Works of grant allen, p.682
Works of Grant Allen, page 682
Of course, the change from fertilisation by wind to fertilisation by insects could not be accomplished without many structural modifications, whereby the flower became adapted to the new and more specialised agency thus afforded it. Some of these modifications were concerned with the food offered by the flower to the fertilising insect. At first this food doubtless consisted of pollen alone, but after a time there was added that sweet matter known as sugar or honey, which is contained more or less in most plants, and which is especially developed during the two processes of flowering and fruiting. Now sugar, by its crystalline condition, so rare amongst energy-yielding organic products, seems specially adapted for affording pleasurable stimulation to the gustatory nerves of animals; and it has therefore been stored up by plants in all those cases where the attraction of some animal ally is desirable for their protection or the continuance of their species. Certain plants lay it by in glands upon their stems to allure the harmless ants who protect them from the ravages of their leaf-cutting congeners. Others, again, as we shall see in a later chapter, collect it in the pulpy covering of their seed-vessels, and thus induce parrots, pigeons, or monkeys to devour and distribute the undigested kernel. Yet others distil it in the recesses of their blossoms, and so tempt the bee, the butterfly, or the humming-bird to rifle their labyrinthine storehouses, and unconsciously aid in the impregnation of their embryos. The honey thus elaborated by the flower has become at last the main ultimate attraction for all fertilising insects, whose most specialised forms we find in the common hive-bees of domestic economy.
A second class of modifications is connected with the shape of the flower. Most entomophilous blossoms possess, in addition to the pistil and the stamens, two other whorls of floral leaves — the corolla and the calyx. In the simplest form of flower, these whorls consist of separate leaves (petals and sepals), as we see in the buttercup or the dog-rose. But in certain more specialised flowers the long-continued action of the insect-fertilisers has unconsciously selected those blossoms which most easily suited themselves to the form of their visitor, and has thus produced a united corolla, all whose petals are joined into a regular tube or cup, as we see in the Canterbury bell, the convolvulus, and the lily of the valley. A number of these tubular flowers united form the head of the daisy, the marigold, and the sunflower. In still more specialised cases the cup becomes irregularly lobed, so as to suit still more closely the shape of its insect friend — a change whose first steps we see in honeysuckle and foxglove, while its completed stage is shown in mint, dead-nettle, snapdragon, lobelia, and orchids. All these varieties of entomophilous flowers we shall have to examine at greater length hereafter.
But the third class of modifications, the important class for our present subject, is that which refers to the colouring of flowers. By far the most conspicuous difference between entomophilous and anemophilous blossoms is the difference of colour. While wind-fertilised plants have seldom more than a few brownish scales or tiny green sepals around their fructifying organs, insect-fertilised plants are almost always distinguished by the growth of large, brilliantly-coloured petals outside the essential whorls, which act as guides and allurements to the eyes of bees or butterflies. These wide, expanded, and costly structures have absolutely no other purpose in the vegetal economy than that of attracting the fertilising agents; and they afford by themselves a strong presumption of developed colour-sense in the creatures for whose guidance they have been slowly evolved. Let us see by what steps they can gradually have reached their present conspicuous dimensions.
We will suppose that some of the flowering plants in the early ages of the world showed some slight tendency to develop the various attractive structures which we now observe in their completed form. They offered to insects the soft and nourishing pollen, and perhaps, too, small quantities of stimulating saccharine matter. Such saccharine matter we know is always evolved during the opening of flowers, at least in small amounts, for the nurture of the blossom itself; and there is nothing extravagant in the supposition that occasional individuals might produce it in more than the average quantity, and so might attract more than the average share of insect attention. In like manner they may possibly have shown a tendency to develop bright-coloured leaves around their essential organs; and if the eyes of insects were capable of distinguishing these bright colours, in however imperfect a degree, it would naturally follow that the hues would go on deepening from generation to generation among the plants, while the perception would go on sharpening itself from generation to generation among the insects. For while the flowers which thus become more and more readily distinguishable by their fertilisers would thereby better secure the chance of descendants, the insects which most readily distinguished flowers would thereby secure for themselves the greatest amount of the available food-stores. So that, supposing such a tendency once set up on either side, we can see every reason why it should ultimately develop to its present noticeable extent.
This, however, is mere a priori hypothesis. The experimental philosopher will ask at once whether we have any grounds for believing that the tendency in point would ever be set up. I think we have such grounds in abundance, and although the question involves a little closer application and more technical considerations than any we have yet encountered, I shall ask the reader patiently to follow me through the exposition, because it really encloses the whole fundamental basis of the developed colour-sense in every terrestrial animal. The point which we have first to consider is this: Did flowers show an original tendency to the production of coloured adjuncts prior to the selective action of insects? And when we have answered that question we must proceed to the second one: Did insects possess any tendency vaguely to discriminate colours apart from the reactive influence of entomophilous flowers?
The solar radiations, falling upon the green portions of plants, are the sole ultimate source of all the energy existing in the animal or vegetal organism. Under their influence, the plant separates carbon and hydrogen from the oxygen with which they were originally combined, stores them up in some part of its own tissues, and turns the free oxygen adrift upon the atmosphere around. In this process, the kinetic or active energy of the solar undulations has assumed the potential or dormant form. The potential energy thus laid up is associated with the carbon and hydrogen of the plant on the one hand, and with the free oxygen of the atmosphere on the other. “Whenever they may recombine, the dormant energy will assume once more the active form, and be yielded up in the shape of mechanical motion, heat, or light.
This reconversion of contained energy into its mobile mode may be brought about in many ways. Sometimes the plant may be cut down and burnt, as we all see in wood-fires, and then the energy will be given out rapidly as heat and light, while part of it will also go off as motion of the surrounding air. Sometimes the plant may fall as it lies, be changed into peat or coal, and finally burnt, like the wood, in a human grate, with the same concomitant phenomena as in the first case. Sometimes, too, these same materials — wood, coal, peat — may be used to feed a steam-engine, and mainly converted into visible movements of the locomotive or its parts, which are finally dissipated by friction into the circumambient æther. In yet other cases, the plant may be eaten by an animal, and then its elements will recombine within his body with the free oxygen supplied by his lungs or gills, and will give off heat and motion, less conspicuously perhaps, but quite as truly as in the engine. There remains, however, another instance, fully as common as these, yet far less generally observed — the instance in which the elements recombine in the tissues of the living plant, and yield up their dormant energies in producing growth, development, and rearrangement of its parts. This metamorphosis of energy (known as Stoffwechsel or Metastasis) actually takes place in every active portion of a plant which does not itself assimilate nutritive material from the surrounding air. And all such parts of plants may be considered as carrying on essentially animal functions — that is to say, functions by which potential energy becomes kinetic, oxygen unites with carbon to form carbonic anhydride, heat is evolved, and motion is given out.
The most noticeable cases of such quasi-animal processes may be seen in the germination of seeds, the growth of bulbs and tubers, the unfolding of flowers, and the ripening of fruits. In fact, every growing and active part of a plant, unless it be itself assimilating kinetic energy from solar undulations, must necessarily be using up energies assimilated elsewhere. Otherwise, it would be manufacturing new energies for itself out of nothing, which we know to be impossible, and inconceivable as a direct contravention of all physical and mental laws.
Now, the active agent of deoxidation in ordinary plants is that peculiar compound substance known as chlorophyll, the pigment which gives a green colour to healthy leaves. Hence all the active organs of plants are usually green in hue, because the chlorophyll is seen through the transparent cell-walls of the epidermis. But there are reasons for believing that wherever the reverse process of metastasis is taking place, other bodies are frequently formed, which reflect the light in slightly different manners, and so give rise to tints of red, orange, yellow, pink, mauve, purple, or blue. We will examine the evidence in order, and see whether we can gather from it any inference as to the origin of coloured flowers.
In the first place, even in active leaves, the presence of green chlorophyll is often masked by the occurrence of other pigments, which give the foliage a tinge of brown, russet, scarlet, or golden yellow. Cases of this sort are commonly known in the copper beech, the red cabbage, and the various species of purple coleus, crimson-hearted caladium, pink dracæna, or pale mauve begonia. Here the colouring matter doubtless belongs to some one among the many by-products of vegetal physiology, which must necessarily occur from time to time in one part or another as the results of assimilative or metastatic changes. But in the more noticeable cases of coloured juices or pigments, other than green, we shall find that the special colouring matter is almost always more or less connected with those portions of the plant where energy is being liberated, and where accordingly oxidation is necessarily taking place.
The only class of plants in which green rarely — we might almost say never — occurs, is that of fungi. But fungi differ from all other plants (except a few parasites and saprophytes) and agree with animals in this, that they derive their energies not directly from solar undulations, but from organised matter already contained in the soil or matrix on which they grow. And there is something in the vivid orange, yellow, lilac, and crimson of their hues, as well as in the pasty whiteness of their common tissues, which strikingly recalls the possibly adventitious colouring of the lower animal forms, such as sea-anemones, star-fish, and medusæ. This analogy, as we shall note hereafter, is not without a deep significance for our present purpose.
When, however, we go on to those plants which have normally green leaves, we see a like result. In the first place, dying leaves, as we all know, assume the most brilliant tints of red, yellow, orange, and brown. Even in our own damp and uncertain climate, the Virginia creeper glows with the richest crimson, while the forest trees shade off into delicate tones of golden gloss and occasional flashes of deep scarlet light. But in American woodlands these displays assume grander dimensions and more glorious beauties, forming perhaps the most magnificent fields of gorgeous colour in the whole organic world. Now, Macaire-Princep has shown, that as leaves begin to turn yellow they give up the function of deoxidation, while a reverse process at once sets in. Mr. Sorby traces the gradual loss of vitality in the fading leaf from bright green through greenish-brown, red, scarlet, and orange-brown to the final dull and dingy hue of the dry leaf. That this change results from some degradation of energy, in whatever component of the leaf it may take place, is beyond all doubt.
Any injury to leaves causes similar-effects, whether due to disease, external impact, or the attacks of insects. Gall-nuts and rose-blights have generally an outer coating of small reddish excrescences, while feeble plants produce yellow-spotted or pink-speckled leaves. Here, too, oxidation, or some other de-energising action, is most probably the cause of the change observed.
Leaves which have given up their natural functions frequently assume bright hues. Thus the pitchers of the side-saddle flower (Sarracenia rubra) have purple tips. Those of the pitcher-plant (Nepenthe) are “tinted and mottled with red and purple.” The leaves of the curious insectivorous plants, with whose habits Mr. Darwin has made us so familiar, are apt to be speckled with similar hues, especially in the active portions, which show, by their movements and secretions, some approach to animal functions. The common English sundew (Drosera rotundifolia), which may be found in all boggy or peaty places, has bright red glands scattered over its leaves. The Venus fly-trap (Dionæa muscipula) is “thickly covered with minute glands of a reddish or purplish colour,” while the spikes which close upon the insect prey have small projections described as “reddish-brown or orange.” Like organs in Drosophyllum lusitanicum are bright pink, and in Pinguicula lusitanica purple. Our own common butterwort and saxifrage, which share to a less extent the same peculiarity, have also slightly reddish or yellowish foliage.
Parasites which live upon the energetic matter stored up by other plants fall obviously under the same class. Their whole existence consists in a continuous metastasis, that is to say, in the expenditure and liberation of previously accumulated nutriment, under the influence of oxidation. The common European broom-rapes (Orobanche) have no green leaves, but merely pink, purple, brown, yellow, blue, or rose-coloured scales and flowers. Cytinus hypocistis, which grows parasitically on the roots of the Cistus, has a bright orange stem and leaves. The common English dodder is noticeable for its pretty twining red filaments, while its Indian congeners display brilliant hanging masses of golden threads. In fact, almost all true and perfect parasites are remarkable for the absence of green and the presence of other bright hues. Of course, many plants usually included under that name, like the mistletoe family, have foliage of the ordinary colour; but these are in reality only half-parasitical, a kind of stepping-stone between the epiphytic plants (orchids and bromelias) and the thorough-going parasites, such as Rafflesia. To the very end, indeed, the degraded leaves or scales of flowering plants contain some traces of chlorophyll, which, however, like the leaves themselves, must be regarded as mere obsolescent relics of their earlier state. It should also be noticed in passing, that many parasites, like Rafflesia and Hydnora, have exceptionally large and brilliant flowers. The blossom of R. Arnoldi sometimes measures three feet in diameter.
Still more noticeable in hue are the plants known as saprophytes, which live like fungi on the decaying matter contained in dead foliage or other organic remains. These, too, have no real assimilative leaves, while their functions are purely animal, consisting in the absorption of oxygen and the expenditure of previously accumulated energies. The Indian-pipe plant of Canada (Monotropa uniflora) has a pure white scaly stem and flower, exactly resembling a fungus to the untrained eye; it grows under the shade of pine forests, amid the rich débris of their pollen and their fallen foliage. The beautiful Neottia speciosa has a scape and rudimentary leaves of bright scarlet. Corallorhiza and many other saprophytes are equally remarkable for their exquisitely coloured scales. It is true that several, if not all, of these plants contain small quantities of chlorophyll or xanthophyll; but here again, we must regard the pigment as a mere remnant of earlier ancestors; while the plant, as a whole, mainly consists of metastatic materials, or, in other words, of oxidation products.
The resemblance which both parasites and saprophytes bear to fungi is certainly remarkable when we remember their close community of nature and function. All alike live upon previously organised material, and all have the same flabby, succulent, pulpy appearance. The Indian-pipe plant is always described by Canadian farmers as “a kind of toadstool;” the Rafflesia is noted for its fungoid look and animal odour; the Cytinus exhales a meaty flavour; and the Cynomorium coccineum is known to druggists by the technical name of Fungus melitensis or Malta mushroom. Putting these facts by the side of their very similar colouration, we are not unnaturally led to expect some causal connection such as that of which we are now in search. Let us pass on to other coloured portions of ordinary plants, which may throw a little more light upon the question at issue.
Buds contain energetic material, stored up by the plant during the preceding season, and expended, presumably, by union with oxygen, during the spring. The sprouting buds of the hawthorn and of many other plants present exquisite tints of pink and mauve. The bulbils of the tiger-lily are covered by purple scales. The various devices by which plants lay by nutriment during one season for their growth in the next are known as bulbs, tubers, corms, or rhizomes. All of these are apt to produce young sprouts of dainty colouring and bright hue. The growing sprays of the potato, when kept carefully from the light, exhibit distinct tinges of pink, blue, violet, and yellow. Asparagus shoots and blanched sea-kale have scales or leaves of mauve, lilac, and greenish brown. Almost all bulbs, on first producing leaves, show very decided colours, which change to green under the action of light. Beet-root, permitted to sprout in the dark, sends up beautiful bunches of deep crimson foliage. Carrots, under like circumstances, put forth golden sprays, varying from light primrose to bright orange. Sprouting peonies are of a full dark red. Rhubarb has rosy stems and pink or yellow leaves. In many of these cases, the colour is most conspicuous in the thin laminated portion of the young leaves, which offer the best medium for the display of delicate pigments. In every case, exposure to the sunlight brings about reversion to the original assimilative function, and results in the final triumph of green chlorophyll.
