第14章 LEAF-CLIMBERS(2)

Clematis Sieboldi.--A shoot made three revolutions against the sun at an average rate of 3 hrs.11 m.The power of twining is like that of the last species.Its leaves are nearly similar in structure and in function, excepting that the sub-petioles of the lateral and terminal leaflets are sensitive.A loop of thread, weighing one-eighth of a grain, acted on the main petiole, but not until two or three days had elapsed.The leaves have the remarkable habit of spontaneously revolving, generally in vertical ellipses, in the same manner, but in a less degree, as will be described under C.microphylla.

Clematis calycina.--The young shoots are thin and flexible: one revolved, describing a broad oval, in 5 hrs.30 m., and another in 6hrs.12 m.They followed the course of the sun; but the course, if observed long enough, would probably be found to vary in this species, as well as in all the others of the genus.It is a rather better twiner than the two last species: the stem sometimes made two spiral turns round a thin stick, if free from twigs; it then ran straight up for a space, and reversing its course took one or two turns in an opposite direction.This reversal of the spire occurred in all the foregoing species.The leaves are so small compared with those of most of the other species, that the petioles at first seem ill-adapted for clasping.Nevertheless, the main service of the revolving movement is to bring them into contact with surrounding objects, which are slowly but securely seized.The young petioles, which alone are sensitive, have their ends bowed a little downwards, so as to be in a slight degree hooked; ultimately the whole leaf, if it catches nothing, becomes level.I gently rubbed with a thin twig the lower surfaces of two young petioles; and in 2 hrs.30 m.they were slightly curved downwards; in 5 hrs., after being rubbed, the end of one was bent completely back, parallel to the basal portion;in 4 hrs.subsequently it became nearly straight again.To show how sensitive the young petioles are, I may mention that I just touched the under sides of two with a little water-colour, which when dry formed an excessively thin and minute crust; but this sufficed in 24hrs.to cause both to bend downwards.Whilst the plant is young, each leaf consists of three divided leaflets, which barely have distinct petioles, and these are not sensitive; but when the plant is well grown, the petioles of the two lateral and terminal leaflets are of considerable length, and become sensitive so as to be capable of clasping an object in any direction.

When a petiole has clasped a twig, it undergoes some remarkable changes, which may be observed with the other species, but in a less strongly marked manner, and will here be described once for all.The clasped petiole in the course of two or three days swells greatly, and ultimately becomes nearly twice as thick as the opposite one which has clasped nothing.When thin transverse slices of the two are placed under the microscope their difference is conspicuous: the side of the petiole which has been in contact with the support, is formed of a layer of colourless cells with their longer axes directed from the centre, and these are very much larger than the corresponding cells in the opposite or unchanged petiole; the central cells, also, are in some degree enlarged, and the whole is much indurated.The exterior surface generally becomes bright red.But a far greater change takes place in the nature of the tissues than that which is visible: the petiole of the unclasped leaf is flexible and can be snapped easily, whereas the clasped one acquires an extraordinary degree of toughness and rigidity, so that considerable force is required to pull it into pieces.With this change, great durability is probably acquired; at least this is the case with the clasped petioles of Clematis vitalba.The meaning of these changes is obvious, namely, that the petioles may firmly and durably support the stem.

Clematis microphylla, var.leptophylla.--The long and thin internodes of this Australian species revolve sometimes in one direction and sometimes in an opposite one, describing long, narrow, irregular ellipses or large circles.Four revolutions were completed within five minutes of the same average rate of 1 hr.51 m.; so that this species moves more quickly than the others of the genus.The shoots, when placed near a vertical stick, either twine round it, or clasp it with the basal portions of their petioles.The leaves whilst young are nearly of the same shape as those of C.viticella, and act in the same manner like a hook, as will be described under that species.

But the leaflets are more divided, and each segment whilst young terminates in a hardish point, which is much curved downwards and inwards; so that the whole leaf readily catches hold of any neighbouring object.The petioles of the young terminal leaflets are acted on by loops of thread weighing 0.125th and even 0.0625th of a grain.The basal portion of the main petiole is much less sensitive, but will clasp a stick against which it presses.