand turn these round and round, thus imitating the revolving movement of a twining plant, and the strings do not become twisted.