Popular Science Monthly/Volume 13/September 1878/The Origin of Fruits
|THE ORIGIN OF FRUITS.|
By Professor GRANT ALLEN.
IN the whole museum of Nature the eye of the artist can find nothing lovelier than flowers; but the second rank in beauty may be fairly claimed on behalf of fruits. Whether we look at the golden oranges, the pink-cheeked mangoes, the purple star-apples, and the scarlet capsicums of the south, or at our own crimson cherries, blushing grapes, bright holly-berries, and rosy apples, we are equally struck with the delicacy of their melting tints and the graceful curves of their rounded form. Our painters have reveled in their rich coloring; and even our sculptors, whose fastidious art compels them to reject that meretricious charm, have loved to chisel their swelling contours in snowy stone. As they hang pendent from their native boughs, clustering in brilliant masses, or scattered here and there as points of brighter light amid the dark foliage which throws up in strong relief their exquisite hues, we may recognize in their beauty the ultimate source of all that refined pleasure which mankind derives from the varied shades of earth and sea and sky, of flower and bird and butterfly, and even of the "human face divine" itself. From the contemplation of ruddy or snowy berries in primeval forests the frugivorous ancestors of our race first acquired the taste for brilliant hues, whose final outcome has produced at length our modern picture-galleries and palaces, our flower-gardens and conservatories, our household ornament and our decorative art.
In a previous paper on "The Origin of Flowers," we endeavored to trace the mutual reactions of insects and blossoms upon one another's forms and hues. But we then deferred for a while the consideration of the further question—"Why do human beings admire these bright whorls of colored leaves, whose primitive function consisted in the attraction of bees and butterflies? Through what community of origin or nature does the eye of man find itself agreeably stimulated by the tints which were first developed to suit the myriad facets of primeval insects?" The answer to this question we have now to attempt, by showing the various steps through which the coverings of certain seeds acquired, for the vertebrate orders—the birds and quadrupeds—exactly the same allurements of color, scent, and taste, which flowers had already acquired for the articulate orders—the bees and butterflies. To the attractive hues of fruit, I believe, we must ultimately trace back our whole artistic pleasure in the pure physical stimulation of beautiful colors, displayed by natural objects or artificial products.
Our present inquiry, then, will yield us some account of that primitive delight in red, purple, orange, and yellow, which we usually take for granted as an innate instinct of humanity, savage or civilized. When, some few months back, we analyzed the various elements of pleasure which make up our aesthetic enjoyment of a daisy, we were compelled, for the time being, to leave the original beauty of its pink-and-white rays wholly unexplained. We regarded the delight in color, relatively to the subject we were then examining, as an ultimate and indecomposable factor in our developed consciousness. To-day, however, I hope we shall be able to go a little further back, and to show that this delight, like all other feelings of our nature, is no mere chance and meaningless accident, but the slow result of a long adaptation whereby man has gradually become fitted to the high and responsible station which he now occupies at the head of organic existence.
The sole object of flowering is the production of seeds—that is to say, of embryo plants, destined to replace their parents, and continue the life of their species to future generations. Flowers and seeds go together; every flower producing seed, and every seed springing from a flower. Ferns and other like plants, which have no blossoms, bring forth spores which grow into shapeless little fronds, instead of true seeds containing a young plantlet. But all flowering species produce some kind of genuine fruit, supplied with more or less nutriment for the tender embryo in its earlier days. And this matter of nutriment is so important to a right comprehension of our subject that I venture, even at the imminent peril of appearing dull, to digress a little into the terrible mysteries of Energy, which comprise the whole difficulty of the question.
Wherever movement is taking place in any terrestrial object, the energy which moves it has been directly or indirectly supplied from the sun. In the green parts of plants, the solar rays are perpetually producing a separation of carbon and oxygen, the former element being stored up in the tissues themselves, while the latter is turned loose upon the atmosphere in a free state. Whenever they recombine, motion and heat will result, as we see alike in our grates, our steam-engines, and our own bodies. An animal is a sort of machine—viewed from a purely physical standpoint—in which the energetic materials laid up by plants are being reconverted into the warmth which reveals itself to our touch, and the evident movement which we see in its limbs. The vegetable or animal substances which are capable of yielding these energies to our bodies we know as food or nutriment. They perform exactly the same part in the physical economy of men or beasts as that which fuel performs in the physical economy of the steam-engine. Of course, from the mental point of view, we have the immense difference between a self-conscious, self-guiding organism, and a dead machine requiring to be supplied and regulated by an external consciousness; yet in the fundamental physical necessity for energetic material, either as food or as fuel, both mechanisms follow essentially the self-same mechanical laws.
But what has all this to do with the origin of fruits? Very little at first sight, indeed, yet everything when we look at the bottom of the question. In fact, what is thus true of animals and steam-engines is equally true of plants. No motion can take place in a growing shoot without the aid of solar energy, directly supplied by the sunshine, or indirectly laid by in the older tissues. In the green parts of a plant this energy is immediately derived from the bounteous light which bathes and vivifies the leaves on every side; but in many other portions of the vegetable organism, energies previously accumulated by older organs are perpetually being utilized, for the production of movement and growth, by lazy structures which cannot work for themselves, and so feed upon the useful materials collected for them by more industrious members of the plant-commonwealth. Especially is this the case with those expensive organs which are concerned in perpetuating the species to future generations. A flower or a seed cannot directly transform waves of light into chemical separation of atoms; they depend for their growth and the due performance of their important functions upon similar separations already carried on for their behoof by the green leaves on whose bounty they rely for proper subsistence. Carbon, set free from oxygen in the leaves, has been carried to them in loose combinations by the sap; and as the bud unfolds or the seed germinates, the oxygen once more unites with this carbon (just as it unites in the furnace of the steam-engine, or the recesses of the animal body), and motion is thereby rendered possible. But without such an access of free oxygen to recombine with the energetic materials, the blossom or the embryo could never grow at all. So we may regard these portions of a plant, incapable of self-support, and dependent for their due function upon energetic compounds laid by elsewhere, as the exact analogues of the animal or the steam-engine. They are in fact similar mechanisms, where food is being used up, and fuel is being consumed; and we find accordingly, as we might naturally expect, not only that motion results, but also that heat is evolved in quantities quite sufficient to be measured by very delicate thermometers.
Now, every growing portion of a plant shares, more or less, in this animal function of feeding upon previously-fabricated nutriment. But there are two sets of organs, both intended ultimately to subserve the same purpose, in which that function becomes especially apparent. The first is in the case of the whole regular reproductive mechanism, including in that term buds, flowers, fruits, and seeds; the second is in the case of such subsidiary reproductive devices as tubers, rhizomes, corms, and all the other varieties of underground stems or roots, which botanists divide into so many puzzling technical classes, while ordinary people are content to lump them roughly together as bulbs. If we glance briefly at each of these two cases, we shall be able to comprehend more fully their connection with the doctrine of energy, and also to see more clearly the problem before us when we endeavor to unravel the origin of fruits.
A germinating pea or a young blade of wheat is supplied by its parent with a large stock of nutriment in the shape of starch, albumen, or other common food-stuffs. If we were to burn the wheat instead of planting it, the energy contained in its substance would be given off during the act of combustion as light and heat. If, again, we were to adopt a more usual course, by grinding, baking, and eating it, then the inclosed energy would minister to the warmth of our bodies, and do its little part in enabling us to walk a mile or to lift a heavy weight. But if, in lieu of either plan, we follow the original design of Nature by covering the seed with moist earth, the chemical changes which take place within it, still resulting in heat and motion, produce that special form of movement which we know as germination. New cells form themselves about the feathery head, a little sprout pushes timidly its way through the surrounding soil, and soon a pair of rounded leaves or a spike of pointed blades may be seen spreading a mass of delicate green toward the open sunlight overhead. By the time that all the stored-up nutriment contained in the seed has been thus devoured by the young plantlet, these green surfaces are in a position to assimilate fresh material for themselves, from the air which bathes them on every side, under the energetic influence of the sunbeams that fall each moment on their growing cells. But I need hardly point out the exact analogy which we thus perceive between the earliest action of the young plant and the similar actions of the frugivorous animals which subsist upon the food intended for its use.
If, however, we look at the second great case, that of bulbs and tubers, we shall see the same truth still more clearly displayed. You cannot grow a blade of wheat or a sprouting pea in the dark. The seed will germinate, it is true; but, as soon as the primitive store of nutriment has been used up, it will wither away and die. Naturally enough, when all its original energy is gone, and no new energy is afforded to it from without in the form of sunshine, it cannot miraculously make growth for itself out of nothing. But if you put a hyacinth bulb in a dark cellar, and supply it with a sufficiency of water, it will grow and blossom almost as luxuriantly as in a sunny window. Now, what is the difference between these two cases? Simply this: the wheat-grain or the pea has only nutriment enough supplied it by the parent-plant to carry it over the first few days of its life, until it can shift for itself; while the hyacinth has energetic materials stored up in its capacious bulb to keep it in plenty during all the days of its summer existence. If we plant it in an open spot where it can bask in the bright sunshine, it will produce healthy green leaves, which help it to flower and to carry on its other physiological actions without depending entirely upon its previous accumulations; but if we place it in some dark corner, away from the sun, though its leaves will be blanched and sickly-looking, it will still have sufficient nutriment of its own to support it through the blossoming season without the external aid of fresh sunshine.
Where did this nutriment come from, however? It was stored up, in the case of the seed, by the mother-plant; in the case of the bulb, by the hyacinth itself. The materials produced in the leaves were transferred by the sap into the flower or the stem, and were there laid by in safety till a need arose for their expenditure. All last year—perhaps for many years before—the hyacinth-leaves were busily engaged in assimilating nutritive matter from the air about them, none of which the plant was then permitted to employ in the production of a blossom, but all was prudently treasured up by the gardener's care in the swelling bulb. This year, enough nourishment has been laid by to meet the cost of flowering, and so our hyacinth is enabled to produce, through its own resources, without further aid from the sun, its magnificent head of bright-colored and heavily-scented purple bells.
Each species of plant must, of course, solve for itself the problem, during the course of its development, whether its energies will be best employed by hoarding nutriment for its own future use in bulbs and tubers, or by producing richly-endowed seeds which will give its offspring a better chance of rooting themselves comfortably, and so surviving in safety amid the ceaseless competition of rival species. The various cereals, such as wheat, barley, rye, and oats, have found it most convenient to grow afresh with each season, and to supply their embryos with an abundant store of food for their sustenance during the infant stage of plant-life. Their example has been followed by peas and other pulses, by the wide class of nuts, and by the majority of garden-fruits. On the other hand, the onion and the tiger-lily store nutriment for themselves in the underground stem, surrounded by a mass of overlapping or closely-wound leaves, which we call a bulb; the iris and the crocus lay by their stock of food in a woody or fleshy stalk; the potato makes a rich deposit of starch in its subterraneous branches or tubers; the turnip, carrot, radish, and beet, use their root as the storehouse for their hoarded food-stuffs; while the orchis produces each year a new tubercle by the side of its existing root, and this second tubercle becomes in turn the parent of the next year's flowering stem. Perhaps, however, the common colchicum or meadow-saffron affords the most instructive instance of all; for during the summer it sends up green leaves alone, which devote their entire time to the accumulation of food-stuffs in a corm at their side; and, when the autumn comes round, this corm produces, not leaves, but a naked flower-stalk, which pushes its way through the moist earth, and stands solitary before the October winds, depending wholly upon the stock of nutriment laid up for it in the corm.
If we look at the parts of plants which are used as food by man of other mammals, we shall see even more clearly the community of nature between the animal functions and those of seeds, flowers, and bulbs. It is true that the graminivorous animals, like deer, sheep, cows, and horses, live mainly off the green leaves of grasses and creeping plants. But we know how small an amount of food they manage to extract from these fibrous masses, and how constantly their whole existence is devoted to the monotonous and imperative task of grazing for very life. Those animals, however, who have learned to live at the least cost to themselves always choose the portions of a plant which it has stored with nourishment for itself or its offspring. Men and monkeys feed naturally off fruits, seeds, and bulbs. Wheat, maize, rye, barley, oats, rice, millet, peas, vetches, and other grains or pulses, form the staple sustenance of half mankind. Other fruits largely employed for food are plantains, bananas, bread-fruit, dates, cocoanuts, chestnuts, mangoes, mangostines, and papaws. Among roots, tubers, and bulbs, stored with edible materials, may be mentioned beets, carrots, radishes, turnips, swedes, ginger, potatoes, yam, cassava, onions, and Jerusalem artichokes. But if we look at the other vegetables used as food, we shall observe at once that they are few in number, and unimportant in economical value. In cabbages, Brussels sprouts, lettuce, succory, spinach, and water-cress, we eat the green leaves; yet nobody would ever dream of making a meal off any of these poor food-stuffs. The stalk or young sprout forms the culinary portion of asparagus, celery, seakale, rhubarb, and angelica, none of which vegetables are remarkable for their nutritious properties. In all the remaining food-plants, some part of the flowering apparatus supplies the table, as in true artichokes, where we eat the receptacle, richly stocked with nutriment for the opening florets; or in cauliflower, where we choose the young flower-buds themselves. In short, we find that men and the higher animals generally support themselves upon those parts of plants in which energy has been accumulated either for the future growth and unfolding of the plant itself, or for the sustenance of its tender offspring.
And now, after this long preamble, let us come back to our original question, and seek to discover what is the origin of fruits.
In botanical language, every structure which contains the seeds resulting from the fertilization of a single blossom is known as a fruit, however hard, dry, and unattractive, may be its texture or appearance. But I propose at present to restrict the term to its ordinary meaning in the mouths of every-day speakers, and to understand by it some kind of succulent seed-covering, capable of being used as food by man or other vertebrates. And our present object must be simply to discover how these particular coverings came to be developed in the slow course of organic evolution,
Doubtless the earliest seeds differed but little from the spores of ferns and other flowerless plants in the amount of nutriment with which they were provided and the mode in which they were dropped upon the nursing soil beneath. But as time went on, during the great secondary and tertiary ages of geology, throughout whose long course first the conifers and then the true flowering plants slowly superseded the gigantic horsetails and tree-ferns of the coal-measures, many new devices for the dispersion and nutrition of seeds were gradually developed by the pressure of natural selection. Those plants which merely cast their naked embryos adrift upon the world to shift for themselves in the fierce struggle of stout and hardy competitors must necessarily waste their energies in the production of an immense number of seeds. In fact, calculations have been made which show that a single scarlet corn-poppy produces in one year no less than 50,000 embryos; and some other species actually exceed this enormous figure. If, then, any plant happens by a favorable combination of circumstances to modify the shape of its seed in such a manner that it can be more readily conveyed to open or unoccupied spots, it will be able in future to economize its strength, and thus to give both itself and its offspring a better chance in the struggle for life. There are many ways in which natural selection has effected this desirable consummation.
The thistle, the dandelion, and the cotton-bush, provide their seeds with long tufts of light hair, thin and airy as gossamer, by which they are carried on the wings of the wind to bare spaces, away from the shadow of their mother-plant, where they may root themselves successfully in the vacant soil. The maple, the ash, and the pine, supply their embryos with flattened wings, which serve them in like manner not less effectually. Both these we may classify as wind-dispersed seeds. A second set of plants have seed-vessels which burst open explosively when ripe, and scatter their contents to a considerable distance. The balsam forms the commonest example in our European gardens; but a well-known tropical tree, the sand-box, displays the same peculiarity in a form which is almost alarming, as its large, hard, dry capsules fly apart with the report of a small pistol, and drive out the disk-shaped nuts within so forcibly as to make a blow on the cheek decidedly unpleasant. These we may designate as self-dispersed seeds. Yet a third class may be conveniently described as animal-dispersed, divisible once more into two sub-classes, the involuntarily and the voluntarily aided. Of the former kind we have examples in those seeds which, like burs and cleavers, are covered with little hooks, by whose assistance they attach themselves to the fur or wool of passers-by. The latter or voluntarily aided sort are exemplified in fruits proper, the subject of our present investigation, such as apples, plums, peaches, cherries, haws, and bramble-berries. Every one of these plants is provided with hard and indigestible seeds, coated or surrounded by a soft, sweet, pulpy, perfumed, bright-colored, and nutritious covering, known as fruit. By all these means the plant allures birds or mammals to swallow and disperse its undigested seed, giving in, as it were, the pulpy covering as a reward to the animal for the service thus conferred. But before we go on to inquire into the mode of their development we must glance aside briefly at a second important difference in the constitution of seeds.
If we plant a grain of mustard-seed in moist earth and allow it to germinate, we shall see that its young leaves begin from the very first to grow green and assimilate energetic matter from the air around them. They are, indeed, compelled to do so, because they have no large store of nutriment laid up in the seed-leaves for their future use by the mother-plant. But if we treat a pea in the same manner, we shall find that it long continues to derive nourishment from the abundant stock of food treasured up in its big, round seed-leaves. Now of course any plant which thus learns to lay by in time for the wants of its offspring gives its embryo a far better chance of surviving and leaving descendants in its turn, than one which abandons its infant plants to their own unaided resources in a stern battle with the unkindly world. Exactly the same difference exists between the two cases as that which exists between the wealthy merchant's son, launched on life with abundant capital accumulated by his father, and the street Arab, turned adrift, as soon as he can walk alone, to shift or starve for himself in the lanes and alleys of a great city.
So, then, as plants went on varying and improving under the stress of over-population, it would naturally result that many species must hit independently upon this device of laying by granaries of nutriment for the use of their descendants. But side by side with the advancing development of vegetable life, animal life was also developing in complexity and perfect adaptation to its circumstances. And herein lay a difficult dilemma for the unhappy plant. On the one hand, in order to compete with its neighbors, it must lay up stores of starch and oil and albumen for the good of its embryos; while, on the other hand, the more industriously it accumulated these expensive substances, the more temptingly did it lay itself open to the depredations of the squirrels, mice, bats, monkeys, and other clever thieves, whose number was daily increasing in the forests round about. The plant becomes, in short, like a merchant in a land exposed to the inroads of powerful robbers. If he does not keep up his shop with its tempting display of wares, he must die for want of custom; if he shows them too readily and unguardedly, he will lay himself open to be plundered of his whole stock in-trade. In such a case, the plant and the merchant have recourse to the self-same devices. Sometimes they surround themselves with means of defense against the depredators; sometimes they buy themselves off by sacrificing a portion of their wealth to secure the safety of the remainder. Those seeds which adopt the former plan we call nuts, while to those which depend upon the latter means of security we give the name of fruits.
A nut is a hard-coated seed, which deliberately lays itself out to escape the notice and baffle the efforts of monkeys and other frugivorous animals. Instead of bidding for attention by its bright hues, like the flower and fruit, the nut is purposely clad in a quiet coat of uniform green, indistinguishable from the surrounding leaves, during its earlier existence; while afterward it assumes a dull-brown color as it lies upon the dusky soil beneath. Nuts are rich in oils and other useful food-stuffs; but to eat these is destructive to the life of the embryo; and therefore the nut commonly surrounds itself with a hard and stony shell, which defies the stoutest teeth to pierce its thickened walls. Outside this solid coating it often spreads a softer covering with a nauseous, bitter taste, so familiar to us all in the walnut, which at once warns off the enemy from attacking the unsavory morsel. Not content with all these protective devices, of color, taste, and hardness, the nut in many cases contains poisonous juices, and is thickly clad in hooked and pointed mail, which wounds the hands or lips of the would-be robber. In brief, a nut is a seed which has survived in the struggle for life by means of multiplied protections against the attacks of enemies. We cannot have a better instance of these precautions than the common cocoanut palm. Its seed hangs at a great height from the ground, on a tall and slender stem, unprovided with branches which might aid the climber, and almost inaccessible to any animal except the persevering monkey. Its shell is very thick and hard, so extremely impermeable that a small passage has to be left by which the germinating shoot may push its way out of the stronghold where it is born. Outside this shell, again, lies a thick matting of hairy fibres, whose elasticity breaks its fall from the giddy height at which it hangs. Yet, in spite of all these cunning precautions, even the cocoanut is not quite safe from the depredations of monkeys, or, stranger still, of tree-climbing crabs. The common Brazil nuts of our fruiterers' shops are almost equally interesting, their queer, shapeless forms being closely packed together, as they hang from their native boughs, in a hard outer shell, not unlike that of the cocoanut. It must be very annoying to the unsuspecting monkey, who has succeeded after violent efforts in breaking the external coat, to find that he must still deal with a mass of hard, angular, and uncanny nuts, which sadly cut his tender gums and threaten the stability of his precious teeth—those invaluable tools which serve him well in the place of knives, hammers, scissors, and all other human implements.
A fruit, on the other hand, lays itself open in every way to attract the attention of animals, and so to be dispersed by their aid, often amid the nourishing refuse of their meals. It is true that, with the fruit as with the nut, to digest the actual seed itself would be fatal to the life of the young plant. But fruits get over this difficulty by coating their seeds first with a hard, indigestible shell, and then with a soft, sweet, pulpy, and nutritious outer layer. The purely accidental or functional origin of this covering is testified by the immense variety of ways in which it has been developed. Sometimes a single seed has shown a slight tendency to succulence in its outer coat, and forthwith it has gone on laying up juices from generation to generation, until it has developed into a one-seeded berry. Sometimes a whole head of seeds has been surrounded by a fleshy stem, and the attention of animals has thenceforward encouraged its new habit by insuring the dispersion of its embryos. A few of the various methods by which fruits attain their object we shall examine in detail further on; it will suffice for the present to point out that any property which secured for the seed dispersion by animal agency would at once give it an advantage over its fellows, and thus tend to be increased in all future generations.
So, then, as birds, squirrels, bats, monkeys, and the higher animals generally, increased on the face of the earth, every seed which showed a tendency to surround itself with succulent pulp would obviously gain a point thereby in its rivalry with other species. Accordingly, as we might naturally expect, fruits, which have been developed to suit the taste of birds and mammals, are of much more recent geological origin than flowers, which have been developed to suit the taste of insects. For example, there is no family of plants which contains a greater number of fruity seeds than the rose tribe, in which are comprised the apple, pear, plum, cherry, blackberry, raspberry, strawberry, quince, medlar, loquat, peach, apricot, and nectarine, besides the humbler hips, haws, sloes, and common hedge-fruits, which, though despised by lordly man, form the chief winter sustenance of such among our British birds as do not migrate to warmer climates during our chilly December days. Now, no trace of the rose tribe can be discovered until late in tertiary times; in other words, no fruit-bearers appear before the evolution of the fruit-eaters who called them into existence: while, on the other hand, the rapid development and variation of the tribe in the succeeding epoch show how great an advantage it derived from its tendency to produce edible seed-coverings.
But not only must these coverings be succulent and nutritious, they must also be conspicuous and alluring. For the attainment of these objects the fruit has recourse to just the same devices which had already been so successfully initiated by the insect-fertilized flowers. It collects into its pulpy substance a quantity of that commonly-diffused vegetable principle which we call sugar. Now sugar, from its crystalline composition, is peculiarly adapted for acting upon the exposed nerves of taste in the tongue of vertebrates; and the stimulation which it affords, like all healthy and normal ones, when not excessive in amount, is naturally pleasurable to the excited sense. Of course, in our own case, the long habituation of our frugivorous ancestors to this particular stimulant has rendered us peculiarly sensitive to its effects. But even from the first, there can be little doubt that a body so specially fitted to arouse sensation in the gustatory nerve must have afforded pleasure to the unspecialized palates of birds and rodents: for we know that even in the case of naturally carnivorous animals, like dogs, a taste for sugar is extremely noticeable. So, then, the sweet juices of the fruit were early added to its soft and nutritive pulp as an extra attraction for the animal senses.
Perfume, of course, follows in the wake of sweetness. Indeed, the difference between taste and smell is much smaller than most people imagine. When tiny floating particles of a body, in the gaseous state, affect certain exposed nerves in the cavity of the nose, we call the resulting sensation an odor; when larger particles of the same body, in the liquid or dissolved state, affect similar exposed nerves in the tongue, we call the resulting; sensation a taste. But the mechanism of the two senses is probably quite similar, while their exciting causes and their likes or dislikes are almost identical. As our great psychological teacher, Mr. Herbert Spencer, well puts it, "smell is anticipatory taste." So we need not be surprised to find that the delicate fragrance of peaches, strawberries, oranges, and pineapples, is a guide to their edibility, and a foreshadowing of their delicious flavor, leading us on by an instinctive action to place the savory morsels between our lips.
But the greatest need of all, if the plant would succeed in enticing the friendly parrot or the obsequious lemur to disperse its seed, is that of conspicuousness. Let the fruit be ever so luscious and ever so laden with sweet sirups, it can never secure the suffrages of the higher animals if it lies hidden beneath a mass of green foliage, or clothes itself in the quiet garb of the retiring nut. To attract from a distance the eyes of wandering birds or mammals, it must dress itself up in a gorgeous livery of crimson, scarlet, and orange. The contrast between nuts and fruits is exactly parallel to the contrast between the wind fertilized and the insect-fertilized flowers. An apple-tree laden with its red-cheeked burden, an orange-bough weighed down with its golden spheres, a rowan or a holly-bush displaying ostentatiously its brilliant berries to the birds of the air, is a second edition of the roses, the rhododendrons, and the May-thorns, which spread their bright petals in the spring before the fascinated eyes of bees and butterflies. Some gay and striking tint, which may contrast strongly with the green foliage around, is needed by the developing fruit, or else its pulpiness, its sweetness, and its fragrance, will stand it in poor stead beside its bright-hued compeers.
How fruits began to acquire these brilliant tints is not difficult to see. We found already in the case of flowers that all external portions of a plant, except such green parts as are actually engaged in assimilating carbon, under the influence of solar energies, show a tendency to assume tints other than green. This tendency would, of course, be checked by natural selection in those seeds which, like nuts, are destroyed by animals, and so endeavor to escape their notice; while it would be increased by natural selection in those seeds which, like fruits proper, derive benefit from the observation of animals, and so endeavor to attract their attention. But it is noticeable that fruits themselves are sour, green, and hard, during their unripe stage—that is to say, before the seeds are ready to be severed from the mother plant; and that they only acquire their sweet taste, brilliant color, and soft pulp, just at the time when their mature seeds become capable of a separate existence.
Perhaps, however, the point which most clearly proves the purely functional origin of fruits is found in the immense variety of their structure, a variety far surpassing that of any other vegetable organ. It does not matter at all what portion of the seed-covering or its adjacent parts happens first to show the tendency toward succulence, sweetness, fragrance, and brilliancy. It serves the attractive purpose equally well whether it be calyx, or stalk, or skin, or receptacle. Just as, in the case of flowers, we found that the colored portion might equally well consist of stamens, petals, sepals, bracts, or spathe—so, but even more conspicuously, in the case of fruits, the attractive pulp may be formed of any organ whatsoever which exhibits the least tendency toward a pulpy habit, and an accumulation of saccharine deposits.
Thus, in the pomegranate, each separate seed is inclosed in a juicy testa or altered shell; in the nutmeg and the spindle-tree, an aril or purely gratuitous colored mass spreads gradually over the whole inner nut; in the plum and cherry, a single part, the pericarp, divides itself into two membranes, whereof the inner or protective coat is hard and stony, while the outer or attractive coat is soft, sweet, and bright colored; in the strawberry, the receptacle, which should naturally be a mere green bed for the various seed-vessels, grows high, round, pulpy, sweet, and ruddy; in the rose, the fruit-stem expands into a scarlet berry, containing the seed-vessels within, which also happens in a slightly different manner with the apple, pear, and quince; while in the fig, a similar stem incloses the innumerable seeds belonging to a whole colony of tiny blossoms, which thus form a compound fruit, just as the daisy-head, with its mass of clustered florets, forms a composite flower. Strangest of all, the common South American cashew-tree produces its nut (which is the true fruit) at the end of a swollen, pulpy, colored stalk, and so preserves its embryo by the vicarious sacrifice of a fallacious substitute. These are only a few out of the many ways in which the selective power of animals has varied the surroundings of different seeds to serve a single ultimate purpose.
Nor is any plan too extravagant for adoption by some aberrant species. What seed-organ could seem less adapted for the attraction of animals than a cone like that of pines and fir-trees? Yet even this hard, scaly covering has been modified, in the course of ages, so as to form a fruit. In the cypress, with its soft young cones, we can see dimly the first step in the process; in the juniper, the cone has become quite succulent and berry-like; and finally, in the red fruit of the yew, all resemblance to the original type is entirely overlaid by its acquired traits.
Equally significant is the fact that closely-allied species often choose totally different means for attracting or escaping observation. Thus, within the limits of the rose tribe itself we get such remarkable variations as the strawberry, where the receptacle forms the fruit; the apple, which depends on the peduncle, or swollen stalk, for its allurement; the raspberry, where each seed-vessel of the compound group has a juicy coating of its own, and so forth: while, on the other hand, the potentilla has no fruit at all, in the popular sense of the word; and the almond actually diverges so far from the ordinary habits of the tribe as to adopt the protective tactics of a nut. Similarly, in the palm tribe, while most species fortify themselves against monkeys by shells of extravagant hardness, as we see in the vegetable ivory, and the solid coquilla-nuts from which door-handles are manufactured, a few kinds, like the date and the doom-palm, trust rather to the softness and sweetness of their pulp, as aids to dispersion. The truth which we learn from these diverse cases may be shortly summed up thus: Whatever peculiarities tend to preserve the life of a species, in whatever opposite ways, equally aid it in the struggle for life, and may be indifferently produced in the most closely-related types.
And now let us glance for a moment—less fully than the subject demands, for this long exposition has run away with our space—at the reactive effects of fruit upon the animal eye. We took it for granted above that birds and mammals could discriminate between the red or yellow berry and the green foliage in whose midst it grows. Indeed, were other proof wanting, we should be justified in concluding that animals generally are possessed of a sense for the discrimination of color, from the mere fact that all those fruits and flowers which depend for their dispersion or fertilization upon animal agency are brightly tinted, while all those which depend upon the wind, or other insentient energies, are green or dull-brown in hue. But many actual observations, too numerous to be detailed here, also show us, beyond the possibility of error, that the higher animals do, as a matter of fact, possess a sense of color, differing in no important particular from that of civilized man.
Whether this sense was developed, however, by the constant search for berries and insects, or whether it was derived from a still earlier ancestry, it would be very difficult to decide. It is possible that, as we saw reason to believe in the case of the flowers and the insect vision, the colors of fruits and the color-sense of birds and mammals may have gone on developing side by side; each plant surviving in proportion as its seeds grew more and more distinctive, and each animal, in turn, standing a better chance of food in proportion as its discrimination of such seeds grew more and more acute. But as there are excellent reasons for crediting fishes and reptiles also with a high faculty for the perception of color, it may be safer to conclude that the sense was inherited by birds and mammals from our common vertebrate progenitors, being only quickened and intensified by the reactive influence of fruits.
Yet it must be remembered that the earliest fruit-eaters, though they might find the scarlet, crimson, or purple coats of their food an aid to discrimination in the primeval forests, would not necessarily derive any pleasure from the stimulation thus afforded That pleasure has been slowly begotten in all frugivorous races by the constant use of these particular nerves in the search for food, which has at last produced in them a calibre and a sensitiveness answering pleasurably to the appropriate stimulation. Just as the peach, which a dog would reject, has become delicious to our sense of taste; just as the pineapple, at which he would sniff unconcernedly, has become exquisite to our sense of smell—so the pure tints of the plum, the orange, the mango, and the pomegranate, which he would disregard, have become lovely to our sense of color. And, further still, just as we transfer the tastes formed in the first two cases to the sweetmeats of the East, or to the violets, hyacinths, and heliotropes of our gardens, so do we transfer the taste formed in the third case to our gorgeous peonies, roses, dahlias, crocuses, tiger-lilies, and chrysanthemums; to our silks, satins, damasks, and textile fabrics generally; to our vases, our mosaics, our painted windows, our frescoed walls, our Academies, our Louvres, and our Vaticans. Even as we put sugar and spices into insipid dishes to gratify the gustatory nerves, whose sensibility was originally developed by the savor of tropical fruits, so do we put red, blue, and purple, into our pottery, our decoration, and our painting, to gratify the visual nerves, whose sensibility was originally developed by the rich tint of grapes and strawberries, star-apples and oranges.
And here again, as in the case of flowers, the feeling once aroused has found for itself new objects in the voluntary selection of beautiful mates—that is to say, of mates whose coloring gratified the rising delight in pure tints. The taste formed upon blossoms produced, by its reaction, crimson butterflies and burnished beetles, the sun-birds of the East, and the humming-birds of the West. So, too, the taste formed upon fruits produced, by a like reaction, parrots, cockatoos, toucans, birds-of-paradise, nutmeg-pigeons, and a thousand other tropical creatures of exquisite plumage and delicate form. As we mount up through the mammalian series, we scarcely come upon any hues brighter than dull-brown or tawny-yellow among the marsupials, the carnivores, the ruminants, or the thick-skinned beasts; but when we arrive at the seed-eating classes, such as the rodents, the bats, and the quadrumana, we find a profusion of color in many squirrels, flying-foxes, and monkeys; while Mr. Darwin does not hesitate in attributing to the same selective action the rosy cheeks, pearly teeth, blue eyes, and golden hair, of the human species.
Nor is it only in the choice of mates that the nascent taste for color displays itself. Even below the limits of humanity bright-hued objects afford a passing pleasure to more than one aesthetically-endowed species. Monkeys love to pull crimson flowers in pieces, dart in pursuit of brilliant tropical birds, and are attracted by the sight of red or yellow rags. Those queer little creatures, the bower-birds, carry the same feeling a step further by collecting fragments of brilliantly-colored objects to decorate their gaudy meeting-places. But, when we reach the race of man, we find the love of color producing far more conspicuous secondary results. The savage daubs his body with red or blue paint, and plants his garden with the scarlet hibiscus or the purple. Soon, with the rise of pottery and cloth-making, he learns the use of pigments and the art of dyeing. Next, painting proper follows, with all the decorative appliances of Egypt, India, China, and Japan, until at last our whole life comes to be passed in the midst of clothing and furniture, wall-papers and carpets, books and ornaments, vases and tiles, statuettes and pictures, every one of which has been specially prepared with dyes or pigments, to gratify the feeling originally derived from the contemplation of woodland berries by prehistoric man, or his frugivorous ancestors. And all these varied objects of civilized life may be traced back directly to the reaction of colored fruits upon the structure of the mammalian eye.
What a splendid and a noble prospect for humanity in its future evolutions may we not find in this thought that, from the coarse animal pleasure of beholding food, mankind has already developed, through delicate gradations, our modern disinterested love for the glories of sunset and the melting shades of ocean, for the gorgeous pageantry of summer flowers, and the dying beauty of autumn leaves, for the exquisite harmony which reposes on the canvas of Titian, and the golden haze which glimmers over the dreamy visions of Turner! If man, base as he yet is, can nevertheless rise to-day in his highest moments so far above his sensuous self, what may he not hope to achieve hereafter, under the hallowing influence of those chaster and purer aspirations which are welling up within him even now toward the perfect day!—Cornhill Magazine.
- See Popular Science Monthly Supplement, No. XIV., p. 151.
- I trust that in the sequel the critical botanist will excuse me for having neglected the strict terminology of carpological science, and made no distinction between seeds and fruits. Some little simplification is absolutely necessary for general readers in this the most involved department of structural botany.