Page:Darwin - On the movements and habits of climbing plants.djvu/39

 "dwarf crimson Nasturtium" had no power of revolving; but they moved during the day to the light, and from it at night, in a rather irregular course. The petioles, when well rubbed, showed no power of curving; nor could I see that they ever clasped any neighbouring support. We have seen in this genus a gradation from species such as T. tricolorum, which have exquisitely sensitive petioles, and internodes which have rapid revolving powers and can spirally twine up a support, to other species, such as T. elegans and T. tuberosum, the petioles of which are much less sensitive, and the internodes of which have very feeble revolving powers and cannot spirally twine round a support, to this last species, which has entirely lost or never acquired these faculties. From the general character of the genus, the loss of power seems the more probable alternative.

In this species and in T. elegans, and probably in others, the flower-peduncles, as soon as the seed-capsule begins to swell, spontaneously bend abruptly downwards and become somewhat convoluted: when a stick lies in the path, it is to a certain extent clasped; but, as far as I have been able to observe, the movement of the peduncle is quite independent of the stimulus from contact.

.—In this tribe (Lindley) of the Scrophulariaceæ, at least four of the seven included genera have leaf-climbing species.

Maurandia Barclayana.—A thin, slightly bowed shoot made two revolutions, following the sun, each in 3 h. 17 m.; this same shoot, the day before, revolved in an opposite direction. The shoots do not spirally twine, but climb excellently by the aid of the young sensitive petioles. These petioles, when lightly rubbed, move after a considerable interval of time, and subsequently become straight again; a loop of thread weighing $1/8$th of a grain caused them to bend.

Maurandia semperflorens.—This freely growing species climbs exactly like the last, by its sensitive petioles. A young internode made two circles, each in 1 h. 46 m.; so that it moves almost twice as rapidly as the last species. But I should not have noticed the present species, had it not been for the following unique case. Mohl says (S. 45) that "the flower-peduncles, as well as the petioles, are wound into tendrils;" and he adds nothing more about the genus. But it must be observed that Mohl classes as tendrils even such objects as the spiral flower-stalks of the Vallisneria. Nevertheless this remark, and the well-known fact that the flower-peduncles of this Maurandia are flexuous, led me