第5章 TWINING PLANTS(4)

When a revolving shoot consists of several internodes, the lower ones bend together at the same rate, but one or two of the terminal ones bend at a slower rate; hence, though at times all the internodes are in the same direction, at other times the shoot is rendered slightly serpentine.The rate of revolution of the whole shoot, if judged by the movement of the extreme tip, is thus at times accelerated or retarded.One other point must be noticed.Authors have observed that the end of the shoot in many twining plants is completely hooked; this is very general, for instance, with the Asclepiadaceae.

The hooked tip, in all the cases observed by me, viz, in Ceropegia, Sphaerostemma, Clerodendron, Wistaria, Stephania, Akebia, and Siphomeris, has exactly the same kind of movement as the other internodes; for a line painted on the convex surface first becomes lateral and then concave; but, owing to the youth of these terminal internodes, the reversal of the hook is a slower process than that of the revolving movement. This strongly marked tendency in the young, terminal and flexible internodes, to bend in a greater degree or more abruptly than the other internodes, is of service to the plant; for not only does the hook thus formed sometimes serve to catch a support, but (and this seems to be much more important) it causes the extremity of the shoot to embrace the support much more closely than it could otherwise have done, and thus aids in preventing the stem from being blown away during windy weather, as Ihave many times observed.In Lonicera brachypoda the hook only straightens itself periodically, and never becomes reversed.I will not assert that the tips of all twining plants when hooked, either reverse themselves or become periodically straight, in the manner just described; for the hooked form may in some cases be permanent, and be due to the manner of growth of the species, as with the tips of the shoots of the common vine, and more plainly with those of Cissus discolor--plants which are not spiral twiners.

The first purpose of the spontaneous revolving movement, or, more strictly speaking, of the continuous bowing movement directed successively to all points of the compass, is, as Mohl has remarked, to favour the shoot finding a support.This is admirably effected by the revolutions carried on night and day, a wider and wider circle being swept as the shoot increases in length.This movement likewise explains how the plants twine; for when a revolving shoot meets with a support, its motion is necessarily arrested at the point of contact, but the free projecting part goes on revolving.As this continues, higher and higher points are brought into contact with the support and are arrested; and so onwards to the extremity; and thus the shoot winds round its support.When the shoot follows the sun in its revolving course, it winds round the support from right to left, the support being supposed to stand in front of the beholder; when the shoot revolves in an opposite direction, the line of winding is reversed.As each internode loses from age its power of revolving, it likewise loses its power of spirally twining.If a man swings a rope round his head, and the end hits a stick, it will coil round the stick according to the direction of the swinging movement; so it is with a twining plant, a line of growth travelling round the free part of the shoot causing it to bend towards the opposite side, and this replaces the momentum of the free end of the rope.

All the authors, except Palm and Mohl, who have discussed the spiral twining of plants, maintain that such plants have a natural tendency to grow spirally.Mohl believes (p.112) that twining stems have a dull kind of irritability, so that they bend towards any object which they touch; but this is denied by Palm.Even before reading Mohl's interesting treatise, this view seemed to me so probable that Itested it in every way that I could, but always with a negative result.I rubbed many shoots much harder than is necessary to excite movement in any tendril or in the foot-stalk of any leaf climber, but without any effect.I then tied a light forked twig to a shoot of a Hop, a Ceropegia, Sphaerostemma, and Adhatoda, so that the fork pressed on one side alone of the shoot and revolved with it; Ipurposely selected some very slow revolvers, as it seemed most likely that these would profit most from possessing irritability; but in no case was any effect produced. Moreover, when a shoot winds round a support, the winding movement is always slower, as we shall immediately see, than whilst it revolves freely and touches nothing.

Hence I conclude that twining stems are not irritable; and indeed it is not probable that they should be so, as nature always economizes her means, and irritability would have been superfluous.

Nevertheless I do not wish to assert that they are never irritable;for the growing axis of the leaf-climbing, but not spirally twining, Lophospermum scandens is, certainly irritable; but this case gives me confidence that ordinary twiners do not possess any such quality, for directly after putting a stick to the Lophopermum, I saw that it behaved differently from a true twiner or any other leaf-climber.