On the movements and habits of climbing plants/Part 4
Hook-climbers.—In my introductory remarks, I staled that, besides the great class of twining plants, with the subordinate divisions of leaf-climbers and tendril-bearers, there were hook- and root-climbers. I mention the former only to say that with the few which I have examined, namely, Galium aparine, Rubus australis, and some climbing Roses, there is no spontaneous revolving movement. If indeed they possessed this power, and were capable of twining, such plants would be placed in the previous great class: thus the Hop, which is a twiner, has reflexed hooks as large as those of the Galium; some other twiners have stiff reflexed hairs; Dipladenia has a circle of blunt spines at the base of its leaves; one tendril-bearing plant alone, as far as I have seen, namely, Smilax aspera, is furnished with spines. Some few plants, which apparently depend solely on their hooks, are excellent climbers, as certain Palms in the New and Old Worlds. Even some of the climbing Roses will ascend the walls of a tall house, if covered with a trellis: how this is effected I know not; for the young shoots of one such Rose, when placed in a pot in a window, bent irregularly towards the light during the day and from it during the night, like any other plant; so that it is not easy to understand how the shoots can get under a trellis close to a wall.
Root-climbers.—A good many plants come under this class, and are excellent climbers. One of the most remarkable is the Marcgravia umbellata, which in the tropical forests of South America, as I hear from Mr. Spruce, grows in a curiously flattened manner against the trunks of trees, here and there putting forth claspers (roots), which adhere to the trunk, and, if the latter be slender, completely embrace it. When this plant has climbed to the light, it sends out free and rounded branches, clad with sharp-pointed leaves, wonderfully different in appearance from those borne by the stem, as long as it is adherent. This surprising difference in the leaves I have observed in a plant of M. dubia in my hothouse. Root-climbers, as far as I have seen, namely, the Ivy (Hedera helix), Ficus repens, and F. barbatus, have no power of movement, not even from the light to the dark. As previously stated, the Hoya carnosa (Asclepiadaceæ) is a spiral twiner, and can likewise adhere by rootlets even to a flat wall; the tendril-bearing Bignonia Tweedyana emits roots, which curve half round and adhere to thin sticks. The Tecoma radicans (Bignoniaceæ), which is closely allied to many spontaneously revolving species, climbs by rootlets; but its young shoots apparently move about rather more than can be accounted for by the varying action of the light.
I have not closely observed many root-climbers, but can give one curious little fact. Ficus repens climbs up walls just like Ivy; when the young rootlets were made to press lightly on slips of glass, they emitted (and I observed this several times), after about a week's interval, minute drops of clear fluid, not in the least milky like that exuded from a wound. This fluid was slightly viscid, but could not be drawn out into threads; it had the remarkable property of not drying. One drop, about the size of half a pin's head, I slightly spread out, and scattered on it some minute grains of sand. The slip of glass was left exposed in a drawer during hot and dry weather, and, if the fluid had been water, it would certainly have dried in one or two minutes; but it remained fluid, closely surrounding each grain of sand, during 128 days: how much longer it would have remained I cannot say. Some other rootlets were left in contact with the glass for about ten days or a fortnight, and the drops of fluid secreted by them were rather larger, and so viscid that they could be drawn out into threads. Some other rootlets were left in contact during twenty-three days, and these were firmly cemented to the glass. Hence we may conclude that the rootlets first secrete a slightly viscid fluid, and that they subsequently absorb (for we have seen that it will not dry by itself) the watery parts, and ultimately leave a cement. When the rootlets were torn from the glass, atoms of yellowish matter were left on it, which were partly dissolved by a drop of bisulphide of carbon; and this extremely volatile fluid was rendered, by what it had dissolved, very much less volatile.
As the bisulphide of carbon has so strong a power of softening indurated caoutchouc[1], I soaked in it during a short time many rootlets of a plant which had grown up a plaistered wall. Attached to two sets of rootlets on the same branch, I found very many extremely thin threads of a transparent, not viscid, excessively elastic substance, precisely like caoutchouc. These threads, at one end, proceeded from the bark of the rootlet, and at the other end were firmly attached to transparent particles of silex and other hard substances. There could be no mistake in this observation, for I played with the threads for a long time, under the microscope, drawing them out with the dissecting-needles and letting them spring back again. Yet, as I looked repeatedly at other rootlets, similarly treated, and could never discover these elastic threads, I infer that the branch had probably been slightly moved from the wall at some critical period, whilst the fluid secreted from the rootlets was in the act of drying and of changing its nature through the absorption of its watery parts. The genus Ficus abounds with caoutchouc, and from the facts here given we may infer that this substance, at first in solution and ultimately modified into an unelastic cement, is used by Ficus repens to cement its rootlets to any object which it may ascend. Whether most other plants, which climb by their rootlets, emit any cement I do not know; but the rootlets of the Ivy, placed against glass, barely adhered to it, yet secreted a little yellowish matter. I may add, that the rootlets of Marcgravia dubia can adhere firmly to smooth painted wood.
Vanilla aromatica emits aërial roots a foot in length, which point straight down to the ground. According to Mohl (S. 49), these crawl into crevices, and, when they meet with a thin support, wind round it, like tendrils. A plant which I kept was young, and did not form long roots; but on placing thin sticks in contact with them, they certainly bent, in the course of about a day, a little to that side, and adhered by their rootlets to the wood; but they did not bend quite round the sticks, and afterwards they repursued their downward course. If these rootlets are really sensitive to contact and bend to the touched side, in this case the class of root-climbers blends into that of tendril-bearers. According to Mohl, the rootlets of certain species of Lycopodium likewise act as tendrils.
Concluding Remarks.
Plants become climbers, in order, it may be presumed, to reach the light, and to expose a large surface of leaves to its action and to that of the free air. This is effected by climbers with wonderfully little expenditure of organized matter, in comparison with trees, which have to support a load of heavy branches by a massive trunk. Hence, no doubt, it arises that there are in all quarters of the world so many climbing plants belonging to so many different orders. These plants have been here classed under three heads:—Firstly, hook-climbers, which are, at least in our temperate countries, the least efficient of all, and can climb only in the midst of an entangled mass of vegetation. Secondly, root-climbers, which are excellently adapted to ascend naked faces of rock: when they climb trees, they are compelled to keep much in the shade; they cannot pass from branch to branch, and thus cover the whole summit of a tree, for their rootlets can adhere only by long-continued and close contact with a steady surface. Thirdly, the great class of spiral-twiners, with the subordinate divisions of leaf-climbers and tendril-bearers, which together far exceed in number and in perfection of mechanism the climbers of the two previous classes. These plants, by their power of spontaneously revolving and of grasping objects with which they come in contact, can easily pass from branch to branch, and securely ramble over a wide and sun-lit surface.
I have ranked twiners, leaf- and tendril-climbers as subdivisions of one class, because they graduate into each other, and because nearly all have the same remarkable power of spontaneously revolving. Does this gradation, it may be asked, indicate that plants belonging to one subdivision have passed, during the lapse of ages, or can pass, from one state to the other; has, for instance, a tendril-bearing plant assumed its present structure without having previously existed as either a leaf-climber or a twiner? If we consider leaf-climbers alone, the idea that they were primordially twiners is forcibly suggested. The internodes of all, without exception, revolve in exactly the same manner as twiners; and some few can still twine well, and many others in a more or less imperfect manner. Several leaf-climbing genera are closely allied to other genera which are simple twiners. It should be observed, that the possession by a plant of leaves with their petioles or tips sensitive, and with the consequent power of clasping any object, would be of very little use, unless associated with revolving internodes, by which the leaves could be brought into contact with surrounding objects. On the other hand, revolving internodes, without other aid, suffice to give the power of climbing; so that, unless we suppose that leaf-climbers simultaneously acquired both capacities, it seems probable that they were at first twiners, and subsequently became capable of grasping a support, which, as we shall presently see, is a great additional advantage.
From analogous reasons, it is probable that tendril-bearing plants were primordially twiners, that is, are the descendants of plants having this power and habit. For the internodes of the majority revolve, like those of twining plants: and, in a very few, the flexible stem still retains the capacity of spirally twining round an upright stick. With some the internodes have lost even the revolving power. Tendril-bearers have undergone much more modification than leaf-climbers; hence it is not surprising that their supposed primordial revolving and twining habits have been lost or modified more frequently than with leaf-climbers. The three great tendril-bearing families in which this loss has occurred in the most marked manner are the Cucurbitaceæ, Passifloraceæ, and Vitaceæ. In the first the internodes revolve; but I have heard of no twining form, with the exception (according to Palm, S. 29. 52) of Momordica balsamina, and this is only an imperfect twiner. In the other two families I can hear of no twiners; and the internodes rarely have the power of revolving, this power being confined to the tendrils; nevertheless the internodes of Passiflora gracilis have this power in a perfect manner, and those of the common Vine in an imperfect degree: so that at least a trace of the supposed primordial habit is always retained by some members of the larger tendril-bearing groups.
On the view here given, it may be asked, Why have nearly all the plants in so many aboriginally twining groups been converted into leaf-climbers or tendril-bearers? Of what advantage could this have been to them? Why did they not remain simple twiners? We can see several reasons. It might be an advantage to a plant to acquire a thicker stem, with short internodes bearing many or large leaves; and such stems are ill fitted for twining. Any one who will look during windy weather at twining plants will see that they are easily blown from their support; not so with tendril-bearers or leaf-climbers, for they quickly and firmly grasp their support by a much more efficient kind of movement. In those plants which still twine, but at the same time possess tendrils or sensitive petioles, as some species of Bignonia, Clematis, and Tropæolum, we can readily observe how incomparably more securely they grasp an upright stick than do simple twiners. From possessing the power of movement on contact, tendrils can be made very long and thin; so that little organic matter is expended in their development, and yet a wide circle is swept. Tendril-bearers can, from their first growth, ascend along the outer branches of any neighbouring bush, and thus always keep in the full light; twiners, on the contrary, are best fitted to ascend bare stems, and generally have to start in the shade. In dense tropical forests, with crowded and bare stems, twining plants would probably succeed better than most kinds of tendril-bearers; but the majority of twiners, at least in our temperate regions, from the nature of their revolving movement, cannot ascend a thick trunk whereas this can be effected by tendril-bearers, if the trunks carry many branches or twigs; and in some cases they can ascend by special means a trunk without branches, but with rugged bark.
The object of all climbing plants is to reach the light and free air with as little expenditure of organic matter as possible; now, with spirally ascending plants, the stem is much longer than is absolutely necessary; for instance, I measured the stem of a kidney-bean, which had ascended exactly two feet in height, and it was three feet in length: the stem of a pea, ascending by its tendrils, would, on the other hand, have been but little longer than the height gained. That this saving of stem is really an advantage to climbing plants I infer from observing that those that still twine, but are aided by clasping petioles or tendrils, generally make more open spires than those made by simple twiners. Moreover, such plants very generally, as was observed over and over again with the several leaf-climbers, after taking one or two turns in one direction, ascend for a space straight, and then reverse the direction of their spire. By this means they ascend to a considerably greater height, with the same length of stem, than would otherwise be possible; and they can do it with safety, as they secure themselves at intervals by their clasping petioles.
We have seen that tendrils consist of various organs in a modified state, namely, leaves and flower-peduncles, and perhaps branches and stipules. The position alone generally suffices to show when a tendril has been formed from a leaf; and in Bignonia the lower leaves are often perfect, whilst the upper ones terminate in a tendril in place of a terminal leaflet; in Eccremocarpus I have seen a lateral branch of a tendril replaced by a perfect leaflet; and in Vicia sativa, on the other hand, leaflets are sometimes replaced by tendril-branches; and many other such cases could be given. But he who believes in the slow modification of species will not be content simply to ascertain the homological nature of different tendrils; he will wish to learn, as far as possible, by what steps parts acting as leaves or as flower-peduncles can have wholly changed their function, and have come to serve as prehensile organs.
In the whole group of leaf-climbers abundant evidence has been given that an organ, still subserving its proper function as a leaf, may become sensitive to a touch, and thus grasp an adjoining object. In several leaf-climbers true leaves spontaneously revolve; and their petioles, after clasping a support, grow thicker and stronger. We thus see that true leaves may acquire all the leading and characteristic qualities of tendrils, namely, sensitiveness, spontaneous movement, and subsequent thickening and induration. If their blades or laminæ were to abort, they would form true tendrils. And of this process of abortion we have seen every stage; for in an ordinary tendril, as in that of the Pea, we ran discover no trace of its primordial nature; in Mutisia clematis, the tendril, in shape and colour, closely resembles a petiole with the denuded midribs of its leaflets; and occasionally vestiges of laminæ are retained or reappear. Lastly, in four genera in the same family of the Fumariaceæ we see the whole gradation; for the terminal leaflets of the leaf-climbing Fumaria officinalis are not smaller than the other leaflets; those of the leaf-climbing Adlumia cirrhosa are greatly reduced; those of the Corydalis claviculata (a plant which may indifferently be called a leaf-climber or tendril-bearer) are either reduced to microscopical dimensions or have their blades quite aborted, so that this plant is in an actual state of transition; and, finally, in the Dicentra the tendrils are perfectly characterized. Hence, if we were to see at the same time all the progenitors of the Dicentra, we should almost certainly behold a series like that now exhibited by the above-named four genera. In Tropæolum tricolorum we have another kind of passage; for the leaves which are first formed on the young plant are entirely destitute of laminæ, and must be called tendrils, whilst the later-formed leaves have well-developed laminæ. In all cases, in the several kinds of leaf-climbers and of tendril-bearers, the acquirement of sensitiveness by the mid-ribs of the leaves apparently stands in the closest relation with the abortion of their laminæ or blades.
On the view here given, leaf-climbers were primordially twiners, and tendril-bearers (of the modified leaf division) were primordially leaf-climbers. Hence leaf-climbers are intermediate in nature between twiners and tendril-bearers, and ought to be related to both. This is the case: thus the several leaf-climbing species of the Antirrhineæ, of Solanum, of Cocculus, of Gloriosa are related to other genera in the same family, or even to other species in the same genus, which are true twiners. On the other hand, the leaf-climbing species of Clematis are very closely allied to the tendril-bearing Naravelia: the Fumariaceæ include closely allied genera which are leaf-climbers and tendril-hearers. Lastly, one species of Bignonia is both a leaf-climber and a tendril-bearer, and other closely allied species are twiners.
Tendrils of the second great division consist of modified flower-peduncles. In this case likewise we have many interesting transitional states. The common Vine (not to mention the Cardiospermum) gives us every possible grade from finely developed tendrils to a bunch of flower-buds, bearing the single usual lateral flower-tendril. And when the latter itself bears some flowers, as we know is not rarely the case, and yet retains the power of clasping a support, we see the primordial state of all those tendrils which have been formed by the modification of flower-peduncles.
According to Mohl and others, some tendrils consist of modified branches: I have seen no such case, and therefore of course know nothing of any transitional states, if such occur. But Lophospermum at least shows us that such a transition is possible; for its branches spontaneously revolve, and are sensitive to contact. Hence, if the leaves of some of the branches were to abort, they would be converted into true tendrils. Nor is it so improbable as it may at first appear that certain branches alone should become modified, the others remaining unaltered; for we have seen with certain varieties of Phaseolus that some of the branches are thin and flexible and twine, whilst other branches on the same plant are stiff and have no such power.
If we inquire how the petiole of a leaf, or the peduncle of a flower, or a branch first becomes sensitive and acquires the power of bending towards the touched side, we get no certain answer. Nevertheless an observation by Hofmeister[2] well deserves attention, namely, that the shoots and leaves of all plants, whilst young, move after being shaken; and it is almost invariably young petioles and young tendrils, whether formed of modified leaves or flower-peduncles, which move on being touched; so that it would appear as if these plants had utilized and perfected a widely distributed and incipient capacity, which capacity, as far as we can see, is of no service to ordinary plants. If we further inquire how the stems, petioles, tendrils, and flower-peduncles of climbing plants first acquired their power of spontaneously revolving, or, to speak more accurately, of successively bending to all points of the compass, we are again silenced, or at most can only remark, that the power of movement, both spontaneous and from various stimuli, is far more common with plants, as we shall presently see, than is generally supposed to be the case by those who have not attended to the subject. There is, however, the one remarkable case of the Maurandia semperflorens, in which the young flower-peduncles spontaneously revolve in very small circles, and bend themselves, when gently rubbed, to the touched side; yet this plant certainly profits in no way by these two feebly developed powers. A rigorous examination of other young plants would probably show some slight spontaneous movements in the peduncles and petioles, as well as that sensitiveness to shaking observed by Hofmeister. We see at least in the Maurandia a plant which might, by a little augmentation of qualities which it already possesses, come first to grasp a support by its flower-peduncles (as with Vitis or Cardiospermum) and then, by the abortion of some of its flowers, acquire perfect tendrils.
There is one interesting point which deserves notice. We have seen that some tendrils have originated from modified leaves, and others from modified flower-peduncles; so that some are foliar and others axial in their homological nature. Hence it might have been expected that they would have presented some difference in function. This is not the case. On the contrary, they present the most perfect identity in their several remarkable characteristics. Tendrils of both kinds spontaneously revolve at about the same rate. Both, when touched, bend quickly to the touched side, and afterwards recover themselves and are able to act again. In both the sensitiveness is either confined to one side or extends all round the tendril. They are either attracted or repelled by the light. The latter case is seen in the foliar tendrils of Bignonia capreolata and in the axial tendrils of the Ampelopsis, both of which move from the light. The tips of the tendrils in these two plants become, after contact, enlarged into disks, which are at first adhesive by the secretion of some cement. Tendrils of both kinds, soon after grasping a support, contract spirally; they then increase greatly in thickness and strength. When we add to these several points of identity the fact of the petiole of the Solanum jasminoides assuming the most characteristic feature of the axis, namely, a closed ring of woody vessels, we can hardly avoid asking, whether the difference between foliar and axial organs can be of so fundamental a nature as is generally supposed to be the case[3].
We have attempted to trace some of the stages in the genesis of climbing plants. But, during the endless fluctuations in the conditions of life to which all organic beings have been exposed, it might have been expected that some climbing plants would have lost the habit of climbing. In the cases given of certain South African plants belonging to great twining families, which in certain districts of their native country never twine, but reassume this habit when cultivated in England, we have a case in point. In the leaf-climbing Clematis flammula, and in the tendril-bearing Vine, we see no loss in the power of climbing, but only a remnant of that revolving-power which is indispensable to all twiners, and is so common, as well as so advantageous, to most climbers. In Tecoma radicans, one of the Bignoniaceæ, we see a last and doubtful trace of the revolving-power.
With respect to the abortion of tendrils, certain cultivated varieties of Cucurbita pepo have, according to Naudin[4], either quite lost these organs or bear semi-monstrous representatives of them. In my limited experience, I have met with only one instance of their natural suppression, namely, in the common Bean. All the other species of Vicia, I believe, bear tendrils; but the Bean is stiff enough to support its own stem, and in this species, at the end of the petiole where a tendril ought to have arisen, a small pointed filament is always present, about a third of an inch in length, and which must be considered as the rudiment of a tendril. This may be the more safely inferred, because I have seen in young unhealthy specimens of true tendril-bearing plants similar rudiments. In the Bean these filaments are variable in shape, as is so frequently the case with all rudimentary organs, being either cylindrical, or foliaceous, or deeply furrowed on the upper surface. It is a rather curious little fact, that many of these filaments when foliaceous have dark-coloured glands on their lower surfaces, like those on the stipules, which secrete a sweet fluid; so that these rudiments have been feebly utilized.
One other analogous case, though hypothetical, is worth giving. Nearly all the species of Lathyrus possess tendrils; but L. nissolia is destitute of them. This plant has leaves, which must have struck every one who has noticed them with surprise, for they are quite unlike those of all common papilionaceous plants, and resemble those of a grass. In L. aphaca the tendril, which is not highly developed (for it is unbranched, and has no spontaneous revolving-power), replaces the leaves, the latter in function being replaced by the large stipules. Now if we suppose the tendrils of L. aphaca to become flattened and foliaceous, like the little-rudimentary tendrils of the Bean, and the large stipules, not being any longer wanted, to become at the same time reduced in size, we should have the exact counterpart of L. nissolia, and its curious leaves are at once rendered intelligible to us.
It may be added, as it will serve to sum up the foregoing views on the origin of tendril-bearing plants, that if these views be correct, L. nissolia must be descended from a primordial spirally-twining plant; that this became a leaf-climber; that first part of the leaf and then the whole leaf became converted into a tendril, with the stipules by compensation greatly increased in size[5]; that this tendril lost its branches and became simple, then lost its revolving-power (in which state it would resemble the tendril of the existing L. aphaca), and afterwards losing its prehensile power and becoming foliaceous would no longer be called a tendril. In this last stage (that of the existing L. nissolia) the former tendril would reassume its original function of a leaf, and its lately largely developed stipules, being no longer wanted, would decrease in size. If it be true that species become modified in the course of ages, we may conclude that L. nissolia is the result of a long series of changes, in some degree like those just traced.
The most interesting point in the natural history of climbing plants is their diverse powers of movement; and this led me on to their study. The most different organs—the stem, flower-peduncle, petiole, mid-ribs of the leaf or leaflets, and apparently aërial roots—all possess this power.
In the first place, the tendrils place themselves in the proper position for action, standing, for instance in the Cobæa, vertically upwards, with their branches divergent and their hooks turned outwards, and with the young terminal shoot thrown on one side; or, as in Clematis, the young leaves temporarily curve themselves downwards, so as to serve as grapnels.
Secondly, if the young shoot of a twining plant, or if a tendril, be placed in an inclined position, it soon bends upwards, though completely secluded from the light. The guiding stimulus to this movement is no doubt the attraction of gravity, as Andrew Knight showed to be the case with germinating plants. If a succulent shoot of almost any plant be placed in an inclined position in a glass of water in the dark, the extremity will, in a few hours, bend upwards; and if the position of the shoot be then reversed, the now downward-bent shoot will reverse its curvature; but if the stolon of a Strawberry, which has no tendency to grow upwards, be thus treated, it will curve downwards in the direction of, instead of in opposition to, the force of gravity. As with the Strawberry, so it is generally with the twining shoots of the Hibbertia dentata, which climbs laterally from bush to bush; for these shoots, when bent downwards, show little and sometimes no tendency to curve upwards.
Thirdly, climbing plants, like other plants, bend towards the light by a movement closely analogous to that incurvation which causes them to revolve. This similarity in the nature of the movement was well seen when climbing plants were kept in a room, and their first movements in the morning towards the light, and their subsequent revolving movements, were traced on a bell-glass. We have also seen that the movement of a revolving shoot, and in some cases of a tendril, is retarded or accelerated in travelling from or to the light. In a few instances tendrils bend in a conspicuous manner towards the dark. Many authors speak as if the movement of a plant towards the light was as directly the result of the evaporation or of the oxygenation of the sap in the stem, as the elongation of a bar of iron from an increase in its temperature. But, seeing that tendrils are either attracted to or repelled by the light, it is more probable that their movements are only guided and stimulated by its action, in the same manner as they are guided by the force of attraction from or towards the centre of gravity.
Fourthly, we have in stems, petioles, flower-peduncles, and tendrils the spontaneous revolving movement which depends on no outward stimulus, but is contingent on the youth of the part and on its vigorous health, which again of course depends on proper temperature and the other conditions of life. This is perhaps the most interesting of all the movements of climbing plants, because it is continuous. Very many other plants exhibit spontaneous movements, but they generally occur only once during the life of the plant, as in the movements of the stamens and pistils, &c., or at intervals of time, as in the so-called sleep of plants.
Fifthly, we have in the tendrils, whatever their homological nature may be, in the petioles and tips of the leaves of leaf-climbers, in the stem in one case, and apparently in the aërial roots of the Vanilla, movements—often rapid movements—from contact with any body. Extremely slight pressure suffices to cause the movement. These several organs, after bending from a touch, become straight again, and again bend when touched.
Sixthly, and lastly, most tendrils, soon after clasping a support, but not after a mere temporary curvature, contract spirally. The stimulus from the act of clasping some object seems to travel slowly down the whole length of the tendril. Many tendrils, moreover, ultimately contract spontaneously even if they have caught no object; but this latter useless movement occurs only after a considerable lapse of time.
We have seen how diversified are the movements of climbing plants. These plants are numerous enough to form a conspicuous feature in the vegetable kingdom; every one has heard that this is the case in tropical forests; but even in the thickets of our temperate regions the number of kinds and of individual plants is considerable, as will be found by counting them. They belong to many and widely different orders. To gain some crude idea of their distribution in the vegetable series, I marked, from the lists given by Mohl and Palm (adding a few myself, and a competent botanist, no doubt, could add many more), all those families in 'Lindley's Vegetable Kingdom' which include plants in any of our several subdivisions of twiners, leaf-climbers, and tendril-bearers; and these (at least, some in each group) all have the power of spontaneously revolving. Lindley divides Phanerogamic plants into fifty-nine Alliances; of these, no less than above half, namely thirty-five, include climbing plants according to the above definition, hook- and root-climbers being excluded. To these a few Cryptogamic plants must be added which climb by revolving.
When we reflect on this wide serial distribution of plants having this power, and when we know that in some of the largest, well-defined orders, such as the Compositæ, Rubiaceæ, Scrophulariaceaæ, Liliaceæ, &c., two or three genera alone, out of the host of genera in each, have this power, the conclusion is forced on our minds that the capacity of acquiring the revolving-power on which most climbers depend is inherent, though undeveloped, in almost every plant in the vegetable kingdom.
It has often been vaguely asserted that plants are distinguished from animals by not having the power of movement. It should rather be said that plants acquire and display this power only when it is of some advantage to them; but that this is of comparatively rare occurrence, as they are affixed to the ground, and food is brought to them by the wind and rain. We see how high in the scale of organization a plant may rise, when we look at one of the more perfect tendril-bearers. It first places its tendrils ready for action, as a polypus places its tentacula. If the tendril be displaced, it is acted on by the force of gravity and rights itself. It is acted on by the light, and bends towards or from it, or disregards it, whichever may be most advantageous. During several days the tendril or internodes, or both, spontaneously revolve with a steady motion. The tendril strikes some object, and quickly curls round and firmly grasps it. In the course of some hours it contracts into a spire, dragging up the stem, and forming an excellent spring. All movements now cease. By growth the tissues soon become wonderfully strong and durable. The tendril has done its work, and done it in an admirable manner.
- ↑ Mr. Spiller has recently shown (Chemical Society, Feb. 16, 1865), in a paper on the oxidation of india-rubber, that this substance, when exposed to the air in a fine state of division, gradually becomes converted into brittle, resinous matter, very similar to shell-lac.
- ↑ Quoted by F. Cohn, in his remarkable memoir, "Contractile Gewebe im Pflanzenreiche," Abhand. der Schlesichen Gesell. 1861, Heft i. S. 35.
- ↑ Mr. Herbert Spencer has recently argued ('Principles of Biology,' 1865, p. 37 et seq.) with much force that there is no fundamental distinction between foliar and axial organs in plants.
- ↑ Annales des Sc. Nat. 4th series, Bot. tom. vi. 1856, p. 31.
- ↑ Moquin-Tandon (Éléments dc Tératologie, 1841, p. 156) gives the case of a monstrous Bean, in which a case of compensation of this nature was suddenly effected; for the leaves had completely disappeared and the stipules had grown to an enormous size.