Page:Darwin - On the movements and habits of climbing plants.djvu/112

 parts acting as leaves or as flower-peduncles can have wholly changed their function, and have come to serve as prehensile organs.

In the whole group of leaf-climbers abundant evidence has been given that an organ, still subserving its proper function as a leaf, may become sensitive to a touch, and thus grasp an adjoining object. In several leaf-climbers true leaves spontaneously revolve; and their petioles, after clasping a support, grow thicker and stronger. We thus see that true leaves may acquire all the leading and characteristic qualities of tendrils, namely, sensitiveness, spontaneous movement, and subsequent thickening and induration. If their blades or laminæ were to abort, they would form true tendrils. And of this process of abortion we have seen every stage; for in an ordinary tendril, as in that of the Pea, we ran discover no trace of its primordial nature; in Mutisia clematis, the tendril, in shape and colour, closely resembles a petiole with the denuded midribs of its leaflets; and occasionally vestiges of laminæ are retained or reappear. Lastly, in four genera in the same family of the Fumariaceæ we see the whole gradation; for 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 the Corydalis claviculata (a plant which may indifferently be called a leaf-climber or tendril-bearer) are either reduced to microscopical dimensions or have their blades quite aborted, so that this plant is in an actual state of transition; and, finally, in the Dicentra the tendrils are perfectly characterized. Hence, if we were to see at the same time all the progenitors of the Dicentra, we should almost certainly behold a series like that now exhibited by the above-named four genera. In Tropæolum tricolorum we have another kind of passage; for the leaves which are first formed on the young plant are entirely destitute of laminæ, and must be called tendrils, whilst the later-formed leaves have well-developed laminæ. In all cases, in the several kinds of leaf-climbers and of tendril-bearers, the acquirement of sensitiveness by the mid-ribs of the leaves apparently stands in the closest relation with the abortion of their laminæ or blades.

On the view here given, leaf-climbers were primordially twiners, and tendril-bearers (of the modified leaf division) were primordially leaf-climbers. Hence leaf-climbers 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