CHAPTER III.--TENDRIL-BEARERS.

Nature of tendrils--BIGNONIACEAE, various species of, and their different modes of climbing--Tendrils which avoid the light and creep into crevices--Development of adhesive discs--Excellent adaptations for seizing different kinds of supports.--POLEMONIACEAE--Cobaea scandens much branched and hooked tendrils, their manner of action-- LEGUMINOSAE--COMPOSITAE--SMILACEAE--Smilax aspera, its inefficient tendrils--FUMARIACEAE--Corydalis claviculata, its state intermediate between that of a leaf-climber and a tendril-bearer.

By tendrils I mean filamentary organs, sensitive to contact and used exclusively for climbing. By this definition, spines, hooks and rootlets, all of which are used for climbing, are excluded. True tendrils are formed by the modification of leaves with their petioles, of flower-peduncles, branches, {24} and perhaps stipules. Mohl, who includes under the name of tendrils various organs having a similar external appearance, classes them according to their homological nature, as being modified leaves, flower-peduncles, &c. This would be an excellent scheme; but I observe that botanists are by no means unanimous on the homological nature of certain tendrils. 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. The species to be described belong to ten families, and will be given in the following order: --Bignoniaceae, Polemoniaceae, Leguminosae, Compositae, Smilaceae, Fumariaceae, Cucurbitaceae, Vitaceae, Sapindaceae, Passifloraceae. {25}

BIGNONIACEAE.--This family contains many tendril-bearers, some twiners, and some root-climbers. The tendrils always consist of modified leaves. Nine species of Bignonia, selected by hazard, are here described, in order to show what diversity of structure and action there may be within the same genus, and to show what remarkable powers some tendrils possess. The species, taken together, afford connecting links between twiners, leaf-climbers, tendril-bearers, and root-climbers.

Bignonia (an unnamed species from Kew, closely allied to B. unguis, but with smaller and rather broader leaves).--A young shoot from a cut-down plant made three revolutions against the sun, at an average rate of 2 hrs. 6m. The stem is thin and flexible; it twined round a slender vertical stick, ascending from left to right, as perfectly and as regularly as any true twining-plant. When thus ascending, it makes no use of its tendrils or petioles; but when it twined round a rather thick stick, and its petioles were brought into contact with it, these curved round the stick, showing that they have some degree of irritability. The petioles also exhibit a slight degree of spontaneous movement; for in one case they certainly described minute, irregular, vertical ellipses. The tendrils apparently curve themselves spontaneously to the same side with the petioles; but from various causes, it was difficult to observe the movement of either the tendrils or petioles, in this and the two following species. The tendrils are so closely similar in all respects to those of B. unguis, that one description will suffice.

Bignonia unguis.--The young shoots revolve, but less regularly and less quickly than those of the last species. The stem twines imperfectly round a vertical stick, sometimes reversing its direction, in the same manner as described in so many leaf-climbers; and this plant though possessing tendrils, climbs to a certain extent like a leaf-climber. Each leaf consists of a petiole bearing a pair of leaflets, and terminates in a tendril, which is formed by the modification of three leaflets, and closely resembles that above figured (fig. 5). But it is a little larger, and in a young plant was about half an inch in length. It is curiously like the leg and foot of a small bird, with the hind toe cut off. The straight leg or tarsus is longer than the three toes, which are of equal length, and diverging, lie in the same plane. The toes terminate in sharp, hard claws, much curved downwards, like those on a bird's foot. The petiole of the leaf is sensitive to contact; even a small loop of thread suspended for two days caused it to bend upwards; but the sub- petioles of the two lateral leaflets are not sensitive. The whole tendril, namely, the tarsus and the three toes, are likewise sensitive to contact, especially on their under surfaces. When a shoot grows in the midst of thin branches, the tendrils are soon brought by the revolving movement of the internodes into contact with them; and then one toe of the tendril or more, commonly all three, bend, and after several hours seize fast hold of the twigs, like a bird when perched.

The Movements and Habits of Climbing Plants Page 31

Charles Darwin

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Charles Darwin

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