第17章 LEAF-CLIMBERS(5)

Clematis vitalba.--The plants were in pots and not healthy, so that Idare not trust my observations, which indicate much similarity in habits with C.flammula.I mention this species only because I have seen many proofs that the petioles in a state of nature are excited to movement by very slight pressure.For instance, I have found them embracing thin withered blades of grass, the soft young leaves of a maple, and the flower-peduncles of the quaking-grass or Briza.The latter are about as thick as the hair of a man's beard, but they were completely surrounded and clasped.The petioles of a leaf, so young that none of the leaflets were expanded, had partially seized a twig.

Those of almost all the old leaves, even when unattached to any object, are much convoluted; but this is owing to their having come, whilst young, into contact during several hours with some object subsequently removed.With none of the above-described species, cultivated in pots and carefully observed, was there any permanent bending of the petioles without the stimulus of contact.In winter, the blades of the leaves of C.vitalba drop off; but the petioles (as was observed by Mohl) remain attached to the branches, sometimes during two seasons; and, being convoluted, they curiously resemble true tendrils, such as those possessed by the allied genus Naravelia.

The petioles which have clasped some object become much more stiff, hard, and polished than those which have failed in this their proper function.

TROPAEOLUM.--I observed T.tricolorum, T.azureum, T.pentaphyllum, T.peregrinum, T.elegans, T.tuberosum, and a dwarf variety of, as Ibelieve, T.minus.

Tropaeolum tricolorum, var.grandiflorum.--The flexible shoots, which first rise from the tubers, are as thin as fine twine.One such shoot revolved in a course opposed to the sun, at an average rate, judging from three revolutions, of 1 hr.23 m.; but no doubt the direction of the revolving movement is variable.When the plants have grown tall and are branched, all the many lateral shoots revolve.The stem, whilst young, twines regularly round a thin vertical stick, and in one case I counted eight spiral turns in the same direction; but when grown older, the stem often runs straight up for a space, and, being arrested by the clasping petioles, makes one or two spires in a reversed direction.Until the plant grows to a height of two or three feet, requiring about a month from the time when the first shoot appears above ground, no true leaves are produced, but, in their place, filaments coloured like the stem.The extremities of these filaments are pointed, a little flattened, and furrowed on the upper surface.They never become developed into leaves.As the plant grows in height new filaments are produced with slightly enlarged tips; then others, bearing on each side of the enlarged medial tip a rudimentary segment of a leaf; soon other segments appear, and at last a perfect leaf is formed, with seven deep segments.So that on the same plant we may see every step, from tendril-like clasping filaments to perfect leaves with clasping petioles.After the plant has grown to a considerable height, and is secured to its support by the petioles of the true leaves, the clasping filaments on the lower part of the stem wither and drop off;so that they perform only a temporary service.

These filaments or rudimentary leaves, as well as the petioles of the perfect leaves, whilst young, are highly sensitive on all sides to a touch.The slightest rub caused them to curve towards the rubbed side in about three minutes, and one bent itself into a ring in six minutes; they subsequently became straight.When, however, they have once completely clasped a stick, if this is removed, they do not straighten themselves.The most remarkable fact, and one which Ihave observed in no other species of the genus, is that the filaments and the petioles of the young leaves, if they catch no object, after standing for some days in their original position, spontaneously and slowly oscillate a little from side to side, and then move towards the stem and clasp it.They likewise often become, after a time, in some degree spirally contracted.They therefore fully deserve to be called tendrils, as they are used for climbing, are sensitive to a touch, move spontaneously, and ultimately contract into a spire, though an imperfect one.The present species would have been classed amongst the tendril-bearers, had not these characters been confined to early youth.During maturity it is a true leaf-climber.

Tropaeolum azureum.--An upper internode made four revolutions, following the sun, at an average rate of 1 hr.47 m.The stem twined spirally round a support in the same irregular manner as that of the last species.Rudimentary leaves or filaments do not exist.The petioles of the young leaves are very sensitive: a single light rub with a twig caused one to move perceptibly in 5 m., and another in 6m.The former became bent at right angles in 15 min., and became straight again in between 5 hrs.and 6 hrs.A loop of thread weighing 0.125th of a grain caused another petiole to curve.