第20章 LEAF-CLIMBERS(8)

Rhodochiton volubile.--A long flexible shoot swept a large circle, following the sun, in 5 hrs.30 m.; and, as the day became warmer, a second circle was completed in 4 hrs.10 m.The shoots sometimes make a whole or a half spire round a vertical stick, they then run straight up for a space, and afterwards turn spirally in an opposite direction.The petioles of very young leaves about one-tenth of their full size, are highly sensitive, and bend towards the side which is touched; but they do not move quickly.One was perceptibly curved in 1 hr.10 m., after being lightly rubbed, and became considerably curved in 5 hrs.40 m.; some others were scarcely curved in 5 hrs.30 m., but distinctly so in 6 hrs.30 m.A curvature was perceptible in one petiole in between 4 hrs.30 m.and 5 hrs., after the suspension of a little loop of string.A loop of fine cotton thread, weighing one sixteenth of a grain (4.05 mg.), not only caused a petiole slowly to bend, but was ultimately so firmly clasped that it could be withdrawn only by some little force.The petioles, when coming into contact with a stick, take either a complete or half a turn round it, and ultimately increase much in thickness.They do not possess the power of spontaneously revolving.

Lophospermum scandens, var.purpureum.--Some long, moderately thin internodes made four revolutions at an average rate of 3 hrs.15 m.

The course pursued was very irregular, namely, an extremely narrow ellipse, a large circle, an irregular spire or a zigzag line, and sometimes the apex stood still.The young petioles, when brought by the revolving movement into contact with sticks, clasped them, and soon increased considerably in thickness.But they are not quite so sensitive to a weight as those of the Rhodochiton, for loops of thread weighing one-eighth of a grain did not always cause them to bend.

This plant presents a case not observed by me in any other leaf-climber or twiner, namely, that the young internodes of the stem are sensitive to a touch.When a petiole of this species clasps a stick, it draws the base of the internode against it; and then the internode itself bends towards the stick, which is caught between the stem and the petiole as by a pair of pincers.The internode afterwards straightens itself, excepting the part in actual 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 twice or thrice lightly rubbing with a thin twig several internodes; and in about 2 hrs., but in one case in 3 hrs., all were bent: they became straight again in about 4 hrs.afterwards.An internode, which was rubbed as often as six or seven times, became just perceptibly curved in 1 hr.15 m., and in 3 hrs.the curvature increased much; it became straight again in the course of the succeeding night.I rubbed some internodes one day on one side, and the next day either on the opposite side or at right angles to the first side; and the curvature was always towards the rubbed side.

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

SOLANACEAE.--Solanum jasminoides.--Some of the species in this large genus are twiners; but the present species is a true leaf-climber.Along, nearly upright shoot made four revolutions, moving against the sun, very regularly at an average rate of 3 hrs.26 m.The shoots, however, sometimes stood still.It is considered a greenhouse plant;but when kept there, the petioles took several days to clasp a stick:

in the hothouse a stick was clasped in 7 hrs.In the greenhouse a petiole was not affected by a loop of string, suspended during several days and weighing 2.5 grains (163 mg.); but in the hothouse one was made to curve by a loop weighing 1.64 gr.(106.27 mg.); and, on the removal of the string, it became straight again.Another petiole was not at all acted on by a loop weighing only 0.82 of a grain (53.14 mg.) We have seen that the petioles of some other leaf-climbing plants are affected by one-thirteenth of this latter weight.

In this species, and in no other leaf-climber seen by me, a full-grown leaf is capable of clasping a stick; but in the greenhouse the movement was so extraordinarily slow that the act required several weeks; on each succeeding week it was clear that the petiole had become more and more curved, until at last it firmly clasped the stick.