by Grant Allen
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 afterwards it assumes a dull brown colour 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 unsavoury morsel. Not content with all these protective devices of colour, 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 cocoa-nut 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 cocoa-nut 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 cocoa-nut. 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-proper, 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 ensuring 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 Miocene 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 shows 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-fertilised 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 tongues 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 unspecialised 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.
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 syrups, 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-fertilised and the insect-fertilised 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 maythorns, 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 endeavour 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 endeavour 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 colour, and
soft pulp just at the time when their mature seeds become capable of a separate existence.
The connection of these changes with the process of oxidation is far more certain and more marked in the case of fruits than in that of flowers. During their early state, pulpy fruits have the structure and chemical composition of leaves. But as they mature, they gradually pass through the acid stage, and finally reach that of ripening, when their gum, their cellulose, and their acids are partially converted into sugar. These alterations are accompanied by “a loss of watery fluid, a slight increase of temperature, and an evolution of carbonic acid.” “Saussure and Couverchel state that grapes, apples, and pears, when separated from their respective plants, and kept at a temperature of about 60° F., gave out carbonic acid. Fremy found that ripe fleshy fruits gave out a large quantity of carbonic acid when boiled in a saline solution. Berard thinks that these changes in fruits depend essentially on the action of the oxygen of the air. Fleshy fruits, he says, may be preserved with little alteration for many weeks in vacuo, in nitrogen, and in hydrogen gas; peaches, plums, and apricots may be kept from twenty to thirty days, and pears and apples for three months, in a sealed bottle containing a little sulphate of iron, lime, and water, which remove the oxygen of the air. Fremy found that the ripening of the fruit was arrested by covering it with varnish, which he supposes to act partly by preventing the access of air, and partly by stopping the transpiration, and thus checking the flow of sap into the fruit.”
It may also be added that here, as in the case of flowers, an original tendency towards colouration in seed-coverings, quite apart from any selective action, may be distinctly noted. Not only are the spore-cases of many mosses prettily tinted with pink or yellow, but the fruits of many flowering plants which have no succulent pulp yet exhibit a decided turn for coloured juices. Instances may be found in the dock, and less markedly in almost all capsuled fruits. But with fruits, as with flowers, we may say roughly that all the bright-coloured species depend for their diffusion upon animals, while all those which do not depend upon animals are dull. “The smaller plants,” says Mr. Wallace, “whose seeds simply drop upon the ground, as in the grasses, sedges, composites, umbelliferæ, &c., always have dry and obscurely-coloured capsules, and small brown seeds. Others, whose seeds are ejected by the bursting open of their capsules, as with the oxalis and many of the Caryophyllaceæ, Scrophulariaceæ, &c., have their seeds small, and rarely or never edible.”
In the case of the attractively coloured fruits, however, Mr. Wallace points out that the actual seeds are of such a nature as to escape destruction when the fruit itself is eaten. “They are generally very small and comparatively hard, as in the strawberry, gooseberry, and fig; if a little larger, as in the grape, they are still harder and less eatable; in the fruit of the rose (or hip), they are disagreeably hairy; in the orange tribe, excessively bitter. When the seeds are larger, softer, and more edible, they are protected by an excessively hard and stony covering, as in the plum and peach tribe; or they are enclosed in a tough horny core, as with crabs and apples. . . . These fruits may also be swallowed by some of the larger frugivorous birds; just as nutmegs are swallowed by pigeons for the sake of the mace which encloses the nut, and which, by its brilliant red colour, is an attraction as soon as the fruit has split open, which it does upon the tree.”
But exactly as we saw that some flowers attract insects by a delusive hope of honey where no honey is really to be had, so do some fruits hold out attractions of colour to birds or mammals where little or no food is to be had in return. Thus many beans have beautiful coverings, which must be purely deceptive in their nature for though some animals may perhaps be able to eat them, yet these can be of no benefit to the plant, and it cannot be for their sake, therefore, that the bright integument has been developed. An extreme case is that of the hard little rosary bean (Abrus precatoria), so well known as the seed from which the prisoners in Cayenne manufacture their pretty ornaments. “It may be,” says Mr. Wallace, “that birds, attracted by the bright colour of the seeds, swallow them, and that they pass through their bodies undigested, and so get dispersed.” If so, the ingenious naturalist suggests that the device may only succeed with “young and inexperienced birds.” I am myself inclined to think, however, that some plants, such as our English cuckoo-pint and the famous West Indian manchineel, actually derive a benefit from their poisonous properties; because if eaten by birds or small mammals, they might destroy their host, and the seeds would thus have an opportunity of germinating in the midst of a rich manure-heap, consisting of its decomposing body.
Another analogy with entomophilous flowers may be found in the very variable nature of the pulpy and coloured substance. It does not matter at all what portion of the seed-covering or its adjacent parts happens first to show the tendency towards 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 coloured 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 towards a pulpy habit, and an accumulation of saccharine deposits.
Thus, in the pomegranate, each separate seed is enclosed in a juicy testa or altered shell; in the nutmeg and the spindle-tree, an aril, or purely gratuitous coloured 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 coloured; 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 encloses 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, coloured 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 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 ca
ses 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.
I have given this large amount of space to the consideration of fruits, because I believe we can hardly over-estimate their importance in quickening the colour-sense of the higher animals, and, above all, in settling the æsthetic tastes of birds, quadrumana, and men. We are apt to forget how considerable an element in the total coloured environment of forestine animals is formed by brilliant fruits. The utilitarian connection of fruits generally has made us cultivate them more for their pulp and sweetness than for their beauty, and in many cases they have actually lost in colour under cultivation; while flowers, being selected entirely for their visual attractiveness, have gone on developing more and more expanded masses of bright petals. But if we look at a few striking instances, we shall find that fruits almost equal in native beauty their earlier rivals, the entomophilous blossoms. Among cultivated varieties commonly grown in Britain, we may take apples, plums, peaches, cherries, grapes, strawberries, raspberries, currants, and pumpkins; while it may be worth while to remind the reader that certain other fruits or seeds, which usually appear on our tables in a green state, like cucumbers and scarlet-runner beans, have brilliant coats in their mature forms. Amongst English wild fruits, sufficient examples will be found in the hips, haws, holly-berries, mistletoe, sloe, mountain ash, barberry, yew, juniper, ivy, spindle-tree, arum, blackberry, iris, saffron, elder, and sea-buckthorn. The tropics and sub-tropical climates, however, supply us with far more gorgeous examples in their oranges, shaddocks, lemons, mangoes, star-apples, pomegranates, capsicums, bananas, nutmegs, achees, egg-fruits, prickly pears, tomatoes, winter cherries, solanums, dates, and passion-flower berries. In fact, we may say that fruits-proper exhibit larger amounts of brilliant colouration than any other class of organic objects except entomophilous flowers.
