Popular Science Monthly/Volume 42/November 1892/Color in Flowering Plants
|COLOR IN FLOWERING PLANTS.|
By ALICE CARTER.
COLOR is as omnipresent as light. Life, the greatest of artists, uses the most common materials to produce masterpieces which sunset clouds can not surpass. The possibilities of almost infinite color variation are present in every green plant, even in its roots and stems. Appropriate conditions only are needed to bring them out; only power to help in the plant economy can intensify and make them hereditary and permanent. There is little doubt that by careful selection leaves would become as wonderfully variegated as flowers. Indeed, this has been done: some of our cultivated maples—masters of chiaroscuro—"are positively rainbow-dyed." Bright-leaved birches, beeches, begonias, and foliage plants are continually improving under man's directive care. These do not appear under natural conditions in our climate, probably because they are of little or no use. Still, there are glorious tree-paintings in our autumn woods. The red of the dogwood, the yellow of the tulip tree, the brilliant purple of the sweet-gum tree might doubtless be enhanced and modified and made to appear at different seasons by intelligent selection and change of conditions; and travelers tell us that in the tropics, where the struggle for existence is most severe, bright-leaved plants are common. Why? What are the uses of color? Since the publication of the works of Sprengel, Müller, Delpino, Hildebrand, and others, its attractive power has been so enthusiastically studied that the thought of its having other meanings has been largely lost sight of.
Flowers are the hope of a plant; their careful protection from injury is of vital importance. They are, as a rule, so short-lived that there must be special adaptation for the speedy fulfillment of their function. On the other hand, the value of crossing is so great that many of them have become partly dependent for its accomplishment upon the aid of other organisms. But the very provision made for the entertainment and attraction of these friends also serves excellently the wants of numerous intruders who would take the treasure without giving any equivalent for its use. Therefore it is that among flowers (and fruits, to which somewhat similar reasoning applies) we find the most marvelous combinations of attractive and protective qualities, and, as with animals, color is an important element in each. The very beauty which is the Elysian field of some happy insect may be the Gehenna of another. The essentials of a heaven are as varied as individual tastes. Hence it is impossible to limit a given color to one function. Use is many-sided, and while the attractive power must still be emphasized, others should not be forgotten. Color, like everything else, is always increasingly developed in proportion to its usefulness, and accordingly most common and most protean among flowers and fruits, although by no means confined to them.
I. Chemical Uses.—It has sometimes merely chemical uses.
The work of (green color) chlorophyl, the study of which is a science in itself, need not be dwelt on here.
According to Pick, red color, too, has an important chemical office, inasmuch as in its presence the food substances manufactured by young, growing shoots (in which it frequently occurs) are transported more rapidly, so making possible the desired quick development of each organ, while those which are made last do not remain in the autumn leaf to be lost with its fall, but are speedily conducted to the winter storehouses.
Photography shows that color changes the properties of light. Stewart suggests that every flower by means of its color may transform the sun's rays in accordance with its own needs.
II. Protective Color.—The knight of the fairy tale who started on his quest with shoes of swiftness, charmed sword, and invisible cloak, may well have been one of Nature's models. Many a time since, the invisible cloak has given the victory to her heroes. In the hand-to-hand battle of life, which is continually waging, many an animal escapes unscathed simply by being so like the objects among which it lives that only the keenest sight can distinguish between them. This is "protective coloring." Walking-sticks resemble twigs; alligators, floating logs; brown bitterns, the rocks among which they stand motionless watching for prey; gay birds-of-paradise are almost invisible among the branches of brilliantly blossomed trees. But the phenomenon is not confined to animals.
A remarkable thing about fruits is the great difference in color between the dry and fleshy kinds. It is hard to think of a conspicuous dry fruit in all our flora, yet they are far the most numerous; for, of the eight hundred and eighty-nine genera of flowering plants given in Gray's Manual, eight hundred and nine have dry fruits. But many of the eighty fleshy-fruited genera are brightly colored: ten have white, eight yellow, eight yellowish, thirteen blue, twenty-three (usually shining) black, sixteen purple, twenty-five red species. Only four have no other color than green.
Surely this is a significant contrast. There is no intrinsic reason why a nut-shell should not be as brightly tinted as a peach skin, but in the light of modern theories of distribution the problem is simplified. It is now known that dry fruits are disseminated by purely mechanical means, by the agencies of wind and water, or by the unconscious help of animals to whose hair, feathers, or feet they adhere. But fleshy fruits are largely, often entirely, dependent upon animals which eat the attractive and palatable covering, and in one way or another scatter the uninjured seeds. As there are wind, water, and insect pollinated flowers, so there are wind, water, and animal carried fruits; and the first two classes of both are inconspicuous, the third commonly beautifully adorned. The negative reason, then, for the absence of color among dry fruits is the needlessness of attractive characters; but there is a positive and perhaps as powerful a cause which has operated to the same end. Dry fruits are by no means unpalatable. The staple vegetable foods, sought after alike by the lower animals and by man, are grains, legumes, and nuts. In their great popularity is their great danger; their treasure must be hidden. The seeds of many Compositæ, greedily eaten by birds, are therefore covered; the distasteful or even poisonous fruits of the parsley family freely exposed. The hard shells of nuts, the hairy or bristly coverings of many pods, defeat the attacks of numerous foes. But the charmed bodies of Siegfried and Achilles did not save them. There never yet was armor without vulnerable point. Nature "arms and equips an organism to find its place and living in the world, and at the same time she arms and equips another to destroy it." Squirrels deftly open the hardest hickory nuts; larvæ penetrate the thickest-shelled almonds. Wallace describes how cleverly the black cockatoo of the Aru Islands breaks into the kanary nut, which is so hard that only a heavy hammer will crack it.
Every little helps, and the absence of the straw may save the camel. Therefore, if in addition to their hard coverings these fruits are also colored so as not to be easily seen, they have still another advantage. Mr. Bailey describes the pods of the sensitive pea as "protectively imitative," much resembling the leaves of the same plant when closed after irritation. Wallace thinks that the dry fruits of herbs "have no doubt often been prevented from acquiring bright colors by natural selection, in order to protect their seeds." And it seems logical that the same purpose may be served by the sober colors of the larger dry fruits.
Fuller describes a kind of figwort as a possible instance of a protectively colored flower. The blossoms are inconspicuous and purplish; the ripening ovary develops a dark purple tint, and somewhat resembles the fallen corolla; the buds, too, are as deeply colored as the opened blossoms, an unusual thing. Since this species is adapted to pollination by wasps, and avoided by other insects which have a respectful awe of their formidable sting, the miniature flowers and the fruits seem to be well protected by resembling the blossoms—an economical method to say the least, and so effective that the wasps themselves, sometimes deceived, were seen to alight on the buds and ovaries.
III. Warning Color.—In the forests of Nicaragua there is a brilliant red and blue frog, which is scornfully rejected by the birds who usually devour frogs and lizards greedily. All the other batrachians are protectively colored and feed only at night; but these little fellows hop boldly around with no thought of fear. A certain hermit crab is found always in shells which are covered with a (usually) bright-colored sponge or sea-anemone, both of which are avoided by the crab's enemies. In such cases striking color is correlated with obnoxious chemical qualities, and the enemies understand it.
It would be decidedly advantageous to plants which are not otherwise fitted to survive the attacks of herbivorous animals to accumulate substances offensive to them and then to advertise their disagreeable qualities as clearly as possible, either by peculiar form or color or odor; and what we are constantly learning of the sagacity of animals leaves little room for doubt that they would quickly recognize these species and shun them.
Kerner and Stahl have proved that tannin is distasteful to snails, goats, rabbits, sheep, and cattle. It is therefore not strange that natural selection has brought about the accumulation of this disagreeable substance in organs and at stages of growth needing special protection; and, in fact, it is most abundant in young leaves, flowers, and unripe fruits.
Wigand has shown that red color often accompanies tannin, and it is also true that red-leaved plants commonly contain oxalic acid. Pick suggests that the union of elements of the oxalic acid and of the red coloring matter, resulting in the formation of crystals of calcium oxalate, prevents the formation of such an excess of the acid as would prevent the action of the ferments by which starch is made diffusible. Raphides are bundles of calcium oxalate crystals. Stahl says that the single-pointed crystals which exist in certain irises are an effectual protection against snails, and that in the pickerel weed there are, besides these, raphides and cells rich in tannin—perhaps a suggestion that raphides may have arisen as the further development by natural selection of such solitary crystals. For these delicate, sharp rods are a formidable defense, piercing the skin of a would-be destroyer of the tissues containing them like so many needles. (Taste Jackin-the-pulpit leaves.)
So there is law in it all. Tannin abounds in plant-tissues. Where it is, red color often appears; and where there is red color, oxalic acid is frequently found; where oxalic acid is, raphides may be formed; and, finally, where there is tannin or raphides or both, there are substances generally disliked by herbivora—a long story, which the red color of the exposed parts of many plants doubtless tells briefly but effectively to their enemies.
For example, various members of the orpine family are not eaten by large animals, because their leaf-tips—the most available parts—contain tannin, "as shown by their dark-red color." Otto Kunze says that the Javanese surround their coffee plantations with a living hedge of red-leaved plants, so keeping off the swine, which abhor this color. The brightly tinted leaves of young oaks, maples, etc., are seldom eaten; and in the tropics, where there is the severest struggle for existence, gay leaves are most abundant.
The mottled leaves of arum, lady's-thumb, some everlastings, and prince's pine, and probably of adder's tongue and cyclamen, are protected by raphides; those of begonia have sour, and of coleus and wild ginger, bitter sap. Such variegations of leaf-surface, which may be imperceptible to the larger animals, may have much significance to smaller ones. Species of caterpillars and of beetles are often confined to particular orders or even genera of plants, presumably because, like the cockatoo to the kanary nut, they have adapted themselves to the peculiar characters of these plants, while kept away from others by effective defenses. (Halicta feeds on the deadly nightshade, which is poisonous to most animals.) I have no doubt that the butterflies, whose wonderful instinct leads them to deposit their eggs upon the plants best adapted to nourish their young, are aided in distinguishing between species and often warned off of the wrong ones by such spots and markings; for they well know how to put two and two together.
The coloring matters of plants are closely connected chemically with the aromatic group of substances (Vines). Naturally, then, odor sometimes takes the place and does the work of color. The presence of distasteful oils, resins, etc., is announced by the odor of some plants no less effectively than that of obnoxious tannin, stinging raphides, or bitter alkaloids, by the colors of others. The most brilliant flowers are not the sweetest; both qualities are not ordinarily needed (though they may exist together, as in the hard-fighting plants of the Mediterranean region); 14·6 per cent of the white flowers are odorous, only 3·2 per cent of the red. And odor, like color, may be at the same time attractive and repellent—a phenomenon probably much more common than we imagine.
Ten of the thirty species mentioned by Kerner as not eaten by herbivorous mammals are either aromatic or strong-scented. Pasture mints and field onions are avoided by cattle. What hungriest mammal would relish a meal of the skunk-cabbage or the spotted arum? Yet flies are attracted to both. If the offensive odor which some beetles and caterpillars emit when handled is a warning, surely that of these plants says more plainly than words, "Nemo me impune lacessit." Herbivora seem even to be repelled by the sweet fragrance of wintergreen, lily of the valley, violets, and of some orchids, which attracts to them their insect friends.
Again, the same animal may be simultaneously attracted and warned by a color. Dr. Ogle found the white and blue varieties of a species of monk's-hood growing in the same district of Switzerland. Almost every opened corolla of the white variety was perforated; none of the blue. The flowers are dependent for pollination upon bees; the perforated white variety therefore produced no seed, and this form was rare; but the blue, entered legitimately and abundantly pollinated, common. So the importance of the warning is evident. The blue corolla invites guests to the feast of honey, but at the same time proclaims that it is for their interest to get it lawfully. The bees understand that unpleasant properties of some kind are associated with the color of this form.
Protective organs or substances are frequently increased in the vicinity of flowers. Long ago Erasmus Darwin wrote: "The flowers or petals of plants are perhaps, in general, more acrid than their leaves; hence they are much seldomer eaten by insects." "Many caterpillars will rather die than eat the flowers of the plant whose leaves are their special food." But insects are not the only foes who steer clear of petals. Kerner gives a long list of plants whose leaves are eaten by herbivora, the flowers untouched. In our own land the blossoms of the waysides and fields—May-weed, buttercups, daisies, dandelions, sorrel, wild carrot, etc.—by their very abundance witness to their immunity from the attacks of grazing animals. The survival of many showy flowers in St. Helena, notwithstanding the introduction of goats, which have destroyed the luxuriant forests, may perhaps be due to the beauty which brands them unpalatable.
The conspicuousness of all of these species is the noteworthy point. They are landmarks, doubtless, to the lower animals as to us. That blossoms are most completely shunned when they are large and showy is almost axiomatic. If bright flowers tasted well, they would be speedily annihilated. The majority of good fodder plants have insignificant flowers.
There is still another class of enemies which may be prevented from attacking flowers by the disagreeable chemical properties of their conspicuous petals. These are the birds. Mr. Brockhurst writes that in the dry summer of 1880 the sparrows, seeking pollen, destroyed his crocuses, preferring the yellows to the purples and whites. They also attacked the primroses, devouring hundreds of them in one morning. Orioles have been seen to bite through the corollas of the trumpet-creeper and golden currant. Young seeds and soft petal tissues would certainly seem to be dainty bird-fare, and would surely be more often so used were they not chemically protected from such injury. As things are, however, other food seems to be preferred when it can be found. In addition to the need of defense against rain, and of adaptation to the form and size of the chosen visitors, necessity of keeping the pollen, nectar, and ovules from destruction by birds may have helped in the formation of tubular and palated corollas. Flowers need the beauty of Helen to attract lovers, the guile of Penelope to discriminate between the true and the false; to provide for the one, and against the others.
IV. Mimicry.—Mr. Bates found in South America and Mr. Wallace in the Malay Archipelago several genera of very abundant, brilliant butterflies which birds refused to eat; and, accompanying these, a few individuals of rare species, themselves defenseless and palatable, differing widely from the type of the genera to which they belong, and so closely resembling the others as to be readily mistaken for them even by experienced collectors. Since its discovery, numerous examples of this so-called "mimicry" have been brought to light in many classes of animals, its conditions being always the same. A rare, helpless species is protected from attack by similarity, in external appearance only, to a common, easily recognizable, well-defended one.
There are strong resemblances between widely separated genera of plants. The submerged parts of water-crowfoots are much like those of water-marigolds. "It is almost impossible to distinguish between" the euphorbias of Africa and the cacti of South America, when not in blossom; mare's-tail looks like an equisetum—one is a flowering, the other a flowerless plant; the false goat's-beard closely resembles the true; dalibarda "in aspect and foliage resembles a stemless violet." Observation may find proof of some advantage gained to the feebler plant by the likeness in the last two cases, inasmuch as the plants occupy the same localities, and in each case one of the species is much more limited in distribution than the other. But none of the others are examples of true mimicry, because the similar plants do not inhabit the same regions, and it is hardly supposable that any benefit accrues from the likeness. Similarity of conditions may have much to do with it in some instances, but a deeper cause, and one of a kind which we can not yet conceive, must be sought in explanation of the extraordinary results sometimes reached.
Nevertheless, there is true mimicry in plants.
The only South African balsam is strikingly like an orchid which grows in the same locality and is visited by the same insects! Surely a clear case of plagiarism.
The "cow-wheat" is parasitic upon the roots of wheat, whose seed its own so exactly resembles that the two can only be distinguished by careful botanical examination. So the husbandman who himself sows tares among the wheat, one day wakes to say, "An enemy hath done this."
Sir John Lubbock thinks that the harmless dead-nettle may be protected from grazing cattle by its great likeness to the stinging nettle—a member of a widely different order.
In 1833 Robert Brown conjectured that the remarkable insect-like forms of the flowers of the genus Ophrys (bee orchid, etc.) "are intended to deter, not to attract insects." But Darwin has shown that some of the species are self-sterile, and all of them are constructed as though insects had played an important part in the shaping of the floral organs. The native home of the genus is the Mediterranean region, where all kinds of methods of defense are resorted to. Most insect-like orchids resemble brilliant butterflies, which, are as a rule unpalatable, and therefore enjoy comparative immunity from attack. May not the flowers be protected by the resemblance, as the defenseless butterflies discovered by Wallace are? Some such reason as this makes it more easy to comprehend the need of such elaborate development for which adaptation to small flies or bees, as in the case of lady-slippers, is hardly a satisfactory explanation. Nature is too economical to spend so lavishly for the accomplishment of what has been done much more simply in other ways. The desirability of combined attraction and repulsion brings with it the need of many new wiles. If "all things are fair in love and war," much may be expected of a man or flower engaged at the same time in both pursuits.
Another orchid (Pogonia ophioglossoides) is very difficult to find because of its great likeness to the much more abundant Indian cucumber which lives in the same places. In company with two botanists and a gardener well versed in the ways of the woods, I have spent hours in finding half a dozen specimens. Mr. Gibson, too, met with the same difficulty, actually treading the orchid under foot, "the imitative whorled foliage of the medeolas having beguiled my discrimination." Surely, though, it is the pogonia which is the imitator. It is the rare form, fulfilling all the conditions of mimicry. The two plants dwell together. The rare one differs from its allies; there is no other pogonia, and, indeed, no other orchid of our flora which has its leaves whorled on the stem. The whole appearance of the plant is decidedly non-orchidean, and, so far as pressed specimens show, the flower continues the imitation, for the greatly elongated sepals and three-parted corolla—all green—have decidedly the semblance of the second whorl of leaves always found on the flowering stems of the medeola.
V. Alluring Color.—Color is sometimes a trap.
There is a singular class of beings, half animal, half plant, in their ways of living—a fascinating, uncanny sisterhood—the insectivorous plants, which display marvelous ingenuity in the entrapping of their victims.
The bladder-wort, which abounds in stagnant ditches or ponds, is a member of this class, which has no apparent attractive powers. Yet Darwin says that one species has wonderfully constructed bladders, curiously like an entomostracan crustacean, and, strangely enough, these are the very animals most frequently killed by them.
Pitcher-plants excited the interest of scientists and travelers over a century ago, but the meaning and mechanism of these "plant-saloons" was only discovered within comparatively recent times. Glandular hairs lead up the margin of the pitcher, and through its mouth to a field of such sugar-plums as grow everywhere in fairyland. Flies, ants, and sometimes moths, follow the baited path to feed on the sweets. But getting out is not so easy as going in. Some, reaching the limit of the sugar grove, slip on the glassy surface which is below it; others, satiated with the honey, try to fly away, but are dashed against the opposing lid of the pitfall, fall back into the tube again, and at last they, too, slip from the middle walls to be drained and more or less completely digested by the fluid secreted from myriads of hairs at its base.
The lure of the allied genus, Darlingtonia, is still more perfect. The singular, orange-red, fleshy, two-lobed organ which hangs over the pitcher's mouth much resembles the flower of the same plant, so that visitors which normally pollinate that may be betrayed by the double deceit. This is curiously like Stewart's description of an Asiatic lizard whose body is protectively colored like the sand on which it lives, but at each angle of the mouth a fold is produced into a shape "exactly resembling a little red flower which grows in the sand." Insects approach and are captured.
The elongated, hollow leaf-tips of Nepenthes have the same general purple coloration. The shade of many an insect which has perished in such a drunkard's grave emphasizes the "touch-not, taste-not" law—"he that is careless in his ways shall die."
The sparkling glands of the sun-dew, pinguicula, and Venus's fly-trap are scattered over the flat leaf-blades. A Portuguese genus is called by the villagers "the fly-catcher," and hung up in their cottages as such. A single plant of martynia, about three feet high and as many in diameter, caught seven thousand two hundred small flies. The abundant hairs secrete an exceedingly viscid fluid, whose unpleasant odor comes to the help of color. The disagreeable smell of Arum crinitum also draws many small flies to its spathes, from which escape is made difficult by the sticky downward-pointing hairs of their inner surfaces. Some of the visitors, unable to make their way out, die and are apparently digested. Others crawl up the spadix and fly away to deposit pollen on the stigmas at the base of the next spathe which they enter and in which they will probably die in their turn. So, as perhaps in Darlingtonia, one insect serves two important purposes. "Thrift, thrift, Horatio!" The priests themselves furnish forth the meats of the marriage-tables.
The prevailing colors of the attractive parts of all these plants, with the single exception of the bladder-wort, are the same that we shall find again in fly-pollinated flowers. The lurid red, purple, or pink of the pitcher-plants, sun-dew, etc., recall the blossoms of Dutchman's pipe, Bryophyllum, and Cirrhopetalum, and the spathes of the skunk-cabbage; the glittering, dew-like drops of butterwort and the unicorn plant occur again in the false flower-glands of Parnassia and the deceptive, sparkling ovary of Paris; and that disagreeable odor is a common characteristic in both classes strengthens the belief that in both carrion-loving flies (or beetles) are the objective points of the attraction, with this unessential difference that in the one class the plant feeds the flies, in the other the flies feed the plant.
VI. Attractive Color.—Leaving aside the negative evidence derivable from cleistogamy and the existence of only inconspicuous flowers in places where insects and flower-frequenting birds are absent, it remains to prove that attractive qualities actually have reference to the visits of animals, thereby establishing their usefulness—i. e., their eligibility as characters upon which natural selection may work.
Where there are bright flowers there are color-minded animals. All orders of insects are represented to an altitude of 2,300 metres, butterflies, flies, and certain bees even to 4,600 metres. The number and kind of insects are in close relation to the number and kind of flowers and their hours of waking and sleeping. Climate affects the color of all parts of a plant. Cereal grains are said to be brighter in the North. Fruits are invariably so. Many travelers have observed the intensity in the color of Alpine flowers up to certain limits of temperature. But if the brighter color is useless it will not be retained after a few generations, as the inconspicuous character of the flora of insect-poor Greenland shows; if serviceable, it will be not only preserved, but deepened as time goes on. The absolute number of flowering plants decreases with increase of latitude or altitude. As men flock to cities until the average compensation becomes equal to less than what they can obtain in the country, so all insects would stay on the plains or in the tropics until their number, becoming disproportioned to that of the flowers, better rewards can be obtained in less crowded regions. To this interdependence of insects and plants, and to the constancy of the numerical relations between the two, inherited intensity of color must be largely due. If the insects are greatly in excess of the attractive flowers, inconspicuous and conspicuous blossoms would be searched and fertilized alike in the resulting scarcity of food. If the number of flowers is much larger than that of the insects supported by them; the latter, becoming fastidious, frequent only the brightest or more fragrant, neglecting the others, which accordingly remain inconspicuous and self-fertile. When both insects and flowers are scarce, the former will remain only so long as the greater attractiveness of the latter makes it as easy to obtain the same amount of honey as could be obtained in the same time from the flora of richer regions where competition lessens the rewards of labor.
It is, therefore, necessary that the insect-dependent plants of colder places should have special attractions, and they do. Observations prove that one or more of the three qualities—color, nectar, and fragrance—which attract visitors are naturally increased in Alpine and northern plants, and it is not strange that some insects have been persuaded to leave all and follow these into their colder homes.
McLeod in the Pyrenees, Delpino in Spitzbergen and Nova Zembla, Müller in the Alps, and Verhoeff on the island of Norderney, all concluded that, in correlation with a scarcity of certain classes of insects, the flowers are either more conspicuous, or there is a noticeable increase in number of wind or self fertilized forms.
That climate alone can not account for the lack of beautiful flowers in countries where flower-visitors are rare, is the more evident in the comparison of the weedy flora of Tahiti with the rich one of the neighboring Sandwich Islands, or with that of Juan Fernandez, in both of which honey-sucking birds abound.
Again, there is proof of the actual preference of the different groups of insects and birds for particular kinds and colors of flowers.
The richest and gayest flowers of the world are those of temperate Australia, South Africa, the south European Alps, and South America. Honey-suckers abound in the first (are found nowhere else in the world); most of the sun-birds of the world are found in the second; humming-birds are almost exclusively confined to the last; butterflies and bees characterize the third.
Large, bright-colored, scentless flowers seem to be the favorites of birds and butterflies.
Riley says that "white moths are naturally attracted to white flowers." The difference in color between flowers visited by night-flying moths and by butterflies is very instructive, showing that something more than absence of light has led to the general colorlessness of evening blossoms (compare the day and night-flowering species of lily, etc.), many of which are fragrant, keeping "their sweetness to themselves all day," to "let the delicious secret out" under cover of darkness. So fragrance does the work of the honey-guides which are invariably lacking in evening flowers. Since many of these remain open only a short time—one, two, or three nights—it is the more important that they be easily found by the keen sighted and scented friends, to whom fragrance is as sure a guide as color.
But of all insects, the females of the social bees take the leadership in horticulture. They are the most useful and industrious of flower-guests, because they provide not only for their own needs, but also for those of their numerous progeny. Many of them are wise systematists, as Aristotle noted long ago, who wrote, "A bee on any one expedition does not pass from one kind of plant to another, but confines itself to a single species and does not change until it has first returned to the hive." Color must help them much; but since they visit a great variety of flowers, it is seemingly most useful as a means of distinguishing intermixed species, one color, in itself, being perhaps little more attractive than another.
Kerner, Kronfield, Forbes, and Della Torre have seen bumblebees fly for hours from one flower to another of the same kind, ignoring other species which grew mixed with them. To such persistent and intelligent industry our field and meadow flowers, at least in part, owe their endless variety of shape and color, and as long as bees live there will be fresh modifications for us to wonder at. The "soft sun-brush" directed by the exquisite taste of these little connoisseurs of true art is continually producing new chefs-d'œuvre.
Nor is the work of flies to be despised. Some of them are almost as enterprising and have apparently as keen a sense of beauty as many bees and butterflies. Müller speaks of the largest and most handsome of the saxifrages as the "masterpiece of the Syrphidæ" (the most highly developed of flies). Fly-flowers have often dark-red color and nectar so scanty that it does not pay the bees to take it—e. g., bryophyllum. In New Zealand flies largely take the place of bees, which are there exceedingly scarce. Some species are exclusively dependent on them. Lurid, snaking spots or markings or disgusting odors often tell the secret of fly-attraction (arum, Dutchman's pipe, skunk-cabbage, smilax).
But some flowers, neither showy nor fragrant, are yet abundantly visited both by bees and flies (bryonia, bur-cucumber, etc.). Kunth found that the greenish petals of some of these plants affect a photographic plate as strongly as those of white, violet, or blue flowers. It is not at all unthinkable that the wonderful eyes of insects may be sensitive to colors invisible to our coarser sense. Kunth adds that the glands of these plants may perhaps contain ethereal oils noticeable to insects though imperceptible to man.
Surely, in the face of all these facts, it can not be denied that there is some relation between the conspicuousness or fragrance of flowers and their pollinization and pollinizers, especially since it is possible in the various insect groups to trace a connection between the two, and since, in the absence of animals of one kind, others have sometimes been delegated to do the work, the botanical character of the region changing correspondingly; as, for instance, in the fly-flora of New Zealand and the bird-flora of Juan Fernandez. Indeed, M. Sevali claims that grasshoppers fertilize the Leguminosæ of New Zealand, and Delpino thinks that Rhodea, which is self-sterile, depends for the production of seeds upon snails!
In order that the selection of insects may cause change in the characteristics of flowers, two things are necessary on their part. Their visits must be methodic—we have seen that they are so for the most highly specialized groups. They must be frequent. Any one who has followed a "busy bee" for half a day will be ready to witness that they are. A bumble-bee in mid-Sweden was seen to suck honey from the monk's-hood at the rate of from 960 to 1,200 visits an hour; a butterfly visited 194 violets in six minutes and three quarters; 2,155 bees were actually counted on a single head of the "honey plant" between 5 a. m. and 7 p. m., the thirty heads of one plant furnishing supplies for over 64,000 bees in one day. Some one has calculated that 2,500,000 visits are made to the red clover for every pound of honey. The United States, by the census report of 1880, produced 25,743,208 pounds, representing, therefore, 64,358,020,000,000 visits of hive-bees alone in this country, and this includes only that used for economic purposes, not at all that kept by the bees themselves. Add to these visits those of the wild bees, bumble-bees, flies, butterflies, birds—which are by no means indiscriminate—and surely here is a force whose selective influence must be enormous, which might easily bring about a comparatively rapid evolution. May not this shed some light on the mystery of the rapid floral development of Phanerogams after their initiation in geologic ages? Gardeners and farmers have, in relatively short time, been able to introduce and establish new forms of flowers, fruits, and grains, but the results of the industry of this vast army of workers must have been inconceivably larger.
This brings us to the other side of the question. Something is necessary on the part of the flowers, which must themselves be capable of great variation in color and form in order that selection may have material to choose from. Every one knows that they are so. As the latest production of the vegetable world they are the most plastic, the most easily influenced by alterations of the environment. (Old traits are not easily changed.) Many of them in their individual development pass through a series of changes which sometimes, perhaps, represents the color phylogeny of the species—e. g., several species of honeysuckle are white the first day, become yellow the second, wither the third. A certain hibiscus is white in the morning, rose-color during mid-day, and red in the evening, repeating these transformations each day as long as the flower lasts. Such changes are supposed to be merely chemical, the oxygen accumulated by night being used up by day.
But the most quick-witted insects would notice them, and we find that some plants have adopted them as dialects of the universal "flower-language." Fritz Müller describes a Brazilian lantana whose flowers last three days, and are yellow the first, red the second, purple the third. A few butterflies stick their tongues into the yellow and red, others visit only the red, none the purple. If the flowers fell the first day, the inflorescences would be less conspicuous; if the butterflies did not appreciate the change, they would lose time in searching for honey from old, functionless nectaries. What may be a purely chemical phenomenon in the hibiscus has become a constant and useful character. (Even more remarkable is the case described by Hildebrand of Eremurus, whose flowers open before the reproductive organs are mature. After the corolla is withered, stamens, stigma, and nectaries become fully developed, so the less intelligent insects, decoyed by the bright young flowers and finding there no honey, leave the inflorescence to the friends who love it because "it has opened its heart" to them.)
Tannin, which causes the disagreeable taste of many petals, is peculiarly abundant in cells which exhibit irritability—i. e., it is easily affected by outside agencies. Petals are delicate organs, and whatever irritable substance is in them is therefore the more easily stimulated. Add this to the fact of their youth, and it is natural that flowers should be pre-eminently variable, and altogether possible that certain stages may be seized upon and made hereditary by the selection of the innumerable army of hungry insects.
So, again, there is law all the way through. The operation of entirely natural and conceivable causes leads to the permanent establishment and combination of colors, odors, and forms which may be protective, repulsive, imitative, attractive, or unite several of these functions. And in the "continuous adjustment of internal to external conditions" is the evolution from alga to rose, which shall by no means stop with the rose. The song of the flowers is clear and true:
A power, more strong in beauty, born of us
And fated to excel us, as we pass
In glory that old darkness;
—For, 'tis the eternal law,
- The disagreeable substances are volatile, disappearing when the flowers are dried