第11章 TWINING PLANTS(10)

By what means certain twining plants are adapted to ascend only thin stems, whilst others can twine round thicker ones, I do not know.It appeared to me probable that twining plants with very long revolving shoots would be able to ascend thick supports; accordingly I placed Ceropegia Gardnerii near a post 6 inches in diameter, but the shoots entirely failed to wind round it; their great length and power of movement merely aid them in finding a distant stem round which to twine.The Sphaerostemma marmoratum is a vigorous tropical twiner;and as it is a very slow revolver, I thought that this latter circumstance might help it in ascending a thick support; but though it was able to wind round a 6-inch post, it could do this only on the same level or plane, and did not form a spire and thus ascend.

As ferns differ so much in structure from phanerogamic plants, it may be worth while here to show that twining ferns do not differ in their habits from other twining plants.In Lygodium articulatum the two internodes of the stem (properly the rachis) which are first formed above the root-stock do not move; the third from the ground revolves, but at first very slowly.This species is a slow revolver: but L.

scandens made five revolutions, each at the average rate of 5 hrs.45m.; and this represents fairly well the usual rate, taking quick and slow movers, amongst phanerogamic plants.The rate was accelerated by increased temperature.At each stage of growth only the two upper internodes revolved.A line painted along the convex surface of a revolving internode becomes first lateral, then concave, then lateral and ultimately again convex.Neither the internodes nor the petioles are irritable when rubbed.The movement is in the usual direction, namely, in opposition to the course of the sun; and when the stem twines round a thin stick, it becomes twisted on its own axis in the same direction.After the young internodes have twined round a stick, their continued growth causes them to slip a little upwards.

If the stick be soon removed, they straighten themselves, and recommence revolving.The extremities of the depending shoots turn upwards, and twine on themselves.In all these respects we have complete identity with twining phanerogamic plants; and the above enumeration may serve as a summary of the leading characteristics of all twining plants.

The power of revolving depends on the general health and vigour of the plant, as has been laboriously shown by Palm.But the movement of each separate internode is so independent of the others, that cutting off an upper one does not affect the revolutions of a lower one.When, however, Dutrochet cut off two whole shoots of the Hop, and placed them in water, the movement was greatly retarded; for one revolved in 20 hrs.and the other in 23 hrs., whereas they ought to have revolved in between 2 hrs.and 2 hrs.30 m.Shoots of the Kidney-bean, cut off and placed in water, were similarly retarded, but in a less degree.I have repeatedly observed that carrying a plant from the greenhouse to my room, or from one part to another of the greenhouse, always stopped the movement for a time; hence Iconclude that plants in a state of nature and growing in exposed situations, would not make their revolutions during very stormy weather.A decrease in temperature always caused a considerable retardation in the rate of revolution; but Dutrochet (tom.xvii.pp.

994, 996) has given such precise observations on this head with respect to the common pea that I need say nothing more.When twining plants are placed near a window in a room, the light in some cases has a remarkable power (as was likewise observed by Dutrochet, p.

998, with the pea) on the revolving movement, but this differs in degree with different plants; thus Ipomoea jucunda made a complete circle in 5 hrs.30 m.; the semicircle from the light taking 4 hrs.