第15章 LEAF-CLIMBERS(3)

The leaves, whilst young, are continually and spontaneously moving slowly.A bell-glass was placed over a shoot secured to a stick, and the movements of the leaves were traced on it during several days.Avery irregular line was generally formed; but one day, in the course of eight hours and three quarters, the figure clearly represented three and a half irregular ellipses, the most perfect one of which was completed in 2 hrs.35 m.The two opposite leaves moved independently of each other.This movement of the leaves would aid that of the internodes in bringing the petioles into contact with surrounding objects.I discovered this movement too late to be enabled to observe it in the other species; but from analogy I can hardly doubt that the leaves of at least C.viticella, C.flammula, and C.vitalba move spontaneously; and, judging from C Sieboldi, this probably is the case with C.montana and C.calycina.I ascertained that the simple leaves of C.glandulosa exhibited no spontaneous revolving movement.

Clematis viticella, var.venosa.--In this and the two following species the power of spirally twining is completely lost, and this seems due to the lessened flexibility of the internodes and to the interference caused by the large size of the leaves.But the revolving movement, though restricted, is not lost.In our present species a young internode, placed in front of a window, made three narrow ellipses, transversely to the direction of the light, at an average rate of 2 hrs.40 m.When placed so that the movements were to and from the light, the rate was greatly accelerated in one half of the course, and retarded in the other, as with twining plants.

The ellipses were small; the longer diameter, described by the apex of a shoot bearing a pair of not expanded leaves, was only 4.625inches, and that by the apex of the penultimate internode only 1.125inch.At the most favourable period of growth each leaf would hardly be carried to and fro by the movement of the internodes more than two or three inches, but, as above stated, it is probable that the leaves themselves move spontaneously.The movement of the whole shoot by the wind and by its rapid growth, would probably be almost equally efficient as these spontaneous movements, in bringing the petioles into contact with surrounding objects.

The leaves are of large size.Each bears three pairs of lateral leaflets and a terminal one, all supported on rather long sub-petioles.The main petiole bends a little angularly downwards at each point where a pair of leaflets arises (see fig.2), and the petiole of the terminal leaflet is bent downwards at right angles;hence the whole petiole, with its rectangularly bent extremity, acts as a hook.This hook, the lateral petioles being directed a little upwards; forms an excellent grappling apparatus, by which the leaves readily become entangled with surrounding objects.If they catch nothing, the whole petiole ultimately grows straight.The main petiole, the sub-petioles, and the three branches into which each basi-lateral sub-petiole is generally subdivided, are all sensitive.

The basal portion of the main petiole, between the stem and the first pair of leaflets, is less sensitive than the remainder; it will, however, clasp a stick with which it is left in contact.The inferior surface of the rectangularly bent terminal portion (carrying the terminal leaflet), which forms the inner side of the end of the hook, is the most sensitive part; and this portion is manifestly best adapted to catch a distant support.To show the difference in sensibility, I gently placed loops of string of the same weight (in one instance weighing only 0.82 of a grain or 53.14 mg.) on the several lateral sub-petioles and on the terminal one; in a few hours the latter was bent, but after 24 hrs.no effect was produced on the other sub-petioles.Again, a terminal sub-petiole placed in contact with a thin stick became sensibly curved in 45 m., and in 1 hr.10m.

moved through ninety degrees; whilst a lateral sub-petiole did not become sensibly curved until 3 hrs.30 m.had elapsed.In all cases, if the sticks are taken away, the petioles continue to move during many hours afterwards; so they do after a slight rubbing; but they become straight again, after about a day's interval, that is if the flexure has not been very great or long continued.

The graduated difference in the extension of the sensitiveness in the petioles of the above-described species deserves notice.In C.

montana it is confined to the main petiole, and has not spread to the sub-petioles of the three leaflets; so it is with young plants of C.

calycina, but in older plants it spreads to the three sub-petioles.

In C.viticella the sensitiveness has spread to the petioles of the seven leaflets, and to the subdivisions of the basi-lateral sub-petioles.But in this latter species it has diminished in the basal part of the main petiole, in which alone it resided in C.montana;whilst it has increased in the abruptly bent terminal portion.