Popular Science Monthly/Volume 32/December 1887/American Cinque-Foils
By GRANT ALLEN.
YOUR American cinque-foils are to me a deeply interesting set of plants. Excuse, I beg of you, dear Mr. Reader, this abrupt beginning. I love a causerie: I love to button-bole my audience, as it were, and, sitting down with it mentally on a bowlder in the meadow, to discuss the matter in hand with it tête-à-tête, as if we two were old friends, which I trust, after all, may be really the truth with the public of "The Popular Science Monthly" on the present occasion. For, indeed, a recent visit to America has made me realize you all far better than I ever did before; it has made me feel your individuality as I never hitherto felt it; and it has also renewed with me the acquaintance of many dear old floral favorites whose faces I bad not seen in earnest for many a long and weary year. Among them, the cinque-foils or potentillas are, it is true, but a feeble folk; very different from the glorious orange lilies, and trilliums, and Solomon's-seals, whose bulbs and tubers I have brought home with me to beautify a little out-of-the-way Surrey garden; but still in their own humble fashion most interesting plants, from the implications as to their past history and transformations legibly written by the hand of Nature upon their very faces. I propose, therefore (having got you now fairly button-holed), to discourse somewhat concerning the American potentillas themselves, as well as concerning certain of their near and dear relations not included in the same genus by the artificial and unwise arrangements of our existing botany.
The first potentilla I found in America was by chance the very one that ought naturally to head the tribe in any systematic work, because it is the one which more than any other seems to preserve in the greatest simplicity the original traits of the prime ancestor. And when we consider that from this ancestor are also descended (in all likelihood) the plum, the peach, the cherry, the almond, the apple, the pear, the strawberry, the raspberry, the rose, and the hawthorn, it must immediately be apparent to the meanest understanding that the plant in question deserves the greatest consideration at our hands as the founder of a large and important family. Nevertheless, this rather scrubby weed (Potentilla Norwegica) with its yellow flowers and hairy stem, much resembles the founders of many other distinguished families in being personally mean, sordid, and inconspicuous. But in spite of its meanness, the Norway potentilla shows many signs of its high respectability as the representative of the elder branch of the family in the direct line. To begin with, its blossoms are a shabby yellow; and shabby yellow I take to have been the original color in every instance of the earliest petals of insect-fertilized flowers. Then, again, it is an annual weed, and herbaceous annuals were doubtless the earliest form of all vegetation all the world over. Once more, the leaves are divided into three leaflets; and this type I take from its frequent recurrence not only among the potentillas themselves, but in the strawberries, the lady's-mantle, the simpler brambles, and many other species as well, to have been the original type of foliage for the entire rose family. Finally, certain minute technical characters in the stipules and the styles, with which I need not trouble you at the present moment, lead to the conviction that we have here to deal to some extent with a fair representative of the old ancestral potentilla form.
The Norway potentilla, however, is distinctly weedy—that is to say, it is one of those unpleasant, dusty-looking plants which loiter about on the precincts of the road-sides and in the waste purlieus of human cultivation. It attests its weediness by its bristly hairs, intended doubtless to repel insects and to make it unpalatable to cattle and horses. As its name implies, it is an Old-World form as well as a native-born American citizen; it is, in fact, a member of that ancient circumpolar pre-glacial flora which was driven down from the once mild and genial Arctic regions by the vast ice-sheet of the Glacial epoch to occupy the plain-lands of either hemisphere in these our chilly and degenerate modern summers. In Europe, however, it remains distinctly a more northern type than with you in America, where it spreads as far south as the Virginia hills.
On the Alpine tops of the White Mountains I was lucky enough to light upon another member of the potentilla group, not far removed in essentials from the Norwegian weed, but infinitely prettier, more delicate, and in a word less weedy all round. This is the plant which Asa Gray identifies with our European Potentilla frigida of the Swiss Alps; and I, who have a pious horror of unnecessary splitting and renaming and tinkering, have not the slightest objection to the identification in any way. But it is worth while to notice, what I often observed of almost every American species said to be identical with those of Europe, that the two plants are not absolutely the same: the time that has elapsed since the Great Ice age effectually severed the two continents has sufficed to produce distinct differences in nearly every kind of plant or animal. The flowers in the American specimens are smaller than in the Swiss, and the stems when full-grown are far less hairy.
Potentilla frigida exhibits all the common peculiarities of high Alpine or Arctic plants. It is a dwarf form, not one fifth the size of the Norway species; it is tufted thickly on its low stems, and it has that matted, close, creeping habit which I have already pointed out in this "Monthly" as the distinctive feature of the glacial flora. It sticks still to the three original leaflets, but its flowers, as is common in mountain types, are far larger and handsomer than those of the wayside weed with which we started our examination of the group. This Old-World form, however, occurs nowhere in the United States except here on the topmost summits of the White Mountains, and even there it lingers on in scanty numbers, rapidly diminished by the growing warmth and the incursions of botanists. I took but a tiny spray for my own specimen, from a spot not far from Tuckerman's Ravine, and left the remainder of the plant I found there still growing. It would be a pity if these last survivors of the Glacial epoch, pushed up onto these chilly heights by the secular summer of our own day, should be exterminated by the hands of those who above all others are bound by natural piety to preserve and protect them.
All over Canada and the Northern States there grows a third and very common potentilla, the cinque-foil or "five-finger" of popular botany (P. Canadensis), a pretty, prostrate, creeping weed, with golden yellow flowers springing close to the ground, and five leaflets instead of three to each leaf. Ever since the days of Linnæus this plant has been considered distinct from the common European cinque-foil (P. reptans), and the differences are certainly sufficient to justify their division as separate species, as systematic botany goes nowadays. Nevertheless, it is quite clear that we have here merely to deal with the American descendants of the same old circumpolar plant. No European naturalist who saw the Canadian cinque-foil for the first time would ever take it for a distinct type; if he found it growing in an English meadow, he would certainly pass it by unnoticed as the familiar cinque-foil of our eastern hemisphere. The differences can only be observed when you look closely into the plant, and they are all of easy adaptive character. In fact, we have here just the same tendency as that which we noticed in the mountain species, only carried, perhaps, one step farther. In that instance, the differences were only sufficient for sytematic botanists to rank the plant as a mere variety; in this case they are sufficient to give it the dignity of a distinct species. But at bottom nobody knows what is a variety and what a species, and it is a mere matter of individual judgment whether a particular form should be regarded as one or the other. It varies "according to the taste and fancy of the speller." Oakes considered the White Mountain potentilla a distinct American species, different from the Alpine kind in Europe, and christened it, accordingly, P. Robbinsiama, after the first person who discovered it on these chilly hill-tops. Asa Gray regards it rather as a mere variety, though he hesitates as to whether it comes nearer to the P. frigida of the Alps, or to the dwarf form known as P. minima (itself a very ill-marked species). It is always so when you come to compare the plants or animals over a large area. However distinct they may seem in particular localities, they shade off into one another by such imperceptible degrees at distant points that the task of drawing hard-and-fast lines, so lightly undertaken by the systematic biologist, becomes at last absolutely impossible.
This very Canadian cinque-foil, for example, runs into two extreme forms, which have each been considered by confirmed "splitters" as distinct species. The first (P. sarmentosa, of Muhlenberg) grows for the most part on very dry soil, and like most plants of arid situations runs largely to pronounced hairiness; for it is a general rule that water-haunting kinds are smooth and glabrous, while dry or desert types are intensely hirsute (the reason for this wide distinction, though well known, would carry us too far away, this morning, from our main subject). The second form, erected by Michaux into a separate species (P. simplex) but reduced to subordinate rank as a variety by Torrey and Gray, belongs to moister soil or to deep meadows, where the lush grass prevents evaporation; and this type grows less hairy and greener, and attains a larger and more luxuriant stature. The two forms differ also in other ways, strictly dependent upon their differences of locality. Sarmentosa, the dry type, creeps squat upon the ground, as if to avoid the sun, and sends out long, rooting runners in every direction after the fashion of the strawberry-vine; whereas, simplex, the moister kind, has ascending stems, which rise in competition among the grasses around them, seldom if ever creep, and never produce summer runners. Again, sarmentosa begins to blossom early, and ends early—April to July in the latitude of New York; while simplex comes and stops a month or so later at either end—May to September in the same district. In other words, the dry type flowers early in spring on its basking banks, but retires from the scorching heat of your American summer; while the moist type begins later in its shady habitat, but is less affected by the droughts of August.
Curiously enough, our common European cinque-foil (P. reptans), the exact analogue of your American plant, and fellow-descendant of the self-same pre-glacial ancestor, has also two well-marked forms usually considered as distinct species, but merging into one another by imperceptible gradations. The parent-type (reptans proper) grows in rich pastures or meadows, and answers best to your variety simplex, though it sends out long, creeping stems which root every now and again at the nodes; it has five large petals to each blossom, and the flowers are identical with those of the Canadian cinque-foil. But on open moors, heaths, and dry places, we have a smaller, closer, and more creeping form, the tormentil (P. tormentilla); it is silky-hairy, like your own sarmentosa, and its upper leaves have often only three leaflets instead of five, thus reverting to the ancestral type of foliage, when the plant was rather a tre-foil than a cinque-foil. But oddest of all, the small flowers have only four petals, arranged like a Maltese cross; whereas all their congeners have their full complement of five, in accordance with the old central plan of the entire rose family. Still, the first flower of all on each stem, produced when the plant is in its vigorous youth, has occasionally five petals; a reversionary fact of great interest. The tormentil has also an intermediate variety of its own (Tormentilla reptans of the hair-splitters), which sometimes creeps like the true cinque-foil, and frequently breaks out into five-petaled blossoms. Even Mr. Bentham, that minute and conservative botanist, admits that "intermediate forms" sometimes occur which can not probably be referred to either species.
And yet, though the tormentil and the cinque-foil are thus intimately connected with one another, by imperceptible gradations, so great is the love of petty distinctions in the human breast, that Linnæus actually erected this slight, four-petaled variety, not only into a distinct species, but even into a separate genus (Tormentilla).
Let us return, however, to our immediate subject, the American potentillas. The next species recognized by Asa Gray is the silvery cinque-foil (P. argentea), a pretty little plant, with small, bright-yellow flowers, confined, for the most part, to very dry, barren, or sandy spots, and with thin, wiry, almost woody stems. It is remarkable for the soft, white, silvery down, that clothes the under side of the five-leaved foliage. The use of this down I do not know, though I suspect it to be a protection from some caterpillar or other insect, which attacks leaves on their under surface. At any rate, it is an exaggeration of the usual downiness of dry-soil species. The silvery cinque-foil is common to Europe and America, and I do not notice any perceptible difference between my English and Canadian specimens. It seems, in fact, to be one of the very few plants which have not altered to any recognizable degree on either side of the Atlantic since the end of the great Glacial epoch. As a proof, however, of the narrow way in which this dry-soil species is restricted and limited to the very sandiest or most barren situations, I may mention that it grows on two spots, and two spots only, within reach of my own home here in Surrey, England. Both these spots are knolls of a peculiarly soft and friable sandstone, into which the rain sinks immediately; and they are the only two bits of that particular formation (a subdivision of the Folkestone sands) to be found anywhere in the neighborhood.
I was shown, at Kingston, Canada, a specimen of another more weedy potentilla (P. paradoxa), which has hardly, as yet, made good its place in the Eastern States, but which, nevertheless, possesses a certain interest for naturalists of the Atlantic shore, as a member of the flora by which before long they are almost sure to be overrun. The species belongs to the western half of the continent, but it is already well established as an immigrant along the banks of the Ohio and the Mississippi, and it has been observed near Oneida, and elsewhere on the shores of Lake Ontario. My own specimen was gathered on a common at Kingston, where it seemed to have established itself in full vigor. Now the interest of this species centers in the fact that until lately the weeds of the Eastern States and Canada were almost entirely of European origin; they were the cosmopolitan pests of civilization, which have followed agriculture from Western Asia along the Mediterranean to the shores of the Atlantic, and, crossing the ocean with seed-corn and fodder crops, have clogged the steps of the intrusive white man through all his colonies and settlements elsewhere. These cosmopolitan weeds succeeded in America to the soil once covered by forest-trees, whose indigenous undergrowth could not stand the garish sunlight of the open clearings. But nowadays, the weedier types of the Western prairie-belt are moving eastward, as farms move west; and being accustomed by nature to open plains, they will probably, in many cases, succeed in establishing themselves side by side with the older plagues of the long-suffering farmer. Potentilla paradoxa is one of the first crop of these weedy immigrants, and its appearance already on the shores of Lake Ontario is the signal for its future advance in a formed phalanx against the tilled fields of New York and New England.
This Western immigrant departs widely in one respect from the type of all the potentillas we have yet considered, and that is in the arrangement of its five, seven, or nine leaflets. In the true cinquefoils, and all their like, the leaflets are arranged, as we say, palmately—that is to say, all start together, like the lobes of a horse-chestnut leaf, from one point. In the P. paradoxa they are arranged pinnately—that is to say, they start in opposite pairs or singly, from a common midrib, like the barbs of a feather or the leaflets of a locust-leaf. The same arrangement, a more convenient one for long leaves, reappears in P. Pennsylvanica, which (in spite of the name incorrectly bestowed upon it by Linnæus) is a Northwestern species. But as I have not seen this last-named plant in the living state, and as I do not like to write about what I have only examined in a dried-up herbarium (a bad habit of the old-fashioned, purely structural botanists), I will say no more at present about it.
On the rocky hills of the North and West there occurs in July a rather pretty, half-shrub-like potentilla (P. arguta), which presents several other interesting peculiarities. This plant has brownish, hairy stems, covered with a viscid, clammy exudation, something like that which covers the young branches and buds of the clammy rose acacia (Robinia viscosa). As I observed that insects are often caught in this clammy secretion, exactly as in the case of the common catchflies (Silene noctiflora Virginica, regia, etc.), I have not the least doubt that the potentilla eats and digests the creatures it entraps, in order to supply it with nitrogenous material for its own pollen, ovules, and seeds. This is the more probable, as the clamminess increases near the flower-buds and blossoms, and is scarcely at all noticeable near the base of the stem. How the potentilla digests its food I do not know, but long observation has fully convinced me that whenever a plant has viscid, glandular hairs or secretions upon its penduncles, pedicels, calyx, and flower-buds, it is invariably an insect-catcher, and an insect-eater too. The flowers are the part that require the most nitrogenous food, and near the flowers the nitrogen-catchers are situated.
Another peculiarity of P. arguta lies in its flowers, which are clustered in large and conspicuous masses, and have petals that vary from pale yellow to primrose or almost white. This is a very interesting fact, because the native color of the potentillas is yellow; but the mountain species, and many other kinds, have varied to snow-white blossoms; and here we get a plant, as it were, in the intermediate or undecided stage between the two colors. Notice, too, that P. arguta is an herb of the rocky hill-sides, and therefore half-way toward becoming a mountain species.
Now, on the summit of Mount Willard, just above the Notch of the White Mountains, I found another very beautiful member of this pretty group, the three-toothed cinque-foil (P. tridentata). This is one of your most northerly and mountain-loving potentillas, unknown in Europe, inhabiting the coast of New England from Cape Cod northward, and the mountain-tops of the great chains, from the Alleghanies to the Maine ranges, as well as in Canada, Labrador, and the extreme north of the continent. The three-toothed cinque-foil carries a step farther the same characteristic, for its flowers are pure white, as so often happens with mountain blossoms. Just in the same way, while almost all lowland buttercups are golden yellow, some of the Alpine buttercups are white as milk, and among these very potentillas there are a few lovely snow-white mountain species in Europe and Asia. One beautiful kind that I gathered on the Maritime Alps at Mentone (P. saxifraga) has a blossom as delicately mountainous in type as the saxifrages themselves, from which it takes its scientific name.
Of course, I don't for a moment mean it to be understood that I think P. tridentata is directly derived from P. arguta, or that the latter species is now on its way to merge into the former. My Mount Willard plant has palmate leaves of only three leaflets, while the common P. arguta of the northern hill-sides has pinnate leaves of from three to nine cut-edged divisions; and in many other technical points they differ widely from one another. All I mean to suggest is merely that the yellowish-white P. arguta is now just passing through a stage which the ancestors of P. tridentata must have passed through long ago. On the whole, to put it briefly, the potentillas are a yellow lot; but a few advanced members of the race are white; and still fewer, like the ornamental P. nepalensis and P. atropurpurea of our gardens, are crimson, scarlet, or bright red. So far as I know, no potentilla is ever blue, which is the highest level of floral coloration.
The three-toothed cinque-foil has an almost shrubby and woody root-stock, and displays a tendency to assume the character of a true shrub. But its northern habitat and mountain manners keep it low and tufted, after the common fashion of upland vegetation. There is another of its kind, however (P. fruticosa), which really grows into a regular shrub, with many branches, terminated by large trusses of bright-yellow flowers. Asa Gray says this plant is "common northward" in wet ground, but I was not lucky enough to hit upon it during my visit to America. However, I have seen living specimens from Teesdale in England, and from them I perceive that, in general habit, the plant greatly approaches the rock-roses (Helianthemum), which grow in very similar situations. The leaflets of the shrubby potentilla, long, narrow, and silky beneath, resemble, at first glance, the leaves of the rock-roses, thus showing how similar conditions tend everywhere to produce similar results, even when starting from the most unlike organic forms to begin with.
One other potentilla, the goose-weed or silver-weed (P. anserina), I must needs mention for form's sake, though I have nothing special to say about it. It is a creeping species, growing close to the ground, with long pinnate and prostrate leaves, silvery white below, with silky down. Both in Europe and America it is very common as a road-side weed, and in moist ditches; but with us it is a weedier and scurvier plant than with you—evidently a sufferer from our long civilization. In America it grows mostly by river-banks and in brackish marshes; in Europe, it belongs rather to waste places and stony pastures than to streams or mud-banks. Few temperate plants, however, have a wider distribution. It is a circumpolar weed in both great continents, extending through Russia and Siberia to Alaska and British America, and it reappears once more, under like conditions, in the southern hemisphere. Nothing kills it out, and it will bear both inundation and trampling under foot to a greater degree than any other plant of equal importance.
The handsomest of your American potentillas, however, is the marsh five-finger (P. comarum or palustris), a very bold and elegant waterside plant, bluish-green in stem and leaves, and with loose corymbs of exceedingly pretty though dingy flowers. The calyx, inside, is lurid-red, and the large petals are tinged with a gloomy and peculiar purple. This fine ornamental plant loves cool northern bogs and marshes, being common in Canada and in the Scotch Highlands. But what gives it to me the deepest interest is its exact resemblance in hue and general aspect to a purple avens (Geum rivale), also common to either hemisphere. Both are plants of the cold swamps and peaty places; both depend for fertilization upon water-side insects; both have lurid-reddish calyxes, and both have large and dingy purplish petals. The inference seems to me irresistible that the color has been evolved in both cases by the special tastes of the upland water-creatures to whose aid both owe the impregnation of their ovules. Indeed, it is often easy thus to classify flowers functionally by their color and the tastes of the particular insects that habitually visit them. In Europe, at least, I believe the particular insect in this case to be Rhingia rostrata, which I have observed in great abundance upon both flowers. American naturalists, please verify, or look out for, the corresponding American species.
On the Alpine summits of the White Mountains, and far to the north again in the Labrador region, there grows abundantly a little matted mountain plant, not recognized by the scientific world at large as a potentilla at all, and known by the name of Sibbaldia procumbens. But you may call the plant whatever you like without altering the undeniable fact that it is in all essentials a dwarfed and depressed mountain potentilla, with the flowers so reduced by chilly conditions that very few stamens or carpels remain, and with the usual dense, spreading, tufty habit common to all Alpine vegetation. It is clearly descended from a high hill-side potentilla not unlike the white P. tridentata of Mount Willard aforesaid (only with yellow flowers), for it has the same type of tre-foil leaves, with each leaflet three-toothed at the end, and the same general aspect and habit. Both plants, I do not doubt, are common descendant? of a single antique Arctic ancestor. But little Sibbaldia has grown so very small and degraded in time that its flowers have dwindled away almost to nothing; the green calyx forms its most conspicuous part; the pale-yellowish petals are very tiny, and in many cases are entirely wanting. In the States Sibbaldia is confined to the higher summits of the White Mountains; but in the Scotch Highlands, as in the far north of British America, it often constitutes for miles together the main element of the low and matted mountain greensward.
Last among your American potentillas I may mention the wild strawberries. Though these at first sight seem somewhat different from the rest of the group, I have not the slightest hesitation in saying that to the evolutionary botanist they can not but appear as closely related species of one and the same natural genus. For the strawberries are only potentillas in which the receptacle of the fruit, instead of remaining hard and dry, swells out into a colored and pulpy mass, attractive to birds, who thus aid in dispersing the tiny "achenes" or nutlets (commonly, and for all practical purposes correctly enough, described as seeds). To us in Europe, the essential identity of the two types is made all the more evident, because we happen to possess a little three-leafleted white potentilla (P. fragariastrum) so exactly like a wild-strawberry vine in foliage and. flower that few save botanists or close observers of Nature ever adequately distinguish between them. This white potentilla is, in fact, a strawberry in everything essential except the fruit; and the succulence of the fruit (or rather receptacle) is after all a matter of comparatively little importance except to the men and birds who eat it. I am fully convinced that if the strawberry had not been an edible berry it would always have been classed merely as a potentilla, and considered as very closely analogous to the P. fragariastrum or "barren strawberry" of Northern Europe. It is hardly more, indeed, than a mere variety.
You have in America two slightly divergent forms of the wild strawberry, erected into species by American botanists, for small differences in the appearance of the berry. Had these differences occurred in any other than an edible fruit they would, I am sure, hardly have been noticed: occurring there, they have been suffered to assume a factitious importance in the eyes of systematizers. One of these varieties (Fragaria vesca), which grows in fields and open places, is the common wild strawberry of Europe; but it bears somewhat larger berries with you than with us, and has a somewhat more erect and noble habit. Apparently it is proud of its American citizenship. It is distinguished by having the nutlets merely superficial on the outside of the berry, not sunk in pits, as in the second variety. This last named form (F. Virginiana or F. Canadensis) is peculiar to America, and differs from the European type in the constricted or bottle-shaped neck of the berry, and in the deep depressions for the nutlets, the ribs between which accordingly give the fruit a distinctly pitted or spiny appearance. It is a woodland plant, native to your forests, and far more forestine in aspect and habit than our English vine. In flavor, also, it differs distinctly, and your cultivated Virginia scarlets are its final product in the gardeners' hands. The Western variety (Illinoensis), according to Gray, gives origin to Hovey's seedling, the Boston pine, and many other cultivated strains. No European strawberry can at all equal these native American fruits in delicacy of flavor.
There is a third species of strawberry, undoubtedly distinct, admitted by Gray as a naturalized American, which possesses for me a peculiar interest. This is the Fragaria Indica, or Duchesnea fragarioides, a Himalayan species, established in copses round Philadelphia and at various places in the Southern States. Some years ago a plant of this curious species was sent to me in a box for identification: I set it out, on the off chance of its living, in my garden at Dorking; and it now overruns the whole place, so that I have had abundant opportunities of observing its growth and development to my heart's content. I am certain that F. Indica is not a true strawberry at all; or, in other words, that it is not a common descendant with the other strawberries of any original white-flowered potentilla ancestor, but an independent development of the succulent habit all by itself. It has yellow blossoms, a very different calyx, and a most insipid, pulpy fruit. I have not the slightest doubt that this species has been developed from a yellow Indian potentilla, just as our strawberries have been developed from a white European potentilla, by the unconscious agency of birds in dispersing the nutlets. All that the two plants have in common (beyond their undoubted generic potentilla type) is the mere fact of a succulent receptacle, which might just as easily occur independently in the one case as in the other. If I had to remodel the genus Potentilla on my own account, I would certainly put the common strawberry into the same division as the white-flowered European P. fragarioides while I would put the Indian species into the same division with the yellow-flowered P. friglda of your Mount Washington range.