第18章 LEAF-CLIMBERS(6)

Tropaeolum pentaphyllum.--This species has not the power of spirally twining, which seems due, not so much to a want of flexibility in the stem, as to continual interference from the clasping petioles.An upper internode made three revolutions, following the sun, at an average rate of 1 hr.46 m.The main purpose of the revolving movement in all the species of Tropaeolum manifestly is to bring the petioles into contact with some supporting object.The petiole of a young leaf, after a slight rub, became curved in 6 m.; another, on a cold day, in 20 m., and others in from 8 m.to 10 m.Their curvature usually increased greatly in from 15 m.to 20 m., and they became straight again in between 5 hrs.and 6 hrs., but on one occasion in 3hrs.When a petiole has fairly clasped a stick, it is not able, on the removal of the stick, to straighten itself.The free upper part of one, the base of which had already clasped a stick, still retained the power of movement.A loop of thread weighing 0.125th of a grain caused a petiole to curve; but the stimulus was not sufficient, the loop remaining suspended, to cause a permanent flexure.If a much heavier loop be placed in the angle between the petiole and the stem, it produces no effect; whereas we have seen with Clematis montana that the angle between the stem and petiole is sensitive.

Tropaeolum peregrinum.--The first-formed internodes of a young plant did not revolve, resembling in this respect those of a twining plant.

In an older plant the four upper internodes made three irregular revolutions, in a course opposed to the sun, at an average rate of 1hr.48 min.It is remarkable that the average rate of revolution (taken, however, but from few observations) is very nearly the same in this and the two last species, namely, 1 hr.47 m., 1 hr.46 m., and 1 hr.48 m.The present species cannot twine spirally, which seems mainly due to the rigidity of the stem.In a very young plant, which did not revolve, the petioles were not sensitive.In older plants the petioles of quite young leaves, and of leaves as much as an inch and a quarter in diameter, are sensitive.A moderate rub caused one to curve in 10 m., and others in 20 m.They became straight again in between 5 hrs.45m.and 8 hrs.Petioles which have naturally come into contact with a stick, sometimes take two turns round it.After they have clasped a support, they become rigid and hard.They are less sensitive to a weight than in the previous species; for loops of string weighing 0.82 of a grain (53.14 mg.), did not cause any curvature, but a loop of double this weight (1.64gr.) acted.

Tropaeolum elegans.--I did not make many observations on this species.The short and stiff internodes revolve irregularly, describing small oval figures.One oval was completed in 3 hrs.Ayoung petiole, when rubbed, became slightly curved in 17 m.; and afterwards much more so.It was nearly straight again in 8 hrs.

Tropaeolum tuberosum.--On a plant nine inches in height, the internodes did not move at all; but on an older plant they moved irregularly and made small imperfect ovals.These movements could be detected only by being traced on a bell-glass placed over the plant.

Sometimes the shoots stood still for hours; during some days they moved only in one direction in a crooked line; on other days they made small irregular spires or circles, one being completed in about 4 hrs.The extreme points reached by the apex of the shoot were only about one or one and a half inches asunder; yet this slight movement brought the petioles into contact with some closely surrounding twigs, which were then clasped.With the lessened power of spontaneously revolving, compared with that of the previous species, the sensitiveness of the petioles is also diminished.These, when rubbed a few times, did not become curved until half an hour had elapsed; the curvature increased during the next two hours, and then very slowly decreased; so that they sometimes required 24 hrs.to become straight again.Extremely young leaves have active petioles;one with the lamina only 0.15 of an inch in diameter, that is, about a twentieth of the full size, firmly clasped a thin twig.But leaves grown to a quarter of their full size can likewise act.

Tropaeolum minus (?).--The internodes of a variety named "dwarf crimson Nasturtium" did not revolve, but moved in a rather irregular course during the day to the light, and from the light at night.The petioles, when well rubbed, showed no power of curving; nor could Isee that they ever clasped any neighbouring object.We have seen in this genus a gradation from species such as T.tricolorum, which have extremely sensitive petioles, and internodes which rapidly revolve and 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 the present species, in T.elegans, and probably in others, the flower-peduncle, as soon as the seed-capsule begins to swell, spontaneously bends abruptly downwards and becomes somewhat convoluted.If a stick stands in the way, it is to a certain extent clasped; but, as far as I have been able to observe, this clasping movement is independent of the stimulus from contact.

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