In the experiments necessary for demonstration of the fact of heliotropism, plants had to be selected which were known to be peculiarly sensitive, and also readily available and of convenient size. The young seedlings of the canary grass (Phalaris canariensis) were selected as fulfilling the conditions, just after they appeared above the soil. Of course there was nothing particularly sensational in such experiments; they did not make a great show, and produced no very startling results, nothing greatly beyond the confirmation of what was previously known. Yet they are not without their interest, and especially those which were made with the view of testing the small amount of light necessary to induce heliotropism. A pot of these seedlings, which had been raised in the dark, was placed in a dark room at the distance of four yards from a small lamp. Within three hours the shoots were perhaps slightly curved to the light, but so little as to be doubtful. In rather more than seven hours all were plainly bent towards the light. Now, the light was so feeble at the distance named that the Roman figures could not be distinguished on the white face of a watch, and no shadow was cast by a pencil held upright on a white card, so that the amount of light diffused must have been exceedingly small, and yet it did not fail to exert its power after upwards of seven hours' exposure. Similar experiments were performed at greater and at less distances, and in a variety of ways, but with substantially similar results. The movement is, of course, very slow where the light is dim, but more rapid where the light is more intense. Placed before a bright lamp the tips of the shoots were all curved at right angles towards it in two hours and a quarter. From other experiments it was determined that seedlings of this grass commenced travelling in the direction of the lamp within from six to ten minutes after their first exposure. Also that the rate of progression was irregular, sometimes almost stationary for ten minutes, and then onwards again. It may be mentioned in this connexion that a series of observations was made in order to determine how long the influence of light would continue to be exerted after the source of light was obscured. It was found that the young shoots would continue to bend in the same direction as that from which the illumination proceeded for from a quarter to half an hour after the light was extinguished.
Movements in the direction of the light have been so universally observed, that heliotropism has been accepted as a fact, independently of any recent observations. The experience of any one who has been concerned with vegetation will furnish numerous instances of such phenomena. The farmer, the nurseryman, the gardener, and even the field labourer, will have something to tell of the movements of leaves and flowers towards or away from the light. It is unnecessary, therefore, to multiply examples to prove that which is generally accepted. Each, perhaps, will have his own theory of the reason
CHAPTER IX
TWINERS AND CLIMBERS.
WHOEVER has read the records of travellers in tropical forests will have been struck with the constant recurrence of some reference to climbing plants, even should they fail to remark the general allusion to climbers and twiners as important features in tropical vegetation. We take up the first book at hand, and in it we read as follows:—"Below, the tree trunks were everywhere linked together by sipos; the woody, flexible stems of climbing and creeping trees, whose foliage is far away above, mingled with that of the taller independent trees. Some were twisted in strands like cables; others had thick stems contorted in every variety of shape, entwining snake-like round the tree-trunks, or forming gigantic loops and coils among the larger branches; others again, were of zigzag shape, or indented, like the steps of a staircase, sweeping from the ground to a giddy height. It interested me much afterwards to find that these climbing trees do not form any particular family. There is no distinct group of plants whose especial habit is to climb, but species of many, and the most diverse families the bulk of whose members are not climbers, seem to have been driven by circumstances to adopt this habit. There is even a climbing genus of palms (Desmoncus), the species of which are called in the Tupi language, 'Jacitara.' These have slender thickly-spined and flexuous stems, which twine about the taller trees from one to another, and grow to an incredible length. The leaves, which have the ordinary pennate shape characteristic of the family, are emitted from the stems at long intervals, instead of being collected into a dense crown, and have at their tips a number of long recurved spines. These structures are excellent contrivances to enable the trees to secure themselves by in climbing, but they are a great nuisance to the traveller, for they sometimes hang over the pathway, and catch the hat or clothes, dragging off the one or tearing the other as he passes. The number and variety of climbing trees in the Amazon forests are interesting, taken in connexion with the fact of the very general tendency of the animals, also, to become climbers."
In a similar manner the Rev. Charles Kingsley was impressed with these plants in the forests of the West Indies. "Around your knees are probably Mamures, with creeping stems and fan-shaped leaves, something like those of a young coco-nut palm. You try to brush through them, and are caught up instantly by a string or wire belonging to some other plant. You look up and around, and then you find that the air is full of wires—that you are hung up in a network of fine branches belonging to half a dozen different sorts of young trees, and intertwined with as many different species of slender creepers. You thought at your first glance among the tree stems that you were looking through open air; you find that you are looking through a labyrinth of wire rigging, and must use the cutlass right and left at every five steps."
To these we will add only one other description of the characteristics of a virgin forest. "Its striking characteristics were, the great number and variety of the forest trees, their trunks rising frequently for sixty or eighty feet without a branch, and perfectly straight; the huge creepers, which climb about them, sometimes stretching obliquely from their summits like the stays of a mast, sometimes winding around their trunks like immense serpents waiting for their prey. Here, two or three together, twisting spirally round each other, form a complete living cable, as if to bind securely these monarchs of the forest; there, they form tangled festoons, and, covered themselves with smaller creepers and parasitic plants, hide the parent stem from sight."
In this temperate clime of ours we know nothing of the gigantic climbers of the tropical forests, but we have many plants with a similar habit, on a small scale, quite sufficient to give us an interest in the phenomena concerned in the twining process. It was in 1865 that Mr. Darwin's memoir of the habits of climbing plants first made its appearance, and, as since revised, is now the text-book on the subject.
In this work scandent plants are divided into four classes, applicable alike to our purpose:—(1) Twiners, those which twine spirally round a support unaided by any other movement; (2) Climbers, endowed with irritable organs, which, when they touch any object, clasp it; (3) Scramblers, which ascend merely by the aid of hooks; and (4) Root-climbers, which ascend by means of rootlets attached to their support. Of these four classes the first and second are of most importance, being by far the most numerous, and true climbers; the third and fourth being pseudo-climbers.
We could not desire a more familiar or better illustration of a "twiner" than the hop (Humulus lupulus), and by a little attention to the habits of this plant we may comprehend the general principle on which all twining plants ascend. The first two or three joints or internodes of the hop, after it emerges from the ground, are straight and stationary. We use the term "internode" as expressing that portion of a stem which lies between one node or knot and the next; that is, between the point where one leaf, or pair of leaves, spring from the stem, and the like point next above it. After the two or three straight joints, or internodes, of the young shoot of the hop, another one grows, which, whilst still young, is seen not to be motionless, like its predecessors, but to bend on one side and move slowly round, or rotate, like the hands of a watch, with the sun. We have already become familiar with this kind of rotation in plants, but must be prepared to meet with it in twining plants in a more exaggerated degree. The average rate of rotation during the day in hot weather was found by experiment to be two hours and eight minutes for each revolution. When the next internode is produced the two continue to rotate, and so on with a third. But as the internodes grow old they cease to revolve. It is the terminal two or three of the internodes that exhibit the movement. A shoot was watched when in full rotation with three internodes. The lowest was a little over 8 inches in length, the second was 3 1/2 inches, and the last, or youngest, was 2 1/4 inches. Therefore, the revolving apex of this stem was about 14 inches in length, and it swept round in a circle of 19 inches in diameter, so that the rate of motion must have been but little less than an inch in two minutes and a half, or 23 inches in an hour.
The purpose of this rotation is so self-evident that it scarcely needs explanation. It is undoubtedly directed primarily in search of 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."
By observing the older stems of the hop which have become entwined round a support, or, better still, taking a piece of old hop-bine a few inches in length and splitting it down longitudinally with a knife, we shall observe that the stem is also twisted upon itself; that, in addition to twisting round its support, it twists round its own axis. Mr. Darwin has shown that an internode 9 inches long, and which had revolved thirty-seven times, had become three times twisted round its own axis in the line of the course of the sun. Some have thought in past times that the twisting of the stem caused the rotation, but it is hardly possible that three twists in the stem should have caused thirty-seven rotations. Many twining plants twist in this manner round their own axis. Experiments have demonstrated that when the plant climbs round a smooth support, such as a glass rod, it is much less twisted on itself than when twining round a rough or rugged support. The axial twisting bears some relationship, therefore, to inequalities in the support, as well as to revolving freely without support. Internodes may be observed rotating freely before they have acquired a single twist on their own axis. The axial twisting must, therefore, have some other cause or object than that which has been attributed to it.