Popular Science Monthly/Volume 17/September 1880/Climbing Plants
By FRANCIS DARWIN, F. L. S.
I THINK most people have a general idea of what a climbing plant is. Even in the smoky air of London two representatives of the class flourish. A certain house in Portman Square shows how well the Virginian creeper will grow; and the ivy may be seen making a window-screen for some London dining-rooms.
Many other climbing plants will suggest themselves—the vine, the honeysuckle, the hop, the bryony—as forming more or less striking elements in the vegetation.
If we inquire what qualities are common to these otherwise different plants, we find that they all have weak and straggling stems, and that, instead of being forced, like many weakly-built plants, to trail on the ground, they are all enabled to raise themselves high above it, by attaching themselves in some way to neighboring objects. This may be effected in different ways: by clinging to a flat surface, like the ivy; or twining round a stick, like the hop; or making use of tendrils, like the vine.
These various contrivances have been studied by more than one German naturalist, as well as by my father, in whose book on the "Habits of Climbing Plants" very full details upon this subject will be found.
Climbing plants are, first of all, divided roughly into those which twine and those which do not twine; twiners are represented by the hop and the honeysuckle, and all those plants which climb up a stick by winding spirally round it. Those which are not twiners—that is, which do not wind spirally round a stick—are such as support themselves by seizing hold of any neighboring object with various kinds of grasping organs; these may be simple hooks, or adhering roots, or they may be elaborate and sensitive tendrils, which seize hold of a stick with a rapidity more like the action of an animal than of a plant. We shall come back to this second class of climbing plants, and shall then consider their various kinds of seizing organs. I merely wish now to insist on the importance of distinguishing between these two methods of climbing, in one of which the plant ascends a support by traveling spirally round it; in the other, fixes on to the support by seizing it at one place, and continuing to seize it higher and higher up as its stem increases in length.
I have heard the curator of a foreign botanic garden bitterly complain of his gardeners that they never could learn the difference between these two classes of climbing plants, and that they would only give a few bare sticks to some tendril-bearing plant, expecting it to twine up them like a hop, while the plant really wanted a twiggy branch, up which it might creep, seizing a twig with each of its delicate tendrils, as it climbed higher and higher. These two kinds of climbers—twiners and non-twiners—may be seen growing up their appropriate supports in any kitchen-garden where the scarlet-runners twine spirally up tall sticks, while the peas clamber up the bushy branches stuck in rows in the ground.
A hop-plant will supply a good example of the mode of growth of true twining plants. Let us imagine that we have a young hop-plant growing in a pot; we will suppose that it has no stick to twine up, and that its pot stands in some open place where there are no other plants to interfere with it. A long, thin shoot will grow out, and, not being strong enough to support itself in the upright position, will bend over to one side. So far we have not discovered anything remarkable about our hop; it has sent out a straggling shoot, which has behaved as might be expected, by falling over to one side. But now, if we watch the hop-plant closely, a very remarkable thing will be seen to take place. Supposing that we have noticed the shoot, when it began to bend over, pointed toward the window—say a north window—and that, when we next look at it after some hours, it points into the room, that is to say, south, and again, north after another interval, we shall have discovered the curious fact that the hop-plant has a certain power of movement by which its shoot may sometimes point in one direction, sometimes in another. Bat this is only half the phenomenon, and, if we examine closely, we shall find that the movement is constant and regular, the stem first pointing north, then east, then south, then west, in regular succession, so that its tip is constantly traveling round and round like the hand of a watch, making on an average, in warm August weather, one revolution in two hours. Here, then, is a most curious power possessed by the shoots of twining plants, which is worth inquiring further into, both as regards the way in which the movement is produced, and as to how it can be of any service to the plant. Questions are often asked in gardening periodicals as to how hops or other climbing plants always manage to grow precisely in the direction in which they will find a support. This fact has surprised many observers, who have supposed that climbing plants have some occult sense by which they discover the whereabouts of the stick up which they subsequently climb. But there is in reality no kind of mystery in the matter: the growing shoot simply goes swinging round till it meets with a stick, and then it climbs up it. Now, a revolving shoot may be more than two feet long, so that it might be detained in its swinging-round movements by a stick fixed into the ground at a distance of nearly two feet. There would then be a straight bit of stem leading from the roots of the plant, in a straight line to the stick up which it twines, so that an observer who knew nothing of the swinging-round movement might be pardoned for supposing that the plant had in some way perceived the stick and grown straight at it. This same power of swinging round slowly comes into play in the very act of climbing up a stick.
Suppose I take a rope and swing it round my head: that may be taken to represent the revolving of the young hop-shoot. If, now, I allow it to strike against a rod, the end of the rope which projects beyond the rod curls freely round it in a spiral. And this may be taken as a rough representation of what a climbing plant does when it meets a stick placed in its way. That is to say, the part of the shoot which projects beyond the stick continues to curl inward till it comes against the stick; and, as growth goes on, the piece of stem which is projecting is, of course, all the while getting longer and longer; and, as it is continually trying to keep up the swinging-round movement, it manages to curl round the stick. But there is a difference between the rope and the plant in this—that the rope curls round the stick at the same level as that at which it is swung, so that, if it moves round in an horizontal plane at a uniform height above ground, it will curl round the stick at that level, and thus will not climb up the stick it strikes against, But the climbing plant, although it may swing round when searching for a stick, at a fairly uniform level, yet, when it curls round a stick, does not retain a uniform distance from the ground, but by winding round like a corkscrew it gets higher and higher at each turn. One may find a further illustration of the action of twining in the swinging-rope model. It is a peculiarity of twining plants that they can only ascend moderately thin supports. A scarlet-runner can climb up a bit of string, or a thin stick, an inch or two in diameter, but when it comes to anything thicker than this it fails to do so. Just as, when the swinging-rope strikes against a large trunk of a tree, it would be unable to take a turn round it, and would fall to the ground instead of gripping it with a single turn, as it does a thin stick. The difficulty which a climbing plant has in ascending a thick stick will be better understood by going back to the original swinging-round movement which the plant makes in search of a stick, and considering how the movement is produced.
As plants have no muscles, all their movements are produced by unequal growth; that is, by one half of an organ growing in length quicker than the opposite half. Now, the difference between the growth of a twining plant which bends over to one side and an ordinary plant which grows straight up in the air lies in this, that in the upright shoot the growth is nearly equal on all sides at once, whereas the twining plant is always growing much quicker on one side than the other.
It may be shown by means of a simple model how unequal growth can be converted into revolving movement. The stem of a young hop is represented by a flexible rod, of which the lower end is fixed, the upper one being free to move. At first the rod is supposed to be growing vertically upward, but when it begins to twine one side begins to grow quicker than any of the others: suppose the right side to do so, the result will be that the rod will bend over toward the left side. Now, let the region of quickest growth change, and let the left side begin to grow quicker than all the others, then the rod will be forced to bend back over to the other side. Thus, by an alteration of growth, the rod will bend backward and forward from right to left. But now imagine that the growth of the rod on the sides nearest to and farthest from us enters into the combination, and that, after the right side has been growing quickest for a time, the far side takes it up, then the rod will not bend straight back toward the right, as it did before, but will bend to the near side. Now the old movement, caused by the left side growing quickest, will come in again, to be followed by the near side growing quickest. Thus by a regular succession of growth on all the sides, one after another, the swinging-round movement is produced, and by a continuation of this action, as I have explained, the twining movement is produced.
I have spoken as if the question of how plants twine were a completely solved problem, and in a certain sense it is so. I think that the explanation which I have given will remain as the fundamental statement of the case. But there is still much to be made out. We do not in the least know why every single hop-plant in a field twines like a left-handed screw, while every single plant in a row of beans twines the other way; nor why in some rare instances a species is divided, like the human race, into right-and left-handed individuals, some twining like a left-handed, others like a right-handed screw. Or, again, why some very few plants will twine half-way up a stick in one direction, and then reverse the spiral and wind the other way. Nor, though we know that in all these plants the twining is caused by the change in the region of quickest growth, have we any idea what causes this change of growth. There is still much to work at, and it is to be hoped that there are still plenty of workers to solve the problems. It is by looking to exceptions that the key to a problem is often found. It is the exceptions to general rules that often lead us to understand the meaning and origin of the rules themselves; and it is to such exceptions that any one who wants to work at climbing plants should turn. Now, it is a general rule that a climbing plant twines in the same way that it revolves. It seems an obvious thing that in the case of the rope model, if we swing the rope round our head in the direction of the hands of a watch, it must twine round the stick against which it strikes in the same direction. But in plants it is not always so. In the large majority of cases it is so, for, if this were not the case, the illustration of the rope would not have been applicable; but it is not universally the rule. Every individual of the plant Hibbertia always twines round its stick in the same direction, but, when it is performing the swinging movement in search of a support, it is found that some plants travel round with the sun, others in the opposite direction. This fact forms an exception of a striking kind—and such exceptions are worthy of close study.
There are other facts of a different nature, which seem to show how difficult the problem is, and how delicately balanced is that part of the organization of the plant which is connected with the power of climbing. For instance, if we cut a branch of most shrubs, and put it in water, it goes on growing, apparently as healthily as ever. Indeed, the practice of making cuttings—where a cut-off branch or shoot develops roots and turns into a new plant—shows us that no serious injury is thus caused. But the twining organization is sensitive to such treatment. A cut branch of hop placed in water was observed to make its revolutions in about twenty hours, whereas in its natural condition—growing on the plant—it makes a complete turn in two or three hours. Again, if a plant growing in a pot is moved from one greenhouse to another, the slight shaking thus caused is sufficient to stop the revolving movement for a time—another proof of the delicacy of the internal machinery of the plant.
Some of the problems, as, for instance, why twining plants can not as a rule climb thick stems, may be looked at from the natural-history point of view. Most of our climbing plants die down in the winter, so that, if they were able to climb round big tree-trunks, they would waste all the precious summer weather in climbing a few feet, whereas the same amount of longitudinal growth devoted to twining up a thin stick would have raised them up to the light after which they are striving And as a plant exercises no choice, hut merely swings round till it hits against an object, up which it will then try to twine, it seems as if the inability to climb thick stems might be a positive advantage to a plant, by forcing it to twine up such objects as would best repay the trouble.
In the classification of climbing plants, proposed by my father in his book, he makes a subdivision of "hook-climbers." These may be taken as the simplest representatives of that class of climbers which are not twining plants. The common bramble climbs or scrambles up through thick underwood, being assisted by the recurved spines which allow the rapidly growing shoot to creep upward as it lengthens, but prevent it from slipping backward again; the common goose-grass (Galium) also climbs in this way, sticking like a burr to the side of a hedge-row up which it climbs. Most country boys will remember having taken advantage of this burr-like quality of Galium in making sham birds!-nests, the prickly stems adhering together in the desired form. Such plants as the bramble or Galium exhibit none of the swinging-round movement which I have described in twiners: they simply grow straight on, trusting to their hooks to retain the position gained.
In some species of clematis we find a mechanism which reminds one of a simple hook-climber, but is in reality a much better arrangement. The young leaves projecting outward and slightly backward from the stem may remind us of the hooked spines of a bramble, and like them easily catch on neighboring objects, and support the trailing stem. Or the leaf of the species of clematis given in Fig. 1 may serve as an example of a leaf acting like a hook. The main stalk of the leaf is seen to be bent angularly downward at the points where each successive pair of leaflets is attached, and the leaflet at the end of the leaf is bent down at right angles, and thus forms a grappling apparatus. The clematis does not, like the bramble, trust to mere growth, to thrust itself among tangled bushes, but possesses the same powers of revolving in search of a support which simple or true twining plants possess. Indeed, many species of clematis are actually twining plants, and can wind spirally up a stick placed in their way. And the same revolving movement which enables them thus to wind spirally also helps them to search for some holding-place for their hook-or grapple-like leaves, and in many species the search is carried on by the leaves swinging round, quite independently of the revolving movement of the stem on which they are borne.
If a leaf of a clematis succeed by any means in hooking on to a neighboring object, the special characteristic of leaf-climbing plants comes into play. The stalk of the leaf curls strongly over toward the object touching it, and clasps it firmly. It is obvious how great is the advantage thus gained over a mere hook. A leaf such as that shown
in Fig. 2 might be made to catch on to a neighboring twig by its bent stalk, in such a way that, although it managed to stay where it was, it could bear none of the weight of the plant, and would be liable to be displaced by a strong wind or other disturbance. But, when the stalk of the leaf had curled close round the twig, nothing could displace it, and it could take its share in the work of supporting the plant.
The extreme sensitiveness of the leaf-stalk to slight and gentle touches gives a curious idea of the alertness of the plant in its search for supporting objects. A leaf may be excited to bend by a loop of string weighing only one-sixteenth of a grain. It is an interesting fact that, in such a hook-like leaf as that of Clematis viticella (Fig. 1), the hooked end of the leaf, which has the best chance of coming into contact with obstacles, is the most sensitive part. This has been made out by hanging small weights on different parts of the leaf, and it is found that the terminal leaflet bends in a few hours after a loop of string weighing less than a grain is hung on it, and which produced no effect in twenty-four hours on the other petioles. One may see proof of the sensitiveness of the leaf-stalks of the wild English clematis, which sometimes catches withered leaves or delicate stalks of the quaking-grass. The same thing is shown by a leaf after having been touched with a little water-color, the delicate crust of dry paint being mistaken for something touching the plant. In such cases, or when the leaf has been merely rubbed with a twig, which is taken away before the leaf seizes it, the plant discovers that it has been deceived, and, after bending for a time, it unbends and becomes straight again.
The bending, which enables a leaf to seize a twig, is not the only change which the stimulus of a touch produces. The leaf-stalk swells and becomes thicker and more woody, and turns into a strong, permanent support to the plant. The thickening of the leaf-stalks is to be made out in Fig. 2, which represents a shoot of clematis, bearing two leaves, each of which has seized a twig; in one of the leaf-stalks this thickening has commenced, and is fairly evident. The thickened and woody leaf-stalks remain in winter after the leafy part has dropped off, and in this condition they are strikingly like real tendrils.
The genus Tropœolum, whose cultivated species are often called nasturtiums, also consists of leaf-climbing plants, which climb like clematis by grasping neighboring objects with their leaf-stalks.
In some species of Tropœolum we find climbing organs developed, which can not logically be distinguished from tendrils; they consist of little filaments, not green like a leaf, but colored like the stem. Their tips are a little flattened and furrowed, but never develop into leaves; and these filaments are sensitive to a touch, and bend toward a touching object, which they clasp securely. Filaments of this kind are borne by the young plant, but it subsequently produces filaments with slightly enlarged ends, then with rudimentary or dwarfed leaves, and finally with full-sized leaves; when these are developed they clasp with their leaf-stalks, and then the first-formed filaments wither and die off; thus the plant, which in its youth was a tendril-climber, gradually develops into a true leaf-climber. During the transition, every gradation between a leaf and a tendril may be seen on the same plant. Fig. 3.—Bignonia. An unnamed species from Kew. It is not always the stalk of a leaf which is developed into the clasping organ; the bignonia-leaf shown in Fig. 3 bears tendrils at its free extremity. And in other plants tendrils are formed from flower-stalks, in which the flowers are not developed, or the whole stem of the plant or a single branch may turn into a tendril. In one curious case of monstrosity, what should have been a prickle on a sort of cucumber, grew out into a long, curled tendril.
The family of the Bignonias is one of the most interesting of the class of tendril-climbers, on account of the variety of adaptation which is found among them.
In the above-mentioned Fig. 3 is seen the tendril-bearing leaf of a species of Bignonia. The leaf bears a pair of leaflets, and ends in a tendril having three branches. The main tendril may be compared to a bird's leg with three toes, each bearing a small claw. And this comparison seems apt enough, for, when the tendril comes against a twig, the three toes curl round it like those of a perching bird.
Besides the toes or tendrils, the leaf-stalk is sensitive, and acts like that of a regular leaf-climber, wrapping itself round a neighboring object.
In some cases the young leaves have no tendrils at their tips, but clasp with their stalks, and this is a case exactly the reverse of Tropæolum—a tendril-chamber whose young leaves have no tendrils, instead of a leaf-climber whose young climbing organs are not leaves. Thus the close relationship that exists between leaf-and tendril climbers is again illustrated.
This plant also combines the qualities of another class of climbers, namely, twiners, for it can wind spirally round a support as well as a hop or any other true twiner. Another species (B. Tweedyana) also helps to support itself by putting out roots from its stems, which adhere to the stick up which the plant is climbing. So that here are four different methods of climbing—twining, leaf, tendril, and root climbing—which are usually characteristic of different classes of climbing plants, combined in a single species.
Among the Bignonias are found tendrils with various curious kinds of sensitiveness. The tendrils of one species exhibit, in the highest perfection, the power of growing away from light toward darkness, just the opposite to the habit of most plants. A plant, growing in a pot, was placed so that the light came in on one side. One tendril was pointing away from the light, to begin with, and this did not move; but the opposite tendril, which was pointing toward the light, bent right over and became parallel to the first tendril. The pot was then turned round, so that both pointed toward the light, and they both moved over to the other side, and pointed away from the light. In another case, in which a plant, with six tendrils, was placed in a box, open at one side, all six tendrils pointed like so many weathercocks in the wind—all truly toward the darkest corner of the box. These tendrils also showed a curious power of choice. When it was found that they preferred darkness to light, it was tried whether they would seize a blackened glass tube, or a blackened zinc plate. The tendrils curled round both these objects, but soon recoiled and unwound with what, my father says, he can only describe as disgust. A post with very rugged bark was then put near them; twice they touched it for an hour or two, and twice they withdrew; but at last one of the hooked tendrils caught hold of a little projecting point of bark; and now it had found what it wanted. The other branches of the tendril quickly followed it, spreading out, adapting themselves to all the inequalities of the surface, and creeping into all the little crevices and holes in the bark. Finally a remarkable change took place in the tendrils: the tips which had crept into the cracks swelled up into little knobs, and ultimately secreted a sticky cement, by which they were firmly glued into their places. This plan of forming adhesive disks on its tendrils is one which we shall find used by the Virginia creeper, as its only method of support, and it forms the fifth means of climbing to be met with among the Bignonias. We see now the meaning of the power possessed by the tendrils of moving toward the dark, for in this way they are enabled to find out and reach the trunks of trees to which they then become attached. It seems, moreover, that the tendrils are especially adapted to the moss lichen-covered trees, for the tendrils are much excited by wool, flax, or moss, the fibers of which they can seize in little bundles. The swelling process is so delicate that, when two or three fine fibers rest on the end of a tendril, the swelling occurs in crests, thinner than a hair, which insert themselves between and finally envelop the fibers. This goes on so that the ball at the end of a tendril may have as many as fifty or sixty fibers imbedded in it, crossing each other in different directions.
The tendrils of the Virginia creeper may here be worth noticing. This plant can climb up a flat wall, and is not adapted to seize sticks or twigs; its tendrils do occasionally curl round a stick, but they often let go again. They, like bignonia-tendrils, are sensitive to the light, and grow away from it, and thus easily find out where the wall lies up which they have to climb. A tendril which has come against the wall is often seen to rise and come down afresh, as if not satisfied with its first position. In a few days after a tendril has touched a wall the tip swells up, becomes red, and forms one of the little feet or sticky cushions by which the tendrils adhere, and which are shown in Fig. 5. The adherence is caused by a resinous cement secreted by the cushions, and which forms a strong bond of union between the wall and the tendril. After the tendril has become attached it becomes woody, and is in this state remarkably durable, and may remain firmly attached and quite strong, for as many as fifteen years.
Besides this sense of touch, by which a bignonia-tendril distinguishes between the objects which it touches, there are other instances of much more perfect and incomprehensible sensibility. Thus some tendrils, which are so sensitive that they curl up when a weight of one-thirtieth or even one-fiftieth of a grain is placed on them, do not take the least notice of a shower of rain whose falling drops must cause a much greater shock to the tendrils.
Again, some tendrils seem to have the power of distinguishing between objects which they wish to seize and their brother tendrils which they do not wish to catch. A tendril may be drawn repeatedly over another without causing the latter to contract.
The tendrils of another excellent climber (Cobœa scandens) possess some curious properties. The tendrils are much divided, and end in delicate branchlets, as thin as bristles, and very flexible, each bearing a minute double hook at its tip. These are formed of a hard, woody substance, and are as sharp as needles; a single tendril may bear between ninety and a hundred of these beautiful little grappling-hooks. The flexibility of the tendrils is of service in allowing them to be blown about by a breath of wind, and they can thus be made to seize hold of objects which are out of reach of the ordinary revolving movements. Many tendrils can only seize a stick by curling round it, and this even in the most sensitive tendril must take a minute or two; but with Cobœa the sharp hooks catch hold of little irregularities on the bark the moment the tendril comes into contact with it, and afterward the tendril can curl round and make the attachment permanent. The importance of this power of temporary attachment is shown by placing a glass rod near a cobæa-plant. Under these conditions the tendrils always fail to get hold of the glass, on which its grapple-like hooks can not seize.
The movement of the little hook-bearing branches is very remarkable in this species. If a tendril catches an object with one or two hooks, it is not contented, but tries to attach the rest of them in the same way. Now, many of the branches will chance to be so placed that their hooks do not naturally catch, either because they come laterally, or with their blunt backs against the wood, but after a short time, by a process of twisting and adjusting, each little hook becomes turned, so that its sharp point can get a hold on the wood.
The sharp hook on the tendrils of Cobœa is only a very perfect form of the bluntly curved tip which many tendrils possess, and which serves the same purpose of temporarily holding the object caught until the tendril can curl over and make it secure. There is a curious proof of the usefulness of even this blunt hook in the fact that the tendril is only sensitive to a touch on the inside of the hook. The tendril, when it comes against a twig, always slips up it till the hook catches on it, so that it would be of no use to be sensitive on the convex side. Some
Fig. 4.—A caught tendril of Bryonia dioica, spirally contracted in reversed directions.
tendrils, on the other hand, have no hook at the end, and here the tendrils are sensitive to a touch on any side. These tendrils led my father at first into a curious mistake, which he mentions in his book. He pinched a tendril gently in his fingers, and, finding that it did not move, concluded it was not sensitive. But the fact was that the tendril, being touched on two sides at once, did not know which stimulus to obey, and therefore remained motionless. It was in reality extremely sensitive to a touch on any one of its sides.
There is a remarkable movement which occurs in tendrils after they have caught an object, and which renders a tendril a better climbing organ than any sensitive leaf. This movement is called the "spiral contraction," and is shown in Fig. 4, which represents the spirally contracted tendril of the wild bryony; it may also be seen in Fig. 5, which represents the tendrils of the Virginia creeper. When a tendril first seizes
Fig. 5.—Ampelopsis hederaria. A. Tendril fully developed, with a young leaf on the opposite side of the stem. B. Older tendril, several weeks after its attachment to a wall, with the branches thickened and spirally contracted, and with the extremities developed into disks. The unattached branches of this tendril have withered and dropped off.
an object it is quite straight, with the exception of the extreme tip, which is firmly curled round the object seized. But in a day or two the tendril begins to contract, and ultimately assumes the corkscrew-like form represented in the figures. It is clear that in spirally contracting the tendril has become considerably shorter; and, since the end of the tendril is fixed to a branch, it is obvious that the stem of the bryony must be dragged nearer to the object which its tendril has caught. Thus, if a shoot of bryony seizes a support above it, the contraction of the tendril will pull up the shoot in the right direction. So that in this respect the power of spiral contraction gives a tendril-climber an advantage over leaf-climbers which have no contracting power, and therefore no means of hauling themselves up to supporting objects.
But the spiral contraction of tendrils has another use, and this is probably the most important one. This use depends on the fact that a contracted tendril acts like a spiral spring, and is thus converted into a yielding, instead of an unyielding, body. The spirally-wound tendril yields like an elastic thread to a pull which would break the tendril in its original condition. The meaning of this arrangement is to enable the plant to weather a gale which would tear it from its support by snapping the tendrils, if they were not converted into spiral springs.
My father describes how he went in a gale of wind to watch the bryony on an exposed hedge, and how, in spite of the violent wind which tossed the branches of the plant about, the bryony safely rode out the gale, "like a ship with two anchors down, and with a long range of cable ahead, to serve as a spring as she surges to the storm." It may also serve to divide the weight which has to be supported equally among a number of tendrils; and this is the meaning of the spiral contraction seen in the tendrils of the Virginia creeper.
It can be seen in Fig. 4 that all the coils of the spiral are not in the same direction. First, there are two in one direction, then six in the other, and then three again in the first direction, making six turns in one way and five in the other. And this is universally the case; the turns in one direction are always approximately equal in number to those in the opposite direction. It can be shown to be a mechanical necessity that a tendril which has its two ends fixed, and which then coils into a spiral, should behave in this way.
A simple model made to show this mechanical necessity is described by Sachs in his "Text-book." It is made by stretching a strip of India-rubber and cementing it to an unstretched strip. The strips being united in a state of longitudinal strain, form a spiral when released. If the model is held by one end only, the turns of the spiral are all in one direction. And this represents the behavior of a tendril which has not managed to seize a support; for some unknown reason such tendrils contract into spirals, and the turns of such spirals are all in one direction. But, if the India-rubber is held at both ends, half the turns are in one direction, half in the other, just as with a tendril the same thing happens.
Now, let us consider the general relations that exist between twining plants, leaf-climbing plants, and tendril-climbing plants. To an evolutionist the question how these various classes of climbing plants have been developed is perhaps of most interest. What is the relationship between them? Have all classes been developed separately from ordinary non-climbing plants, or has one class been developed out of one of the others; and, if so, which is the oldest form of climbing plant? There can be little doubt on this latter point. I think we may certainly say that the earliest form which existed was a twining plant. We see that twining plants do not possess the essential feature of leaf or tendril-bearers, namely, the sensitiveness to a touch which enables a leaf or tendril to grasp a stick. But, on the other hand, most leaf and tendril-climbers do possess the essential quality of a twiner—the power of revolving or swinging round, which exists in the shoots, leaves, or tendrils of so many of them. This power of revolving merely serves in some leaf-and tendril-climbers to carry on the search for supports; but other leaf-and tendril-climbers, as we have seen, do actually wind spirally round a stick exactly like a true twiner. How twiners originally obtained their power of swinging round we need not now inquire; it seems to be merely an increase of a similar movement which is found to occur in a meaningless manner in other plants. Thus several flower-stems have been observed bowing themselves over and swinging round in small circles, like climbing plants. Here the movement is merely an unintelligible concomitant of growth, for, as we see, the movement is of no advantage to the flower-stem. But the existence of this movement is of great interest to us, for it shows how a twining plant might be developed by a similar movement being found to be advantageous, and being increased by natural selection to the requisite extent.
Another question which may occur to us is this: In what way is climbing by leaves or tendrils a more perfect method than twining? Why, when a plant had become a twining plant, did it not rest satisfied? The fact that leaf-and tendril-climbers have been developed out of twiners, and not vice versa, is a proof that climbing by leaves or tendrils is a more advantageous habit than twining; but we do not see why it should be so. If we inquire why any plant has become a climber, we shall see the reason. Light is a necessity for all green plants; and a plant which can climb is enabled to escape from the shadow of other plants with a far less waste of material than a forest tree, which only pushes its branches into the light by sheer growth. Thus the weak, straggling stem of a climbing plant gets all the advantages gained by the solid, column-like tree-trunk. If we apply this test—which is the most economical plan of climbing, twining, or leaf climbing—we see at once that a plant which climbs by seizing wastes far less material than one which twines. Thus a kidney-bean, which had climbed up a stick to a height of two feet, when unwound from its support was found to be three feet in length, whereas a pea which had climbed up two feet by its tendrils was hardly longer than the height reached. Thus the bean had wasted considerably more material by its method of climbing by twining round a stick, instead of going straight up, supported by its tendrils, like the pea. There are several other ways in which climbing by tendrils is a much better plan than twining. It is a safer method, as any one may convince himself by comparing the security of a tendril-bearer in a heavy wind with the ease with which a twiner is partly blown from its support. Again, by looking at those leaf-climbing plants which still possess in addition the power of twining, it will be seen how incomparably better they grasp a stick than does a simple twiner. And again, a twiner from being best fitted to climb bare stems often has to start in the shade, whereas a leaf-or tendril-climber can ramble for the whole extent of its growth up the sunny side of a bush.
We can thus see plainly how it has been an advantage for twining plants to develop into leaf-climbers. We shall also find reasons why a leaf-climber should find it advantageous to become a tendril-climber.
We have seen how tendrils form a more sensitive, efficient grasping organ than simple leaves. Tendrils possess also the valuable power of shortening themselves by spirally contracting, and thus pulling up after them the stem on which they grow, and afterward serving as springs and breaking the force of the wind. We have had some cases where we see the close relationship between leaf-and tendril-climbers, and where we can see intermediate stages in the process of transition from one method of climbing to the other.
In certain kinds of Fumaria we can follow the whole process. Thus we have one kind, which is a pure leaf-climber, grasping by its leafstalks, which bear leaflets not at all reduced in size. A second genus has the end leaflets very much smaller than the rest. A third kind has the leaflets reduced to microscopical dimensions; and, lastly, a fourth kind has true and perfect tendrils. If we could see the ancestors of this last kind we should undoubtedly have a series of forms connecting it with an extinct leaf-climber, resembling the series which at present connects it with its contemporary leaf-climbing relatives.
To repeat once more the steps which it is believed have occurred in the evolution of climbing-plants: It is probable that plants have become twiners by exaggerating a swinging-round or revolving movement, which occurred in a rudimentary form, and in a useless condition, in some of their ancestors. This movement has been utilized for twining, the stimulus which has driven the process of change in this direction having been the necessity for light.
The second stage has been the development of sensitive leaves by a twining plant. No doubt at first no leaf-climber depended entirely on its leaves—it was merely a twiner which helped itself by its leaves. Gradually the leaves became more perfect, and then the plant could leave off the wasteful plan of growing spirally up a stick, and adopt the more economical and more effective one of pure leaf-climbing.
Finally, from sensitive leaves were developed the marvelously perfect tendrils which can perceive one fiftieth of a grain, and can show distinct curvature within twenty-five seconds after being touched, tendrils, with delicate, sticky ends, or endowed with the power of moving toward the dark, or of creeping into little cracks, or with that mysterious sense of touch by which a tendril can distinguish a brother tendril from an ordinary twig, and can distinguish the weight of a drop of rain hanging to it from a bit of thread—in short, all the delicate contrivances which place tendril-bearers so eminently at the head of the climbing plants.
There is only one more fact connected with the evolution of climbing plants which must be alluded to, namely, the curious way in which the representatives of the class are scattered throughout the vegetable kingdom. Lindley divided flowering plants into fifty-nine classes, called Alliances, and in no less than thirty-five of these climbing plants are found. This fact shows two things: First, how strong has been the motive power—the search after light—which has driven so many distinct kind of plants to become climbers; secondly, that the power of revolving, which is the first step in the ladder of development of the power of climbing, is present in an undeveloped state in almost every plant in the vegetable series.—Popular Science Review.