We have seen that tendrils consist of various organs in a modified state, namely, leaves, flower-peduncles, branches, and perhaps stipules. With respect to leaves, the evidence of their modification is ample. In young plants of Bignonia the lower leaves often remain quite unchanged, whilst the upper ones have their terminal leaflets converted into perfect tendrils; in _Eccremocarpus_ I have seen a single lateral branch of a tendril replaced by a perfect leaflet; in _Vicia sativa_, on the other hand, leaflets are sometimes replaced by tendril-branches; and many other such cases could be given. But he who believes in the slow modification of species will not be content simply to ascertain the homological nature of different kinds of tendrils; he will wish to learn, as far as is possible, by what actual steps leaves, flower-peduncles, &c., have had their functions wholly changed, and have come to serve merely as prehensile organs. In the whole group of leaf-climbers abundant evidence has been given that an organ, still subserving the functions of a leaf, may become sensitive to a touch, and thus grasp an adjoining object. With several leaf-climbers the true leaves spontaneously revolve; and their petioles, after clasping a support grow thicker and stronger. 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 laminæ 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 laminæ are still occasionally retained. In four genera of the Fumariaceæ 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 _Tropæolum tricolorum_ we have another kind of passage; for the leaves which are first formed on the young stems are entirely destitute of laminæ, and must be called tendrils, whilst the later formed leaves have well-developed laminæ. In all cases the acquirement of sensitiveness by