These, whilst young and about an inch in length with the leaflets on the petiole only partially expanded, are highly sensitive; a single light touch with a twig on the inferior or concave surface near the tip caused them to bend quickly, as did occasionally a loop of thread weighing one-seventh of a grain (9.25 mg.).The upper or convex surface is barely or not at all sensitive.Tendrils, after bending from a touch, straighten themselves in about two hours, and are then ready to act again.As soon as they begin to grow old, the extremities of their two or three pairs of branches become hooked, and they then appear to form an excellent grappling instrument; but this is not the case.For at this period they have generally quite lost their sensitiveness; and when hooked on to twigs, some were not at all affected, and others required from 18 hrs.to 24 hrs.before clasping such twigs; nevertheless, they were able to utilise the last vestige of irritability owing to their extremities being hooked.
Ultimately the lateral branches contract spirally, but not the middle or main stem.
Lathyrus aphaca.--This plant is destitute of leaves, except during a very early age, these being replaced by tendrils, and the leaves themselves by large stipules.It might therefore have been expected that the tendrils would have been highly organized, but this is not so.They are moderately long, thin, and unbranched, with their tips slightly curved.Whilst young they are sensitive on all sides, but chiefly on the concave side of the extremity.They have no spontaneous revolving power, but are at first inclined upwards at an angle of about 45 degrees, then move into a horizontal position, and ultimately bend downwards.The young internodes, on the other hand, revolve in ellipses, and carry with them the tendrils.Two ellipses were completed, each in nearly 5 hrs.; their longer axes were directed at about an angle of 45 degrees to the axis of the previously made ellipse.
Lathyrus grandiflorus.--The plants observed were young and not growing vigorously, yet sufficiently so, I think, for my observations to be trusted.If so, we have the rare case of neither internodes nor tendrils revolving.The tendrils of vigorous plants are above 4inches in length, and are often twice divided into three branches;the tips are curved and are sensitive on their concave sides; the lower part of the central stem is hardly at all sensitive.Hence this plant appears to climb simply by its tendrils being brought, through the growth of the stem, or more efficiently by the wind, into contact with surrounding objects, which they then clasp.I may add that the tendrils, or the internodes, or both, of Vicia sativa revolve.
COMPOSITAE.--Mutisia clematis.--The immense family of the Compositae is well known to include very few climbing plants.We have seen in the Table in the first chapter that Mikania scandens is a regular twiner, and F.Muller informs me that in S.Brazil there is another species which is a leaf-climber.Mutisia is the only genus in the family, as far as I can learn, which bears tendrils: it is therefore interesting to find that these, though rather less metamorphosed from their primordial foliar condition than are most other tendrils, yet display all the ordinary characteristic movements, both those that are spontaneous and those which are excited by contact.
The long leaf bears seven or eight alternate leaflets, and terminates in a tendril which, in a plant of considerable size, was 5 inches in length.It consists generally of three branches; and these, although much elongated, evidently represent the petioles and midribs of three leaflets; for they closely resemble the same parts in an ordinary leaf, in being rectangular on the upper surface, furrowed, and edged with green.Moreover, the green edging of the tendrils of young plants sometimes expands into a narrow lamina or blade.Each branch is curved a little downwards, and is slightly hooked at the extremity.
A young upper internode revolved, judging from three revolutions, at an average rate of 1 hr.38 m.; it swept ellipses with the longer axes directed at right angles to one another; but the plant, apparently, cannot twine.The petioles and the tendrils are both in constant movement.But their movement is slower and much less regularly elliptical than that of the internodes.They appear to be much affected by the light, for the whole leaf usually sinks down during the night and rises during the day, moving, also, during the day in a crooked course to the west.The tip of the tendril is highly sensitive on the lower surface; and one which was just touched with a twig became perceptibly curved in 3 m., and another in 5 m.;the upper surface is not at all sensitive; the sides are moderately sensitive, so that two branches which were rubbed on their inner sides converged and crossed each other.The petiole of the leaf and the lower parts of the tendril, halfway between the upper leaflet and the lowest branch, are not sensitive.A tendril after curling from a touch became straight again in about 6 hrs., and was ready to re-act;but one that had been so roughly rubbed as to have coiled into a helix did not become perfectly straight until after 13 hrs.The tendrils retain their sensibility to an unusually late age; for one borne by a leaf with five or six fully developed leaves above, was still active.If a tendril catches nothing, after a considerable interval of time the tips of the branches curl a little inwards; but if it clasps some object, the whole contracts spirally.
SMILACEAE.--Smilax aspera, var.maculata.--Aug.St.-Hilaire
considers that the tendrils, which rise in pairs from the petiole, are modified lateral leaflets; but Mohl (p.41) ranks them as modified stipules.These tendrils are from 1.5 to 1.75 inches in length, are thin, and have slightly curved, pointed extremities.
