by Grant Allen
The young shoots at the end of branches are in the same position, as regards energy, with the sprouts which arise from bulbs or tubers. They cannot yet feed themselves, but they are nourished by energetic materials from the older leaves, whose carbon combines with oxygen in their tissues to yield the energy whereby their growth is carried on. Now the bright tints of these young shoots are very noticeable (as may be especially observed in the fuchsia, the hawthorn, and the rose-apple), and they can be skilfully arranged in such combinations as to produce a visible effect not at all unlike that of flowers.
If we compare these various cases with those of bright-hued entomophilous blossoms and brilliant fruits, we shall find that they have all one quality in common — they occur in parts which are expenders, not accumulators, of energy. Hence we are led to suppose that those portions of plants which subsist upon previous accumulations are apt to assume bright hues of different sorts. To what can we attribute the tendency which we thus observe? Can we give any causal formula for the empirical generalisation at which we have now arrived? I think we can, and in the following manner: —
Chlorophyll, the active deoxidising principle, has a definite composition, which enables it to carry on its proper functions, and a definite mode of reflecting light, which we call green. How far its greenness is bound up with its other physical properties we cannot say. Perhaps, as has been objected, it might equally well perform its physiological purpose were it red or yellow. But more probably its special reaction upon light is intimately connected with its special reaction upon carbonic anhydride under the influence of light. However this may be, at least we know that active chlorophyll is always green; and the more active, the brighter its hue, as Mr. Sorby has abundantly shown. Hence, every part of a plant which performs deoxidising functions has necessarily a green pigment for its foundation. The greenness may indeed be masked by other dyes (perhaps themselves the products of oxidation), as in cell-sap or epidermis, but in the actual active principle itself, greenness is apparently always present as an essential and inherent property.
So leaves as a rule, where exposed to sunlight, are green, but the remaining portions of the plant do not seem to be bound by such a stringent law of colouration. There is no reason why other colours should not appear in them from time to time, and, if they prove useful, be perpetuated through the action of natural selection. How, then, do they arise?
Colour, we have seen already, is merely the mode in which various bodies react upon light, reflecting or absorbing its constituent elements in varying proportions of their several rays. But there is no property of different bodies more variable in its nature than this particular mode of reaction. The slightest change in the molecular constitution of a substance is apt to be accompanied by considerable changes in its hue. Materials which appear chemically almost identical pass through strange varieties of tint with the greatest readiness. And this is particularly the case with organic matter, which differs from all other matter in the striking effects produced upon its physical constitution by apparently trifling causes. Hence we might naturally expect that very small changes in the constituents or contents of plant tissues would be likely to produce great alterations in their colour. And we find accordingly in all non-active parts of a plant that by-products of various tints do actually occur with considerable frequency: take, for example, the bright hues of many stems, barks, and juices, the red under-side of the Victoria regia leaf, the amber nether foliage of the star-apple, and the beautiful scales of the gold and silver ferns.
Whether such colours are always due to oxidation, it would be difficult to say; but in a large number of instances it is quite clear that oxidation is going on in the tissues where the colours appear. Obviously, in all cases of metastasis, the recombination of oxygen with the accumulated hydrocarbons is the only source of the energy whereby growth is carried on. Sometimes as much as 40 or even 50 per cent. by weight of the organic matter contained in seeds which germinate in the dark is lost by conversion into carbonic anhydride and water; and somewhat the same change must take place in bulbs, tubers, corms, and rhizomes. Almost all the above-quoted cases fall apparently under a like generalisation. The red colouring matter of persistent winter leaves, as seen in Sempervivum, Mahonia, Vaccinium, and Sedum, is clue to a substance mainly consisting of tannin. Mr. Sorby does not absolutely say that the colours of autumn foliage are due to oxidation, but he refers them on the whole to decreased vitality, absorption of chlorophyll, and similar causes, which bring into prominence various minor principles otherwise unnoticed. Of the lichnoxanthine series he says expressly, “They are probably only products of the oxidisation of chlorophyll, from which they may be prepared artificially.” Of the erythrophyll series, on the other hand, he merely observes, “They are usually indicative of low constructive energy.” The chrysotannin group, again, “when oxidised, give rise to various brown substances, which are the cause of many of the characteristic tints of autumnal foliage.” But with regard to the pigments of entomophilous flowers his language is much more decided in tone. “The coloured substances in the petals are in many cases exactly the same as those in the foliage from which chlorophyll has disappeared; so that the petals are often exactly like leaves which have turned yellow or red in autumn, or the very yellow or red leaves of early spring. . . . The colour of many crimson, pink, and red flowers is due to the development of substances belonging to the erythrophyll group, and not unfrequently to exactly the same kind as that so often found in leaves . . . The facts seem to indicate that these various substances may be due to an alteration of the normal constituents of leaves, some being probably formed from chlorophyll, others from the xanthophylls, and perhaps some from other constituents. So far as I have been able to ascertain, their development seems as if related to extra oxidisation, modified by light and other varying conditions not yet understood.” In like manner Lory found that parasites, as exemplified by broom-rapes, absorbed oxygen and exhaled carbonic anhydride in all stages of growth, whether exposed to the sun or not. So, also, Morot showed that in etiolated plants the coloured portion of the tissues gave out carbonic anhydride, while the green portion gave out oxygen. In short, without going into the lengthy ultimate question — scarcely soluble at present — whether all bright vegetal pigments (except chlorophyll) are themselves actually oxidation products, we may at least affirm that they occur with exceptional frequency in those plants or parts of plants where oxidation is largely taking place. They may be always directly due to the absorption of oxygen, or they may be merely secondary results of that action; but they certainly show a great tendency to present themselves wherever energy is being expended; and that conclusion is quite sufficient for our immediate object.
Here, to guard against an obvious criticism, it should be added that only a tendency, not a universal law, in such a direction is believed to exist. For example, the leaves of the sensitive plant and the Desmodium, which exhibit movements far more marked than those of the insectivorous species already noticed, are perfectly green. But the whole conclusion here suggested amounts in brief to the following principle: Wherever considerable changes occur in the nature of the vegetal tissues or their contents, they are apt to be accompanied by similar changes in the reaction of the tissues upon the incident sunbeams.
Yet it is a noteworthy fact of great importance, as shedding light upon the origin of the colour-sense, that such brilliant tints are everywhere exceptionally common in the organic world. Besides the green of chlorophyll, the orange and scarlet of autumn leaves, or the varied hues of flowers and fruits, we find unusually bright colouring in many parts of animals, especially the very simplest, such as jelly-fish and sea-anemones. Although, as we shall observe hereafter, many of these are doubtless due to the selective action of sexual preference, acting through the colour-sense itself, yet in the lowest organisms there is some reason to believe that the purity and splendour of the prevailing hues are only due to the adventitious composition of their molecules. And when we further notice th
e brightness of mammalian blood, besides the numerous changeful hues of sundry viscera or their contents, we shall probably be willing to allow that organic bodies habitually display pure and gorgeous tints, which the mineral world only shows us in a few rare and exceptional jewels.
Before we proceed, however, to apply these general principles to the genesis of entomophilous flowers, it will be well to glance briefly at a distinction of considerable importance, already hinted at in the preceding paragraph. Colour, as such, cannot of itself subserve any special function except in connection with the animal eye. The hues of all inorganic and of most organic bodies depend entirely upon the fortuitous molecular constitution of the particular body. But when a colour so reflected happens to produce some specific effect upon the eyes of any animal, whose interference is either useful or noxious to the animal or plant reflecting it, then the principle of natural selection will come into play, and the colour, as such, may be said to subserve the special function of attraction or protection, as the case may be. Henceforward, in the present work, a colour which seems simply to depend upon molecular constitution, apart from function to be subserved, will be described as adventitious; while a colour which also subserves a special function will be described as purposive.
Now all the coloured objects with which we have so far dealt — green leaves, autumn foliage, young shoots, sprouting buds — are purely adventitious in their tints. Flowers, however, which we next approach, are purposive; but, like all other purposive adaptations, they must necessarily have taken their rise in some adventitious circumstance, afterwards increased and developed by selective action.
With such data before us, then, let us proceed to inquire what was the genesis of those bright entomophilous flowers, which present brilliant tints in specialised thin leaves or petals, admirably adapted alike for rapid oxidation, and for the ostentatious display of delicate pigments.
The flower is one of the purely expensive structures which we noticed above as seats of oxidation and liberated energy. The well-known experiments of Saussure, Dutrochet, Vrolik, and De Vriese, detailed in all handbooks of physiological botany, sufficiently prove that during the act of flowering oxygen is consumed, carbonic anhydride evolved, and heat liberated. These experiments have been generally conducted upon various species of Arum, which are insect-fertilised flowers; but similar phenomena have also been observed in the cones of cycads, whose blossoms are strictly anemophilous. Indeed, as the absorption of oxygen is chiefly concerned with the maturation of the pollen, and, to a less extent, of the pistil, it is clear that it can be but little influenced by the nature of the surrounding structures.
Hence we would naturally expect that all floral organs, wind-fertilised or insect-fertilised alike, would show a tendency to the production of bright colours, in accordance with the general principle here laid down. This a priori expectation is fully justified by the actual facts.
In the first place, even among flowerless plants, the purely expensive structures employed in the elaboration of young spores are almost always tinged with some other hue than that of the green pigment which distinguishes the active and assimilating leaves. In mosses the graceful little spore-cases, which rise like miniature fruits at the extremity of the tall spiky stems, are usually pink or reddish brown in colour. The beautiful Splachnum rubrum of the Canadian forests has a cup of brilliant scarlet, which has led the children who pick it to give it the pretty popular name of red-cap moss. Many lycopodiums produce bright golden fructifications, very conspicuous in the lovely exotic L. dendroideum. Ferns generally bear their spores on the under surface of the frond, where their brown or russet colour makes them very noticeable and pretty objects. So that, in spite of their ill-chosen name, the cryptogams themselves exhibit the universal tendency to varied colouration in the reproductive organs.
Next, when we examine the phanerogamous division of plants, we see at once that the actual floral structures themselves are always more or less marked by distinctive colours. The pollen is generally of a rich golden yellow, while the surrounding scales show tints of silvery grey or faint pink. Even among the wind-fertilised blossoms, not a few are thus rendered conspicuous when they hang thickly together in large close-set masses. Many catkins, several grasses, the larch and other conifers, the dock and its congeners, all display blossoms of considerable distinctness, quite uninfluenced by the selection of insects. The inner bracts of the unopened artichoke head are often a brilliant mauve, not less beautiful than that of many flowers. The glumes which surround the floral organs of grasses are ruddy purple. The female flowers of the common hazel are a fine red, as Mr. Darwin reminds me. Evidently we have here a groundwork of differential colouring upon which selection might set to work, and ultimately produce the striking results that we see to-day in every flower-garden.
These, then, are the ultimate elements of our problem. Flowers consist essentially of male and female organs, which really represent aborted leaves, greatly modified for their special function, as Wolff and Goethe long since pointed out. These reproductive organs are situated at the ends of axes, where growth is failing; and Mr. Herbert Spencer observes that such points are just the ones where coloured leaves, as noted above, frequently make their appearance. In anemophilous flowers, as a rule, we find only the two whorls of essential floral organs; but in entomophilous flowers, as a rule, we find two additional whorls, the petals and the sepals, one or both of which are brilliantly coloured, the colouration apparently subserving no other purpose than the attraction of insects who aid in fertilising the flowers. We can hardly resist the inference that the coloured whorls represent an intensification of the natural tint in growing shoots and floral organs, slowly modified by the selective action of the insect eye.
When we look more closely at the nature of showy entomophilous flowers, this conviction becomes greatly strengthened. If colouration depends wholly or in part upon oxidation of previously stored material, it will follow that very large and massive blossoms can only be produced by the aid of considerable prior accumulations in so depend for their support upon bulbs, corms, tubers, or other like bulky reservoirs of energetic material. It will be sufficient to mention the cases of the water-lilies, the lotus, the dahlias, the orchids, the iris, the crocus, the gladiolus, the narcissus, the snowdrop, the daffodil, the tulip, the various lilies, the tuberose, the hyacinth, and the meadow-saffron. In many of these plants the handsomest heads of bloom are secured by cutting off the flower-buds for several successive years, and so preventing the expenditure of material until enough has been accumulated for a gorgeous display of blossom. Certain other flowers, again, depend for support upon starch or other nutriment laid by in the fleshy receptacle from which they spring. This is the case with the artichoke, the dandelion, and many of their sister composites. A third class lives upon materials stored up in the woody branches, as in the almonds, flowering cherries, and other trees, which bloom in the spring before the fresh leaves make their appearance. Yet a fourth sort maintain themselves cheaply upon the manufactured juices of other plants, like the leafless parasite, Rafflesia, whose flower measures three feet in diameter, or the pretty little English dodder, whose suckers fasten themselves tightly upon the growing stems of gorse. A great number of the most beautiful exotics are saprophytes, which live entirely upon the decaying vegetable mould in which they are embedded. Indeed, whenever showy flowers, like poppies and convolvulus, grow without the aid of some such accumulated nutriment, it will generally be found that their petals are thin and papery, so that the total cubical content of the flower-bud is really quite inconsiderable. Such plants, in fact, have learnt to make a very great display at very little actual expense.
Furthermore, flowers often exhibit different colours according to the state of oxygenation which their juices have reached, and these differences, as I shall endeavour to show hereafter, bear a definite relation to the various periods of maturity, and the particular insect whose assistance is required. Almost all blossoms in their early stages contain
green pigments and perform foliar functions; but as they mature, they gradually assume their proper hues of yellow, blue, or red. “The endochrome of the rudimentary petals,” says Mr. Sorby, “approximates in character to that of the leaves; and, during their development, their leaf-like character is gradually lost, and often new colouring matters are formed.” The series of changes may be easily followed in a hyacinth, a tulip, or a daffodil; but perhaps the garden hydrangea (H. hortensis) offers the best opportunity for watching this interesting phenomenon, because the structures in which the mauve or pink pigment finally appears are exposed to view during the whole process of maturation. Other changes also frequently take place after the flower is fully developed. “Cheiranthus chamæleo has at first a whitish flower, then a citron-yellow, then red or slightly violet; the petals of Stylidium fruticosum are pale yellow at first, then lightish rose-coloured; the flowers of Œnothera tetraptera are first whitish, then rose-coloured or nearly red; the corolla of Cobæa scandens is greenish-white the first day, and violet the day following; the flowers of Hibiscus mutabilis appear in the morning of a white colour, towards midday they become flesh-coloured, and at night they are red.” F. Müller has observed a Lantana at Sta. Catherina in Brazil, the flowers of which last three days, “being yellow on the first, orange on the second, purple on the third day;” and his interesting explanation of this peculiarity will find further mention when we come to treat of the parallel adaptation whereby insects have accommodated themselves to the colours of flowers. Indeed, Delpino believes that all such changes of hue are specially intended to inform the fertilising insects of the proper moment for effecting impregnation.
We conclude, then, with much probability, that the bright pigments of entomophilous plants are due originally to the natural oxidation taking place in all purely expensive structures, aided by the selective action of insects. It is noteworthy, as proving the functional origin of these pigments, that both great divisions of flowering plants, the monocotyledons and the dicotyledons, have independently hit upon the very same device of coloured leaves for attracting their insect allies. But this could hardly have happened had not some original groundwork existed in the mere fact of oxidation, upon which selective action might be successfully exerted. Still more clear does this argument become when we recollect that in almost every family under these two great divisions, anemophilous and entomophilous genera may be found side by side, thus proving that the device of colour has been independently adopted by different plants, not twice alone, but a thousand times over. Whenever brilliant leaves showed any tendency to appear in the neighbourhood of the floral organs, no matter what the species, genus, family, or class, it would seem that the plant thereby derived such an advantage as to perpetuate the habit in future, under the constant stimulus of over-population and natural selection, resulting in survival of the fittest.
