We thus see that leaves may acquire all the leading and characteristic qualities of tendrils, namely, sensitiveness, spontaneous movement, and subsequently increased strength.If their blades or laminae were to abort, they would form true tendrils.And of this process of abortion we can follow every step, until no trace of the original nature of the tendril is left.In Mutisia clematis, the tendril, in shape and colour, closely resembles the petiole of one of the ordinary leaves, together with the midribs of the leaflets, but vestiges of the laminae are still occasionally retained.In four genera of the Fumariaceae we can follow the whole process of transformation.The terminal leaflets of the leaf-climbing Fumaria officinalis are not smaller than the other leaflets;those of the leaf-climbing Adlumia cirrhosa are greatly reduced;those of Corydalis claviculata (a plant which may indifferently be called a leaf-climber or a tendril-bearer) are either reduced to microscopical dimensions or have their blades wholly aborted, so that this plant is actually in a state of transition; and, finally, in the Dicentra the tendrils are perfectly characterized.If, therefore, we could behold at the same time all the progenitors of Dicentra, we should almost certainly see a series like that now exhibited by the above-named three genera.In Tropaeolum tricolorum we have another kind of passage; for the leaves which are first formed on the young stems are entirely destitute of laminae, and must be called tendrils, whilst the later formed leaves have well-developed laminae.In all cases the acquirement of sensitiveness by the mid-ribs of the leaves appears to stand in some close relation with the abortion of their laminae or blades.
On the view here given, leaf-climbers were primordially twiners, and tendril-bearers (when formed of modified leaves) were primordially leaf-climbers.The latter, therefore, are intermediate in nature between twiners and tendril-bearers, and ought to be related to both.
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.The Fumariaceae include closely allied genera which are leaf-climbers and tendril-bearers.
Lastly, a species of Bignonia is at the same time both a leaf-climber and a tendril-bearer; and other closely allied species are twiners.
Tendrils of another kind consist of modified flower-peduncles.In this case we likewise have many interesting transitional states.The common Vine (not to mention the Cardiospermum) gives us every possible gradation between a perfectly developed tendril and a flower-peduncle covered with flowers, yet furnished with a branch, forming the flower-tendril.When the latter itself bears a few flowers, as we know sometimes is the case, and still retains the power of clasping a support, we see an early condition of all those tendrils which have been formed by the modification of flower-peduncles.
According to Mohl and others, some tendrils consist of modified branches: I have not observed any such cases, and know nothing of their transitional states, but these have been fully described by Fritz Muller.The genus Lophospermum also shows us how such a transition is possible; for its branches spontaneously revolve and are sensitive to contact.Hence, if the leaves on some of the branches of the Lophospermum were to abort, these branches would be converted into true tendrils.Nor is there anything improbable in certain branches alone being thus modified, whilst others remained unaltered; for we have seen with certain varieties of Phaseolus, that some of the branches are thin, flexible, and twine, whilst other branches on the same plant are stiff and have no such power.
If we inquire how a petiole, a branch or flower-peduncle first became sensitive to a touch, and acquired the power of bending towards the touched side, we get no certain answer.Nevertheless an observation by Hofmeister well deserves attention, namely, that the shoots and leaves of all plants, whilst young, move after being shaken.
Kerner also finds, as we have seen, that the flower-peduncles of a large number of plants, if shaken or gently rubbed bend to this side.
And it is young petioles and tendrils, whatever their homological nature may be, which move on being touched.It thus appears that climbing plants have utilized and perfected a widely distributed and incipient capacity, which capacity, as far as we can see, is of no service to ordinary plants.If we further inquire how the stems, petioles, tendrils, and flower-peduncles of climbing plants first acquired their power of spontaneously revolving, or, to speak more accurately, of successively bending to all points of the compass, we are again silenced, or at most can only remark that the power of moving, both spontaneously and from various stimulants, is far more common with plants, than is generally supposed to be the case by those who have not attended to the subject.I have given one remarkable instance, namely that of the Maurandia semperflorens, the young flower-peduncles of which spontaneously revolve in very small circles, and bend when gently rubbed to the touched side; yet this plant certainly does not profit by these two feebly developed powers.