第6章 TWINING PLANTS(5)

The belief that twiners have a natural tendency to grow spirally, probably arose from their assuming a spiral form when wound round a support, and from the extremity, even whilst remaining free, sometimes assuming this form.The free internodes of vigorously growing plants, when they cease to revolve, become straight, and show no tendency to be spiral; but when a shoot has nearly ceased to grow, or when the plant is unhealthy, the extremity does occasionally become spiral.I have seen this in a remarkable manner with the ends of the shoots of the Stauntonia and of the allied Akebia, which became wound up into a close spire, just like a tendril; and this was apt to occur after some small, ill-formed leaves had perished.The explanation, I believe, is, that in such cases the lower parts of the terminal internodes very gradually and successively lose their power of movement, whilst the portions just above move onwards and in their turn become motionless; and this ends in forming an irregular spire.

When a revolving shoot strikes a stick, it winds round it rather more slowly than it revolves.For instance, a shoot of the Ceropegia, revolved in 6 hrs., but took 9 hrs.30 m.to make one complete spire round a stick; Aristolochia gigas revolved in about 5 hrs., but took 9 hrs.15 m.to complete its spire.This, I presume, is due to the continued disturbance of the impelling force by the arrestment of the movement at successive points; and we shall hereafter see that even shaking a plant retards the revolving movement.The terminal internodes of a long, much-inclined, revolving shoot of the Ceropegia, after they had wound round a stick, always slipped up it, so as to render the spire more open than it was at first; and this was probably in part due to the force which caused the revolutions, being now almost freed from the constraint of gravity and allowed to act freely.With the Wistaria, on the other hand, a long horizontal shoot wound itself at first into a very close spire, which remained unchanged; but subsequently, as the shoot twined spirally up its support, it made a much more open spire.With all the many plants which were allowed freely to ascend a support, the terminal internodes made at first a close spire; and this, during windy weather, served to keep the shoots in close contact with their support; but as the penultimate internodes grew in length, they pushed themselves up for a considerable space (ascertained by coloured marks on the shoot and on the support) round the stick, and the spire became more open.

It follows from this latter fact that the position occupied by each leaf with respect to the support depends on the growth of the internodes after they have become spirally wound round it.I mention this on account of an observation by Palm (p.34), who states that the opposite leaves of the Hop always stand in a row, exactly over one another, on the same side of the supporting stick, whatever its thickness may be.My sons visited a hop-field for me, and reported that though they generally found the points of insertion of the leaves standing over each other for a space of two or three feet in height, yet this never occurred up the whole length of the pole; the points of insertion forming, as might have been expected, an irregular spire.Any irregularity in the pole entirely destroyed the regularity of position of the leaves.From casual inspection, it appeared to me that the opposite leaves of Thunbergia alata were arranged in lines up the sticks round which they had twined;accordingly, I raised a dozen plants, and gave them sticks of various thicknesses, as well as string, to twine round; and in this case one alone out of the dozen had its leaves arranged in a perpendicular line: I conclude, therefore, Palm's statement is not quite accurate.

The leaves of different twining-plants are arranged on the stem (before it has twined) alternately, or oppositely, or in a spire.In the latter case the line of insertion of the leaves and the course of the revolutions coincide.This fact has been well shown by Dutrochet, who found different individuals of Solanum dulcamara twining in opposite directions, and these had their leaves in each case spirally arranged in the same direction.A dense whorl of many leaves would apparently be incommodious for a twining plant, and some authors assert that none have their leaves thus arranged; but a twining Siphomeris has whorls of three leaves.

If a stick which has arrested a revolving shoot, but has not as yet been encircled, be suddenly taken away, the shoot generally springs forward, showing that it was pressing with some force against the stick.After a shoot has wound round a stick, if this be withdrawn, it retains for a time its spiral form; it then straightens itself, and again commences to revolve.The long, much-inclined shoot of the Ceropegia previously alluded to offered some curious peculiarities.