Popular Science Monthly/Volume 30/November 1886/Thistles
| THISTLES. |
By GRANT ALLEN.
THERE is no weed weedier or more ubiquitous than the common thistle. In paradise, it is true, if we may trust John Milton and the Sunday-school books—wise, as usual, beyond what is written—there were no thorns or thistles; the creation and introduction of the noxious tribe upon this once innocent and thornless earth being a direct consequence of the fall of man, and a stern retribution for Adam's delinquency. But since then the thistle has managed so to diffuse itself over the habitable globe that there hardly now remains a spot on earth without its own local representative of that ever-intrusive and conquering genus. Wherever civilized man goes, there the thistle accompanies him as a matter of course in his various wanderings. It adapts itself to all earthly environments. Close up to the Arctic Circle you find it defying the indigenous reindeer with its prickly wings; under an equatorial sky you may observe it accommodating itself most complacently with a sardonic smile to tropical existence, and battling with the prickly cactuses and the thorny acacias, to the manner born, for its fair share of the dry and arid uplands. Even nettles are nowhere in competition with it; in spite of its valuable and irritating sting, the nettle has not the plasticity and adaptability of constitution that mark the stout and sturdy thistle tribe. Garnered and harvested yearly with the farmer's corn, its seeds have been gratuitously distributed by its enemy, man, in all climates; and, when once it gains the slightest foothold, its winged down enables it to diffuse itself ad infinitum through the virgin soil of yet unconquered and unthistly continents. A field of thistles in England itself is a beautiful sight for the enthusiastic botanist (who has usually a low opinion of the agricultural interest); but in the fresh and fallow earth of New Zealand they attain a yet more prodigious and portentous stature, that might well strike awe and dismay into the stout heart of a Berkshire farmer.
The fact is, the thistle is one of those bellicose plants which specially lay themselves out, in the struggle for existence, for the occupation of soils where they are compelled to defend their leaves and stems from the constant attacks of the larger herbivores. On open plains and wide steppes, much browsed over in the wild state by deer or buffalo, and in the degenerate civilized condition by more prosaic cows and donkeys, one may always note that only the prickliest and most defensive plants have any chance of gaining a livelihood. Gorse and blackthorn form the central core of the little bushy clumps on our English commons, grown over thickly with bramble and dog-rose, or interspersed every here and there with occasional taller masses of may and holly. Nay, at times even naturally undefended species assume a protective armor under such special circumstances, as in the case of the pretty little pink rest-harrow, which grows close to the ground with soft stems and leaves where unmolested by cattle, but quickly develops an erect and stiffly thorny variety when invaded by troops of cows or horses. In that case the unarmed specimens get eaten down in a short time by the browsing cattle, and only those which happen to possess any slight tendency in a prickly direction are left to occupy the stubborn soil and produce seed for the next generation. It is this unconscious selective action of the larger herbivores which has at last produced the general prickliness of all the plants that naturally frequent rich and open lowland pastures.
There are differences, however, between prickles and prickles. Some plants are positively aggressive, like the stinging-nettle; others are merely and strictly defensive, like the common thistle, whose proud motto, as everybody well knows, is “Nemo me impune lacessit.” In the very doubtful Latinity of the Licensed Victualers, it goes in strictly for “Defensio non provocatio”; whereas the nettle, it need hardly be said, is often most distinctly provoking, and even goes out of its way to annoy a neighbor. This distinction I take to depend upon a difference in the acquired habits of the two races. The nettle is almost entirely a product of urban civilization; it hangs about the streets and out-houses of small villages, the neighborhood of farm-yards, and the immediate surroundings of rural man. It lives in constant expectation, as it were, of being browsed upon by donkeys, or trampled under foot by cattle, or picked by children, or stubbed up root and all by the ruthless farmer. Hence its temper has become permanently soured, and it has at last developed a restless, feverish, wasp-like habit of stinging everybody who comes within arm's length of it. It is necessary to the safety of the nettle, in fact, that it should give you warning of its presence at once, and induce you to keep well away from it under pain of a serious and lasting smart. Our common English nettle, which grows everywhere along road-sides and waste places, is bad enough in this respect; but the smaller nettle—a foreign importation of more strictly civilized and urban habits, never found far from human habitations—is still crueler and more poisonous; while the South European Roman nettle, accustomed for innumerable generations to the fierce struggle against Italian civilization, has developed an advanced and excruciating sting, which beats the puny efforts of our own species into complete insignificance, as the virus of the hornet beats the virus of the hive-bee.
On the other hand, the thistle family are far more truly rural and agricultural in their habits, being denizens of the open fields and meadows, less dependent than the nettles upon richness of soil, and readily accommodating themselves to all vacant situations. Hence they have only felt the need of arming themselves in a rough-and-ready prickly fashion against the probable assaults of their natural enemies. They have forged darts, but have not learned to poison them. Their prickly leaves and wings are amply sufficient for defense, without the necessity for developing a virulent juice to be injected into the very veins of their savage aggressors. Natural selection can never push any special line of evolution further than is imperatively called for by the wants and circumstances of the particular species. It always necessarily leaves off just at the point where the protection afforded is fully sufficient to guard the kind from the possibility of extinction. The thistles have found in actual practice that prickles alone are quite enough to secure their boasted immunity from extraneous attacks: the nettles have practically discovered for themselves that without stings they would soon be landed in the final limbo of utter nonentity.
Circumstances have still preserved for us a very tolerable series of the successive stages whereby our existing thistles have gradually acquired their present prickly and repellent characteristics. In the good old days, while evolution was still fighting hard for public recognition, it used to be urged by the uninstructed outsider that we never found any “missing links.” As a matter of fact, in ninety-nine cases out of a hundred, the links are not and never were missing at all; and the practical difficulty is rather to establish any well-marked distinctions of kind than to discover long series of intermediate individuals. Just as the white man gradually merges into the negro by slow steps, when we cross Europe, Asia, and Africa, through Italians, Greeks, Levantines, Arabs, Egyptians, Nubians, Abyssinians, and true Soudanese, so the various kinds of thistles merge imperceptibly one into the other by innumerable varieties and natural hybrids. To be sure, there are such things as well-marked species in nature; but there are also groups which it is impossible anywhere to split up into good and distinctly different kinds. The brambles, the wild roses, the St. John's-worts, and the epilobes absolutely defy regular classification: the thistles, though perhaps a little more amenable to the subtile arts of the artificial species-maker, still constantly glide one into the other by strangely graduated intermediate forms. The great crux really lies in the problem of the existence of such natural gradations; for, according to the strict Darwinian principles, the better adapted and more specialized forms ought to crush out the intermediate types, and leave the species well demarkated one from the other by broad intervals. Probably the true explanation of the anomaly is to be found in the wide distribution and high adaptability of these dominant forms; they can accommodate themselves exactly to such an extraordinary variety and diversity of situations, that special intermediate types answer best in every intermediate soil or climate.
The most primitive and unarmed class of the thistle tribe is well represented by the saw-wort of our copses, a true thistly plant in all its general appearance and habits, but absolutely devoid of thorns or prickles. The leaves, indeed, are toothed and pointed, but the points never project into fierce spines, as in the more advanced kinds; and even the little scales that form a cup for the flower-head, though faintly stiff and sharp, are scarcely if at all defensive in character. The flower, of course, is usually the first part to be specially protected, because upon it depend the future seeds and the hope of coming generations of thistles. Just as instinct teaches female animals to fight fiercely and bravely for their young, so natural selection teaches menaced plants to ann themselves stoutly against the threatening depredators of their seeds and blossoms. The reason why the saw-wort and its unarmed South European allies have managed to do without the protective inventions of their more developed relations is no doubt because they live mostly in thickets and woody places, not much overrun by cattle or horses. Their neighbors in the open meadows and pastures have been compelled long since to adopt more military tactics in order to save themselves from premature extinction. Often, indeed, in a close-cropped paddock, you will find only two kinds of tall plant uneaten by the beasts—the meadow buttercup, preserved from harm by its acrid juices, and the creeping thistle, armed all round with its long rows of parallel prickles.
In the mountains of Wales and the north of England there is yet another kind of true thistle, classed as such by technical botanists (for the saw-wort is artificially relegated to a distinct genus), which is also destitute of prickles on the leaves, though it sometimes shows the first faint beginnings of a prickly tendency around the scaly flower-cup, and in the bristly teeth of its crinkled leaves. From this early stage in the evolution of thistledom we can trace the gradual steps in the defensive process, through thistles that grow with prickly leaves, and those in which the prickly margins begin to run a little down the stem, to those which have clad themselves from top to toe in a perfect mail of sharp spines, so that it becomes quite impossible to grasp them anywhere with the hand, and they can only be eradicated by the hoe or plow. It is a significant fact that the most persistent and troublesome of all these highly developed kinds, the creeping thistle, now universally diffused by man over the globe, is a special weed of cultivation, far most frequently found in tilled fields, and seldom disputing with the simpler forms the open moors, mountains, or pastures. It does not trust entirely, like others of its kind, to its floating seeds, blown about everywhere as they are by their light tag of thistle-down; but it creeps insidiously underground for many yards together, sending up from time to time its annual stems, and defying all the attempts of the agricultural interest to exterminate it bodily by violent measures. This is the common and familiar pale purplish thistle of our English corn-fields, and there can be little doubt that it has developed its curious underground habits by stress of constant human warfare, especially with the plowshare. Thus the very efforts we make at fighting Nature defeat themselves: if we persistently hoe down the stems and leaves of an obnoxious weed, the weed retaliates by sending out hidden subterranean suckers, and the last state of the agriculturist is worse than the first.
On the close-cropped chalk downs of our southern counties there is another curious form, the stemless thistle, which shows in another way the hard struggle of Nature to keep up appearances under the most difficult and apparently hopeless circumstances. Among the low sward of those chalky pastures, nibbled off incessantly as fast as it springs up by whole herds of Southdowns, no plant that normally raised its head an inch above the surface would have a chance of flowering without being eaten down at once by its ruthless enemies. So the local dwarf or stemless thistle has adopted a habit of expanding its very prickly leaves in a flat rosette or spreading tuft close to the ground, and bearing its blossoms on the level of the soil, pressed as tight as possible against the short turf beneath. The appearance of these three or four dwarfed and stunted but big flower-heads, bunched thickly together in the middle of their flat leaves, is most quaint and striking when once one's attention is called to their existence: yet so unobtrusive and unnoticeable is the entire plant that few people save regular botanists ever discover the very fact of its presence on the chalk downs. It is only one out of a large group of specialized chalk plants, all of which similarly creep close to the ground, while a few of them actually bury their own seeds in the soil by a corkscrew process, so as to escape the teeth of the all-devouring sheep. The power of producing a stem, however, is rather dormant than lost in the dwarf thistle, for under favorable circumstances and in deep soil it will raise its flowers eight or ten inches above the surrounding turf.
The question what particular plant ought to be identified with the stiff, heraldic Scotch thistle has long been debated, somewhat uselessly, it must be acknowledged, among botanists and antiquaries. For heraldry is not particular as to species and genus: it is amply satisfied with a general rough resemblance which would hardly suit the minute requirements of those microscopical observers who distinguish some forty kinds of native British blackberries. However, it has been amicably decided in the long run that the heraldic symbol of Scotland, that proud plant which no man injures unavenged, is not to be considered a thistle at all, but an onopord, a member of a neighboring though distinct genus, whose Greek name expressly marks it out as the favorite food of—how shall I put it with becoming dignity?—the domestic beast of Oriental monarchs. To what base uses may we come at last! The royal emblem of the north, as identified by Mr. Bentham and other profound authorities, is now at last settled to be nothing more nor less than the cottony donkey-thistle. North of the Tweed this identification should be mentioned, as French newspapers remark, under all reserves.
Almost all the thistles have purple florets, and purple, it may be safely assumed, is the primitive color of the whole thistle-head tribe. Some of them, indeed, fade off gradually into pink and white; but such reversion to a still earlier ancestral hue is everywhere common and easily brought about by stress of circumstances. The thistles in the lump are composites by family, and the apparent flower is really a flower-head, containing an immense number of small, bell-shaped, five-petaled florets, with the petals united at their base into a deep tube. The honey rises high in the throat within, and is sucked chiefly by bees and burnet-moths, who form the principal fertilizers of the entire group. Purple is the favorite color of these advanced flower-haunters, and it seems probable that all the purple blossoms in nature have been evolved by their constant and long-extended selective action. Nothing can be more interesting than to watch a great burly humble-bee (one of the large black sort) bustling about from flower-head to flower-head of the pretty, drooping, welted thistle on a bright summer's day, with his proboscis constantly extended in search of food, and unconsciously carrying the pollen-grains about his head and legs from the florets of one blossom to the sensitive surface of the next in order.
After the flowers have been duly fertilized, the thistle-seeds begin to swell, and the down around them to grow dry and feathery. This down, so familiar to all of us among the autumn fields, has doubtless played no small part in the dispersal of the thistles. It is to their floating seeds (or rather, to be strictly accurate, their fruits) that the entire family owe a great part of their existing vogue and unpopularity. In almost all the composites the tiny calyx grows out into much the same silky down on the ripe fruit, but in hardly any other case save perhaps those of the dandelion and the common sow-thistle, does it form so light and airy a floating apparatus as in the true thistles. Wafted about on the wings of the wind, the thistle-down is blown easily hither and thither, alighting everywhere, far and near, and finding out fresh spots for itself to root and thrive on every side. Not only does this plan insure the proper dispersal of the seeds, however: it also provides for that most important agricultural need, the rotation of crops. Long before scientific farming had hit upon the now familiar rotatory principle, hundreds and hundreds of plants in the wild state had worked it out practically for themselves under stress of the potent modifying agency of natural selection. For thistles can no more grow on the same spot for an indefinite number of generations than corn or turnips can; they require to let the soil on which they live lie fallow for a while from time to time, or be occupied by other and less exhausting crops. Hence it follows that in nature innumerable means exist for favoring or insuring the dispersal of seeds; or, to speak more correctly, only those plants in the long run succeed in surviving which happen to possess some such facility for constant rotation and occupation of fresh districts.
It is very interesting in this respect to compare the devices for the distribution of their seeds in some of the thistle's own nearest and best-known relations. The burdock, for example, is in flower and fruit almost a thistle, though it differs considerably from the thistles proper in its large, broad, heart-shaped foliage. But the burrs, or ripe flower-heads, instead of being surrounded, thistle-fashion, by a very defensive prickly involucre, have developed instead, hooked points to their bracts, which catch at once at the wool of sheep, the legs of cattle, and the dresses or trousers of wayfaring humanity. In this way the entire head of seeds gets carried about from place to place, and rubbed off at last against a hedge or post (at least by its unwilling four-footed carriers), where it forms the nucleus of a fresh colony, and starts in life under excellent auspices, especially if dropped (as it is apt to be) in the immediate neighborhood of a well-manured farm-yard. Hence the burdock has no further need for the down which it inherits, like all its tribe, from some remote common ancestor; it has substituted a new and more practically effective system of transport en bloc, for the old general composite mode of dispersal in single seeds by a feathery floating apparatus. Accordingly, the pappus, or ring of down, though it still exists as a sort of dying rudiment on each fruitlet of the burrs, is reduced greatly in size and expansion, and consists of a mere fringe of short, stiff hairs, useful perhaps in preventing flies from laying the eggs of their destructive grubs upon the swelling seeds. In the common knap-weeds, again, which wait for a high wind to shake out their seeds from the head, this dwarfing of the down has proceeded much further, so that at first sight a careless observer would never notice its existence at all: but if you look close at the ripe fruit with a small pocket lens, you will observe that it is topped by a ring of very minute, scaly bristles, occasionally mixed with a few longer and hairier ones, which are all that now remain of the once broad and feathery down. Among the true thistles, on the other hand, which trust entirely to the gentle summer breezes for dispersal, and which float away often for miles together, innumerable gradations of featheriness exist, some species having the down composed of long, straight, undivided hairs; while in others of a more advanced type it consists of regular feathered blades, barbed on either side with the most delicate beauty. Almost all our commonest and most troublesome English thistles belong to this last-named very feathery type, whose seeds are, of course, enabled to float about on the wind far more readily and to greater distances than the simple-haired varieties.
The thistle pedigree is a long and curious one. The group forms, apparently, the central and most primitive existing tribe of the composite family, and it bears in its own features the visible marks of a vast previous evolutionary history. Starting apparently from blossoms with five distinct and separate yellow petals, like the buttercups, the ancestors of thistlehood gradually progressed, as it seems, by insect selection, to a condition something like that of the harebell or the Canterbury bell, in which the petals have coalesced at their bases into a single large and united tube. Clustering together next into closely serried heads, like those of the scabious, the rampions, and the common blue sheepsbit, they endeavored to make up for the individual minuteness of their dwarfed flowers by the number and mass collected in a group on the summit of each stem. In this way they gradually assumed the distinctive crowded composite form, each floret consisting of a tubular five-lobed corolla, a calyx reduced to hairs or down, and single tiny seed-like fruit. Of this stage in the development of the family, the simpler and less specialized members of the thistle group, such as the unarmed saw-worts and the Alpine saussurea, are now the best surviving representatives. From some such early central form, the evolving composites split up and diversified themselves into all their astonishing and almost incredible existing variety. Some of them, varying but little in minor details from the parent stock, acquired prickly leaves and grew into the thistle kind, or developed hooked and sticky involucres, and were known as burdocks. Others, producing at their edge a row of brilliantly colored and attractive florets, which serve the purpose of petals for the compound head, branched off into all the marvelous wealth of daisies, asters, sunflowers, marigolds, dahlias, golden-rods, ox-eyes, and cinerarias. In yet others the whole mass of the florets, central as well as external, has assumed this ray-like or strap-like form; and to this group belong the dandelions, hawk-weeds, salsifies, lettuces, sow-thistles, chiccories, nippleworts, and cat's-ears. By far the most successful of all flowering plants, the composites have taken possession in one form or another of the whole world; and among the entire wealth of their extraordinary diversity there is no group more universally fortunate than the common thistle. What from the purely agricultural point of view we describe as a very persistent and almost ineradicable weed, from the higher biological point of view we should more properly regard as a dominant and admirably adapted species of plant. The one conception is merely narrow, practical, and human; the other is positive, philosophical, and universal.—Longman's Magazine.