Page:Darwin - On the movements and habits of climbing plants.djvu/42

 some little force. The petioles, when coming into contact with a stick, take either a complete or half turn round it; ultimately they increase much in thickness. Leaves arising on the side of the stem opposite to the light move towards it; and, in doing so, the petioles are sometimes brought into contact with the stem, and consequently clasp it; but the petioles have no true spontaneous movement.

Lophospermum scandens, var. purpureum.—Some long, moderately thin internodes made four revolutions at an average rate of 3 h. 15 m. The course pursued was very irregular—sometimes an extremely narrow ellipse, sometimes a large circle, sometimes an irregular spire or zigzag line, and sometimes the apex stood still. The young petioles, when brought by the revolving movement into contact with a stick, clasp it, and soon increase considerably in thickness; but they are not quite so sensitive to a light weight as those of the Rhodochiton, for loops of thread weighing one-eighth of a grain did not invariably cause them to bend.

This plant presents a case not observed in any other leaf-climber or twiner or tendril-bearer, or in any other plant as far as I know, namely, that the young internodes are sensitive to a touch. When a petiole clasps a stick, it draws the base of the internode against it; and then the internode itself bends towards the stick, which is thus caught between the stem and the petiole as by a pair of pincers. The internode straightens itself again, excepting the part in contact with the stick. Young internodes alone are sensitive, and these are sensitive on all sides along their whole length. I made fifteen trials by lightly rubbing two or three times with a thin twig several internodes; and in about 2 h., but in one case in 3 h., all became bent: they became straight again in about 4 h., subsequently. An internode, which was rubbed as much as six or seven times with a twig, became just perceptibly curved in 1 h. 15 m., and subsequently in 3 h. the curvature increased much; the internode became straight again in the course of the night. I rubbed some internodes one day on one side, and the next day on the opposite side or at right angles; and the curvature was always towards the rubbed side.

According to Palm (S. 63), the petioles of Limaria cirrhosa and, to a limited degree, those of L. elatine have the power of clasping a support.

.—S. jasminoides.—Some of the species of this large genus are twiners; but this is a true leaf-climber. A long, nearly upright shoot made four revolutions, moving against the sun, very