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This is the case: thus the several leaf-climbing species of the Antirrhineae, of Solanum, Cocculus, and Gloriosa, have within the same family and even within the same genus, relatives which are twiners. In the genus Mikania, there are leaf-climbing and twining species. The leaf-climbing species of Clematis are very closely allied to the tendril-bearing Naravelia.
This may be the more safely inferred, as in young and unhealthy specimens of other tendril-bearing plants similar rudiments may occasionally be observed. In the bean these filaments are variable in shape, as is so frequently the case with rudimentary organs; they are either cylindrical, or foliaceous, or are deeply furrowed on the upper surface.
The Vine is the weakest revolver observed by me, apparently exhibiting only a trace of a former power. In the Eccremocarpus the movement is interrupted by many long pauses. Very few tendril-bearing plants can spirally twine up an upright stick.
Bates remarks, likewise abounds according to Mohl and Palm with climbing plants; and of the tendril-bearing plants examined by me, the highest developed kinds are natives of this grand continent, namely, the several species of Bignonia, Eccremocarpus, Cobaea, and Ampelopsis.
There is no such movement in any leaf- climber, with the exception of an occasional trace of it in the petioles of Tropaeolum tricolorum. On the other hand, the tendrils of all tendril-bearing plants, contract spirally after they have caught an object with the following exceptions. Firstly, Corydalis claviculata, but then this plant might be called a leaf-climber.
I have seen only one tendril-bearing plant, namely, Smilax aspera, which is furnished with reflexed spines; but this is the case with several branch-climbers in South Brazil and Ceylon; and their branches graduate into true tendrils. Some few plants apparently depend solely on their hooks for climbing, and yet do so efficiently, as certain palms in the New and Old Worlds.
With the majority of tendril-bearing plants the young internodes revolve in more or less broad ellipses, like those made by twining plants; but the figures described, when carefully traced, generally form irregular ellipsoidal spires.
In the leaf-climbing Clematis flammula, and in the tendril-bearing Vine, we see no loss in the power of climbing, but only a remnant of the revolving power which is indispensable to all twiners, and is so common as well as so advantageous to most climbers. In Tecoma radicans, one of the Bignoniaceae, we see a last and doubtful trace of the power of revolving.
Tendril-bearers have undergone much more modification than leaf-climbers; hence it is not surprising that their supposed primordial habits of revolving and twining have been more frequently lost or modified than in the case of leaf-climbers. The three great tendril-bearing families in which this loss has occurred in the most marked manner, are the Cucurbitaceae, Passifloraceae, and Vitaceae.
Consequently I will describe tendril-bearing plants by natural families, following Lindley's classification; and this will in most cases keep those of the same nature together.
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