Kitabı oku: «The Romance of Plant Life», sayfa 21

But it is not necessary to go to the tropics to find interesting and ingenious climbing plants.

There is a very common little British plant, Stellaria holostea (the Star of Bethlehem, Great Starwort, or Stitchwort), which is common in shady places, light woods, and by hedges. In the spring it grows very quickly, and the pairs of leaves are shut together over the growing point, so that the end of the stem is narrow and can insert itself between the leaves and twigs of the neighbouring plants. As soon as such a growing end gets out of the foliage into the light, each pair of leaves opens out and curves backwards, making a pair of broad, curved hooks excellently suited to hang the stem on to the leaves or twigs. Then another period of growth follows, and again a new pair of hook-like leaves opens out. The stem may be five or six feet long.

In a rather rare Speedwell (Veronica scutellata) a very similar method is used, but the leaves have special little backward-pointing teeth on their edges which assist in the attachment process.

But these leaves are not to be compared as regards perfection of mechanism with the tendrils by means of which plants climb. These tendrils are thin, flexible, twining threads, which may be formed by the modification of whole leaves, in other cases of leaflets, or sometimes of branches. Sweet Peas, Vetches, Passion-flowers, Vines, and many other plants possess them.

They are like twining plants in the way in which they revolve or twine so as to wrap themselves round anything which they touch. They move much faster than twining plants. A Cobæa tendril only takes twenty-five minutes to make a complete turn, Passion-flowers take from half to three-quarters of an hour, and the Vine tendril takes a little over an hour to make one complete turn.

But in one way they differ altogether, for they are sensitive to contact. If tickled, they contract and embrace closely the object which is touching them. They show a most extraordinary sensibility and sensitiveness.

As a matter of fact, these tendrils have a finer sense of touch and a much more delicate feeling of weight than any human being. They detect the weight of twenty-seven inches of a spider's thread.

It is, however, best to explain what happens. A half-grown curved tendril of the Passion-flower is perhaps the most interesting to experiment with, but any sort of tendril does quite well. If one very gently rubs the inner or concave side of its little hook, then in a very few minutes, or even seconds, the tendril distinctly curves. If this has happened naturally, as when for instance it has been rubbing upon a pea-stick, this curve makes it curl round the stick, and the more it touches the more it curls, until the whole tendril is wrapped round the support.

It is, of course, quite impossible to explain it all exactly: the sensitive part on the inside of the curve differs from the outside or convex part of the tendril; the former has a layer of elongated, thin-walled cells, full of the living matter, protoplasm, which are absent on the outer side. Immediately the tendril touches the stick, the outer convex surface begins to grow rapidly. It grows from forty to 200 times as fast as the inner side which touches the stick! Very soon after it has clasped the stick the tendril becomes woody and forms a strong, woody, spiral coil.

These tendrils can be made to curve by a weight exceedingly small. The most sensitive part of our own skins is quite unable to distinguish so small a weight as is perceived by these tendrils. Even the sensation of taste can only be produced by a weight eight times as great as that shown by some of them. Tendrils curve very quickly after they have been touched. In twenty seconds some tendrils curve (Cyclanthera), others (Passiflora) take thirty seconds, and some of them require four to five minutes or even longer before they make up their minds to coil.

Even more remarkable, however, is the fact that they do not coil when raindrops fall on them, giving a much harder blow than small weights. If one tendril touches or rubs against another, it is said not to curve. They are persevering little things also, for Darwin got a passion-flower tendril to curve when struck or rubbed no less than twenty-one times during fifty-four hours.

If one reflects on all these curious facts, it is difficult to help feeling that these plants behave very much in the way that a reasonable animal would do. There are many other cases in which some vegetable does exactly what we should expect of reasonable beings under the circumstances. The tip of the root (see p. 89), the Sensitive Plant, the Monkey and Barberry flowers, are all well-known cases.

So that it is difficult to find anything in science to contradict the comfortable belief that wide-open flowers and stretched-out leaves of plants as they drink in the warm rays of the sunlight are really enjoying themselves, whilst they are doing their day's work.

All these interesting facts are so beautifully described and so carefully summed up by Charles Darwin, that we shall only earnestly recommend our readers to get first that fascinating book The Power of Movement in Plants, and then read all the rest of his works.¹⁴²

There are an extraordinary number of these plants and the tendrils are formed exactly where they will be most useful. Every part of a leaf may become a tendril. The whole leaf is changed into one in some kinds of Lathyrus. In a very beautiful creeper which is not so often grown in greenhouses as it might be (Gloriosa superba), the tip of the leaf only acts as a tendril. Leaflets are often made into tendrils. The Clematis is the most economical of them all, for the leaf-stalk coils round and forms little woody rings which hold up the plant.

Before leaving the subject of tendrils, it may be interesting to notice the queer corkscrew spirals in which they roll themselves up. These spirals are formed after the end of the tendril has tied itself to the support and become woody. The free part between the end and its own stem goes on revolving; now if you tie a piece of string at both ends and make it revolve, you will see at once that it must coil itself into a double spiral, one part in one direction and the other in the opposite way, with a flat piece between them.

One might be disposed to think no more about these double coils; but here comes in one of the curious, inexplicable coincidences which happen so often in plant life. Such a coil is much stronger than a straight bit of wire or string would be, because if pulled out it yields and is springy. That of course makes it less probable that the tendril will be broken. Attached by a series of wiry springs, the plant yields and sways to the wind, and it is not likely that it will be torn away. Besides this, the coiling of the tendril pulls the stem closer to its support, which is also a great advantage.

Certain Virginian Creepers and Vines behave in quite a different manner. The tendrils grow away from the light and so seek the shadow of the leaves. They are also divided into little branches. At the tip of each little branch is a small knob; if this should touch the wall or the trunk of a tree, etc., it immediately secretes a drop of cement and glues itself firmly to the wall. There is a curious difference in different sorts of Ampelopsis in this respect. There is no adhesive pad in one of them (Ampelopsis hederacea) until it touches, whilst A. Veitchii has them more or less ready for gluing before they touch (though they become much larger and better developed as soon as they rub against the wall).¹⁴³

One of the most interesting of our common climbers, "that rare old plant the Ivy green," has not yet been mentioned. It is exceedingly decorative on walls, especially on ruins and on old tree-trunks in winter time, where its dark, brilliant green is most effective.

A violent controversy rages as to whether it does good or harm. Unhappily it does not do any good to trees. It does not suck their sap, for its roots do not get through the bark, but it does choke, with its clinging branches, young tree-stems, and prevents their growing properly.

Also, in winter storms an ivy-covered tree is much more likely to be blown down. But on walls the ivy certainly does good, for it sucks up the moisture, and ivy-covered walls are much more dry inside than those which are exposed to rain.

Its method of climbing is very curious. All along the stem quantities of little roots are produced. They dislike light, like most roots, and creep into crevices and cracks, where they wedge themselves in by growing thicker. Thus the stem is anchored all along its length. It is curious to find that these roots are formed before a twig is actually touching the wall, so as to be ready for any emergencies.¹⁴⁴

One interesting little point in the growth of the ivy on a tree is perhaps worth mentioning. The main stem runs nearly straight up the trunk, and when young is pulled down into the crevices or cracks in the bark. But its branches leave the main stem at an angle of forty-five degrees or so to it; these latter may often grow in this direction for a foot or eighteen inches, but then they gradually begin to turn more and more distinctly up the tree. Still these branches firmly clasp the trunk like arms spread out on either side of it, and make it almost impossible to dislodge the main stem.

Old plants of ivy entirely surround the trunk. The flowering branches grow straight out into the air, and have no tendency to cling to the bark. Their leaves are also different.

The ivy may be considered as a root-climber, although the branches assist by growing round the stem.

A curious instance has been given me of the longevity of ivy and its power of clinging to life. A correspondent mentions the case of a Scotch fir whose life was threatened by an ivy. The trunk of the ivy was sawn through. That did not kill it, at any rate immediately!

Probably the rain soaked up by the leaves, and by the roots in the crevices of the bark, kept it sufficiently fresh to cling to life. As it refused to die, a ladder was brought, and it was dragged off the tree. No doubt it would have died if the weather had been at all dry.

There are some very beautiful tropical plants which also climb by means of their roots. These roots, the so-called girdle roots, grow right round the stem and embrace it, so that the climber is perfectly supported.

It is impossible not to be impressed with the extraordinary variety of all these contrivances by which plants are able to escape the trouble of supporting themselves. But such ways of life involve certain disadvantages. Supposing there is nothing on which to climb, the stems trail feebly on the ground, and are probably soon choked by the surrounding grasses. Curiously enough, there are varieties of the Ivy, Wistaria, and the French Bean which are upright, and do not climb at all. The Tree Ivy has all its leaves like the leaves of the flowering shoot in the common form. In America, Wistaria sinensis is often grown as a standard tree, and does not send out the long shoots, sometimes thirty feet in length, which are common when it grows on walls. The dwarf French Bean has a thick stem and requires no support, yet it often puts out a long slender shoot which tries to twine round something.

In a tropical forest also, the creepers, though they damage the trees, yet manage to find space for their leaves and flowers: more vegetable matter is formed per square yard of ground than would be the case if there were no climbing plants.

CHAPTER XXVI
PLANTS WHICH PREY ON PLANTS

The kinds of cannibals – Bacteria – Spring flowers – Pale, ghostly Wood-flowers – Their alliance with fungi – Gooseberries growing on trees – Orchid-hunting – The life of an orchid – The mistletoe – Balder the Beautiful – Druids – Mistletoe as a remedy – Its parasitic roots – The trees it prefers – The Cactus Loranthus– Yellow Rattle and Eyebright, or Milk-thief, and their root-suckers – Broomrape and toothwort – Their colour and tastes – The scales of the toothwort which catch animalcula – Sir Stamford Raffles – A flower a yard across – The Dodder – Its twining stem and sucker-roots – Parasites rare, degenerate and dangerously situated.

THE word cannibal is often used in a very loose and unscientific way. Amongst some savage tribes it is the custom to eat old people and young children; but this is only in seasons of famine and scarcity, when there is no other food available, and not because they are specially fond of them. But amongst other tribes wars are made for the special purpose of capturing fat young people to cook. Sometimes they have become so accustomed to such delicacies that they are unable to get their food in any other way. Of course, when tribes become "pure cannibals" of this last type they have to be destroyed like wild beasts.

Among plants we find all sorts of transitions and degrees of cannibalism. There are plants which sometimes, and, as it were, accidentally, attack others. But there are also real cannibal plants which live entirely on the life-juices and sap of other plants, and cannot exist by their own labours at all. Moreover, we can find almost every conceivable state of transition. These can be clearly and definitely traced from those plants which depend on the labour of their own roots and leaves to others which have no leaves, and which consist merely of one large flower and a large adhesive sucker fixed on some one else's root.

The difficulty is very often to know where to draw the line. Probably no flowering plant is quite independent of the labour and work of its neighbours. As we have tried to show in another chapter, a long preliminary cultivation by bacteria, lichens, and mosses is required before flowering plants can develop on bare rock. That is also necessary in all cases where the soil is mineral or inorganic, without any organic dust or fragments of vegetable or animal matter. Bacteria must always begin the work by preparing nitrates and other salts.

So that only those bacteria which weather rocks can be called really free and independent. But other bacteria, such as those which cause typhoid, anthrax, hydrophobia, etc., are the best possible examples of pure cannibals, or, as they are usually described, parasites.

This last word is derived from a peculiar class of people in ancient classical times, who used to appear whenever a meal was going to begin, and received food without giving anything in return. They are represented by our tramps or by the "sundowners" in Australia, who appear as soon as the evening meal is ready and when there is no possibility of going any further on their journey.

The way in which plants became parasites or cannibals is a very interesting part of plant life, and we shall try to trace some of the various stages.

To begin with, if one looks out for them in spring one is sure to find a whole series of beautiful spring flowers. There is the Primrose, with its bright, hardy, yellow flowers; the Violet, whose strong perfume much annoys the huntsman, for it spoils the "scent" and shows him that the end of winter has come; the delicate little Moschatel, the Lesser Celandine, the Bluebell or Hyacinth, Dog's Mercury, the Male and the Lady Fern, and many others.

Most of these begin to grow and are in flower early in the season. That is because they are living on the dead leaves of the last year, or rather of two or three years ago. Their roots are breaking up and devouring, with the help of worms, beetles, and insects, the leaf-mould of past seasons.

They are quite dependent on the trees; they cannot exist except where such leaf-mould is formed.

But it is very difficult to tell whether these humble little herbs which live on the scraps that fall from the tall trees are either parasites or clients, which last do some good in return for their share.

Probably they are distinctly useful and good for the forest if this is considered as a whole establishment. They use light which would otherwise be wasted, and their own dead leaves increase the annual deposit of leaf-mould.

There are other plants, such as the Bird's-nest (Neottia) and Coralroot¹⁴⁵ Orchids, as well as Monotropa and others, which also live on the rich, decaying leaf-mould of forests, but these are generally pale in colour, for they possess but little green chlorophyll. They are more directly dependent on the mould and have ceased to do much work for themselves. Most of them in fact have entered into an alliance with fungi, and use these fungi to get their food material from the dead leaves.

Such fungi are always abundant in good, well-grown forests (see p. 86). These Orchids and Monotropa have their roots and underground stems covered and wrapped round by the fungus threads, which extend from them in every direction, breaking up and decomposing the dead leaves.

The colour of Monotropa is a pale waxen yellow, that of the others is usually a ghostly pale, opalescent, steel-blue or coral-like hue, which makes them very distinct in the dim, mysterious shades of the forest.

These plants are undoubtedly of use, for they break up and decompose the leaf-mould.

Another very interesting group are not well represented in this country. Sometimes one may see on an old tree a Gooseberry bush in full foliage quite high up the trunk in the fork of the branches. In sheltered woody ravines, Polypody ferns are often established on old moss-clad branches, where their green fronds hang over to catch as much as they can of the sunlight. But Orchids, Bromeliads, and Ferns which grow upon the branches of great trees are one of the most conspicuous and beautiful features of tropical woods. It is for these tree-orchids that the orchid-hunter braves the head-hunters of Borneo or traverses the precipices and rugged forests of Guatemala and Brazil. It is often necessary to cut down a tall tree in order to get the orchids in its higher branches. Often, however, this is unsuccessful, for the tree is so held up by creepers and other giants of the forest that it never reaches the ground!

Then, after being stripped from the branches, in some out-of-the-way forest-clad range of Burma, Celebes, South America, or Madagascar, these orchids are dried, put up in crates and packed off to London, where they are carefully cultivated in hot-houses and persuaded to flower. They may be worth sixpence or they may be worth £500 each, but no one can tell until they have flowered in London.

But the romance of the orchid-hunter is not exactly what we have to describe here. It is rather the romance of the life of the orchid itself.

It is perched high up on the branches of the tallest trees in the forest, exposed to sun, exposed to wind, and quite unable to gather either salts or rain from the soil. How, then, does it manage to live?

These orchids, it must be remembered, are only found in out-of-the-way and feverish, unhealthy places, where the aboriginal savages still lurk and endure a dreadful existence of hunger and starvation in dense tropical forests.

Now the word "dense" explains the whole story. Those forests are so thick, so full of giant trees and exuberant growth, that civilized man even to-day in 1906 can make nothing of them, and leaves them to the savage. The reason why vegetation is so luxuriant is simply that there are both plentiful moisture and a hot, tropical sun. That makes the life of the orchid possible, and also ensures malaria for the hunter.

It hangs out into the moist air long pendulous roots which act as so many sponges absorbing and soaking in moisture. The tremendous energy of growth covers bark and branches with creeping plants innumerable, with a profusion of moss, liverworts, and ferns such as we cannot imagine from our own experiences in this country. So the roots of our orchid find on the branches rich leaf-mould, and it lives happily and contentedly on the salts and moisture accumulated by the mosses and other plants. Its leaves are fleshy and succulent, rather like those of a desert plant, so that it can store up water against a season of drought.

These plants which grow in this way on other plants, do not, as a rule, greatly injure them, but many have not stopped at this stage. Take, for instance, the Gooseberry growing in the fork of an old tree. Some bird has been eating gooseberries and dropped the seed there. The roots of the gooseberry will grow down into the rotten part of the trunk. Earth and leaf-mould will accumulate there, and it is quite probable that the whole inside of the tree will decay away. The roots of the gooseberry will, if only indirectly, help in this decay.

But it is far otherwise with another set of plants – the Mistletoe and its allies. There is plenty of romance connected with the mistletoe. Dr. M. T. Masters says as follows: "The origin of the modern custom connected with mistletoe is not very clear. Like many other customs, its original significance is only guessed at. If known, perhaps, the innocent merriment now associated with the plant would be exchanged for a feeling of stern disapproval, and the mistletoe would be banished from our homes. In such a case ignorance is bliss."

It will be remembered that all the gods of Iceland were once gathered together so that a general oath might be exacted of every plant "that grew upon the earth," that they would do no harm to Balder the Beautiful. The Mistletoe did not take the oath, because it does not grow upon the earth but upon a tree. Then the enemy fashioned an arrow out of the mistletoe, and killed Balder. There is a modern idea that the story is a myth representing the death of Spring, for a great many similar stories occur in widely distant places.

However, it seems pretty certain that the plant was a sacred one to the Druids in the time of the Romans.

Ovid speaks of this in the line, "Ad Viscum Druidæ cantare solebant." At their solemn meetings, which were held in remote sacred groves, a Druid clad in white robes cut the mistletoe with a golden sickle. Then, apparently, human sacrifices were offered and a general festival took place.

Some remnant of this custom seems to have persisted in Herefordshire until recent times, for the tune "Hey derry down, down down derry" (which means in a circle move we round the oak) is supposed to be a relic of the hymn chanted by the Druids when they had found mistletoe on the oak.

It was said in the Middle Ages to be a useful cure for apoplexy, madness, and giddiness. That is not at present the general view. Indeed, under present conditions it might conceivably promote the last and even the second of these disorders, though in an agreeable way!

The Mistletoe and its allies, Loranthus and Arceuthobium, grow upon the branches of trees like the orchids and gooseberries already mentioned, but they differ altogether in having a special kind of absorbing root which sinks down into the bark until it reaches the wood of the "host" tree. The sap running up the tree is then tapped by this root, and goes to supply the mistletoe with water and salts in solution. It has, however, its own green leaves. Thrushes eat the berries of the mistletoe; they will be left upon a branch with the guano; as the latter dries up, the seed is drawn to the underside of the branch, and sticks in a crack or crevice; it then sends the sinker-root mentioned above into the branch.

Every year afterwards new mistletoe "roots" are formed which grow through the soft part of the bark and send down sinkers into the wood. Cases of Mistletoes forty years old have been recorded. The trees which they prefer are the Apple, and after that Black Poplar, though mistletoe may be found on Silver Fir, various Pines, and others. It is more difficult to get it to grow on the Oak than on any other tree. Indeed, only seven cases of mistletoe growing on oak have been recorded in this country.¹⁴⁶ It is quite a valuable crop in some places, and is sent in tons to the London market.

There are many species of Mistletoe, and at least one kind attacks, and is parasitic upon, another species of Mistletoe.

Most Mistletoes and Loranthus have their own green leaves, and only take from the plant to which they are attached sap and mineral salts. But in Chile there is a beautiful Loranthus that has practically no green leaves at all. Its blood-red flowers grow in dense masses upon the giant Cactus, which is common on the drier hills, and these are always mistaken for the Cactus's own flowers, which are quite different. These almost leafless Loranthus, and the curious Arceuthobium are more parasitic than ordinary mistletoes, for they obviously take other food material (probably sugar and albuminoids) from their "host."

Another series of parasites or cannibals are quite common in Great Britain. One often sees in some meadow that the grasses are growing in a scanty and unhealthy manner; one then notices amongst them numbers of the Yellow Rattle or the Eyebright (which the Germans call Milk-thief). These plants are not very remarkable in any way, but if one examines them closely one sees that the leaves and stems are more purplish-red than is at all usual with our ordinary flowering plants. But if you dig up some specimens very carefully, then the wickedness of the Yellow Rattle and Eyebright becomes apparent; every here and there upon their roots are little whitish swellings which are firmly attached to the roots of other plants (generally of grasses). These two robber plants send from these swellings minute sucker-roots which pierce into the grass-root and intercept the water which the grass has been absorbing for itself.

They are therefore parasites, and indeed they may cause a considerable loss of forage in a meadow.

A good many other British plants are root thieves. Besides these two, there are the Cow-wheat, Red Rattles, Toadflax, Broomrapes, and Toothwort.

A curious point about them is that they differ amongst themselves in the degree in which they are dependent on the work of others. Some are able to grow quite well without any such extraneous help, but the Broomrape and Toothwort are entirely dependent on others' labours. They have extremely little chlorophyll and very small leaves, and are clearly parasites "pure and simple."

There are about 180 species of Broomrape (Orobanche). All of them attack roots, and most confine their attentions to one particular flowering plant. Their colours are generally very striking and unusual. Our British species are reddish, flesh-coloured, or dirty white, but some of the foreign kinds are blue or violet, yellow, or yellowish to dark brown. Generally the seedling Broomrape worms its way down into the earth till its root-tip touches the root of its special favourite host, then the root of the Broomrape fixes itself for life; its suckers grow into the host and absorb all the food material which it requires. Those kinds which attack Tobacco and Hemp are dangerous pests and do considerable damage.

The Toothwort (Lathraea) is so called because its scales have a sort of resemblance to human teeth. With the curious superstition which prevailed in medieval times, it was supposed that the plant must be a remedy for toothache because it resembled teeth. Unfortunately this is not the case.

It is, generally, quite like the Broomrape in its method of growth, but it sends out long thread-like branching roots with suckers on the ends, which become fastened on the Hazel roots. For several years the plant remains underground and forms very odd-looking, white, scaly branches. These scales are rolled back in such a way as to form peculiar and irregular cavities which open to the outside near the tip of the leaf. There is no doubt that animalcula of sorts get into these cavities and probably die there. In that case, their remains will form a useful supplement to the diet of the plant. The following remarks, however, taken from Kerner have been disputed by other botanists.

Certain of the cells lining these cavities "appear to send out delicate filaments.

"When small animals penetrate into the labyrinthine chambers of a Lathraea leaf and touch the organs just described, the protoplasmic filaments are protruded and lay themselves upon the intruders. They act as prehensile arms in holding the smaller prey, chiefly Infusoria, and impede the motion of larger animals so as to cut off their retreat. No special secretion has been observed to be exuded in the foliar chambers of Lathraea. But seeing that some time after the creatures have entered the chambers, the only remains of them that one meets with are claws, legs, bristles, and little amorphous lumps, their sarcode-flesh and blood having vanished and left no trace, we must suppose that the absorption of nutriment from the dead prey here ensues…"¹⁴⁷

But strange as these Broomrapes and Toothworts may be, they are quite inconspicuous as compared with the gigantic parasites found in Sumatra and Java.

In 1818, when Sir Stamford Raffles was making a tour in the interior of Sumatra, his party came across one of those extraordinary plants which have been called after him.

Imagine a gigantic flower in shape resembling a very fleshy forget-me-not, but more than a yard across! The colour is a livid, fleshy tint, and the smell is like that of a charnel-house. This extraordinary Rafflesia Arnoldii is the biggest flower in the world. It has no proper stems or leaves, but consists merely of this huge flower-bud attached to the roots of Figs, etc., which traverse the ground in these forests. It is said to be only found in places frequented by elephants, which are supposed to carry its seeds on their feet.

There are four other kinds known: all of them occur in Sumatra, Java, and other neighbouring islands. R. Padma for example, has a flower about eighteen inches across. The central part is a dirty blood-red, while the lobes have almost the colour of the human skin. This also has a "cadaverous smell, anything but pleasant."

These weird Rafflesias seated on the roots "which wind about on the dark forest ground" have impressed every observer.