by Grant Allen
When we pass on to examine the various parts of the flower which may thus become devoted to the attractive function, we find still plainer evidence to the same effect. The essential floral organs themselves, already so conspicuous in the various catkins, may be specially modified for the sake of displaying brilliant pigments. The common meadow rue depends almost entirely for attraction upon these organs. In the family of Mesembryanthemums, the outer stamens become flattened and petaloid, so as to resemble the corolla of ordinary flowers. In the water-lilies, the tendency towards a similar change is always noticeable. Indeed, if one may hazard a guess in so uncertain a question, analogy would rather lead us to suppose that all petals are modified stamens than that the transition has taken place in the opposite direction. However this may be, the corolla, or petaline whorl, forms in most flowers the main attractive organ. Roses, buttercups, violets, blue-bells, and primroses may stand as sufficient examples. Next in order comes the calyx, or sepaline whorl, usually a protective organ, but often so modified as to aid in the function of alluring the insect guests. In the fuchsia, the bright sepals make the most striking part of the whole blossom; while in the tulip, crocus, and other brilliant monocotyledonous plants, both sepals and petals are coloured alike, so as to be usually lumped together under the common name of perianth pieces. In the marsh marigold, the marvel of Peru, the purple clematis, and the crimson Aristolochia cordata, the petals are wholly wanting, and the calyx alone performs the task of ostentatious chromatic display.
Nor does the process of colouration stop short at the regular floral whorls. The bracts and other secondary adjuncts often aid in the attractive effect. Several euphorbias have separately inconspicuous flowers, enclosed in a common involucre of the most beautiful scarlet hue. Poinsettia pulcherrima bears tiny yellow blossoms, which would doubtless fail by themselves to catch even the microscopic eye of a tropical butterfly; but they are surrounded by a thick mass of gorgeous crimson bracts, so strikingly lovely as to ensure for the plant a place in all our great conservatories. The various arums bear their minute flowers on a yellow spadix, about which grows a huge white or purply-green sheath, known as a spathe, whose large size and bright colour makes up for the relative inconspicuousness of the essential organs. In short, whatever part happened to display a tendency towards bright colouration, and thereby attracted the attention of insects, would naturally grow more and more prominent from generation to generation, till it reached the furthest limit of useful expenditure.
That the colour of the flower is a mere intensification of that prevailing in the stem has long since been recognised by painters. In some cases, as in Peperomia, the hue of the stem becomes itself very noticeable. In others, as in Echeveria, the stalk and bracts are pinkish, gradually growing deeper till we reach the calyx, while the petals themselves appear simply as an intensified form of the surrounding tint. In Epiphyllums, the end of the leaf-like peduncle is often bright red like the blossom itself. Amongst English plants, Echium, Sedum, Chrysosplenium, Rumex, and many other genera, show like phenomena. And when, as in the parasites and saprophytes, the stem and scales have no special reason for greenness, we find such brilliant examples as Lastræa, Monotropa, Neottia, and Corallorhiza, whose rudimentary leaves are quite as beautifully coloured as the flowers themselves.
From whatever point of view we regard the question, then, it seems equally probable that even before insect selection had come into play certain flowers would show a considerable tendency to the production of adventitious colours. Wherever such patches of red or blue shone out among the prevailing green of primitive forests, we may be sure they would act as beacons to the rudimentary eyes of unspecialised insects. At first their colours would doubtless be arranged in very indefinite patches; but as they were gradually selected by their insect visitors, the effects of cross-fertilisation, by weeding out individual peculiarities, would make their shape and hue more and more definite with each new generation. For such definiteness, as we shall observe abundantly hereafter, is a mark of contradistinction between adventitious and purposive colouration. Wherever we find a plant, like the common West Indian Bromelia pinguin, in which the spathes are coloured brightly but irregularly, the crimson fading off into white or green, we may fairly conclude that the selective process has not yet proceeded very far. But when we get a definite bunch of crimson bracts, as in Poinsettia pulcherrima, standing apart as a regular mass from the green foliage below, we may be sure that the selective process has continued for a considerable period of time; while in the three constant coloured leaves which surround the little blossoms of the Bougainvillea, we see a still further progress in numerical definiteness. So, too, if we compare the English cuckoo-pint with the Æthiopian lily (Richardia africana), or the apple with the orange, we shall see reason to believe that the former cases represent a relatively incomplete, and the latter cases a relatively complete, stage of the differentiating action. And we shall observe hereafter, when we come to examine the origin of bright-coloured fruits, that these structures, which have been developed to suit the eyes of birds and mammals, and are therefore comparatively late in geological time, possess on the whole much less definite colours than entomophilous flowers, which have been developed to suit the eyes of insects, and date far back in geological time.
The first step towards definiteness in colouration is gained by that dwarfing of the internodes which gives the floral whorls their circular appearance. The earliest entomophilous flowers probably belonged to the dicotyledonous group, which now exhibits the highest differentiation of any; but they consisted of separate petals, like the common dog-rose, instead of being tubular or bell-shaped, like the honeysuckle or the campanula. Gradually, however, the various petals in certain cases became adnate, that is to say, developed together, so as to form a single indented corolla. The former class of flowers are known as polypetalous, the other as gamopetalous. At a still later date came the irregular flowers, like the labiates and orchids, which are specially adapted to the shapes of insects; while the differentiating process is doubtless still going on under our very eyes whenever a bee visits a blossom in the meadows around us.
Side by side with this differentiation of various flowers went the differentiation of flower-haunting insects. Even in the Carboniferous world some vagrant species of that great class already lived in the hard siliceous underbrush; but Sir John Lubbock believes that Hymenoptera, Hemiptera, and Diptera first came into being during the Cretaceous era; while Lepidoptera, or butterflies, did not appear until the Tertiary times. Beetles first exhibit evident marks of flower-feeding during the Miocene epoch. As for honey-bees, they probably represent the very latest and most highly differentiated members of the whole class, and they could hardly have reached their present form till a very late period. In short, if we look at the correlation of the flowers and the insects, we shall see reason to believe, what is already suspected on purely palæontological grounds, that gamopetalous flowers could not be developed before the rise of specialised insects having a proper proboscis fitted for fertilising their bloom.
Again, the entomophilous monocotyledons are probably far more modern in date than the bright-coloured dicotyledons, and they are also on the whole far more leaf-like and less definite. Most of them consist of six perianth pieces, shaped very much like the ordinary leaves, and seldom having any specialised features. Yet, as they found the field already occupied by bright-hued dicotyledons, it was necessary, if they would secure the attention of insects, to bid for their favour by very large and showy blossoms. Accordingly, these newest comers amongst the insect-fertilised plants form a large proportion of our choicest garden species. It will suffice merely to enumerate the iris, crocus, narcissus, daffodil, snowdrop, amaryllis, aloe, tulip, tiger-lily, fritillary, crown-imperial, tuberose, hyacinth, star of Bethlehem, meadow-saffron, hellebore, arum, and Æthiopian lily, to show how many of the most brilliant flowers belong to this class. Even here, however, a large number of species have advanced to a high degree of differentia
tion, due to the agency of insects. While many lilies have six separate perianth pieces, as we see in the tulip and the fritillary, others, like the lily of the valley, have become quite gamopetalous, or, to speak more correctly, the petaline and sepaline whorls have coalesced into a bell-shaped cup. But the orchid family display the most curious adaptations of all, being modified in an infinite variety of ways to suit the insects of their several countries, and presenting the most marvellous tricks of mimicry, mechanical device, and sportive cunning, which at first sight almost compel us to imagine an inherent consciousness guiding the blind course of their strange developments.
It has been remarked, too, that, as a rule, flowers whose forms are highly modified, so as to admit of fertilisation with considerable certainty by a single insect visitor, do not need the same large display of showy corollas as those which trust almost to chance for the conveyance of their pollen to the proper receptacle. Thus Sprengel contrasts the great size and numerous petals of the water lily, whose shape has no special reference to the organs of the fertilising insect, with the little labiates, whose form ensures the due application of the pollen at every visit. So, too, we may compare the common orchid with the fritillary, the lily of the valley with the tulip, and the composites with the rose family. Of course many interfering causes must be understood as putting a limitation upon the truth of this roughly generalised statement. For example, the great tropical butterflies, the larger bees, and the humming-birds, form fertilising agents who naturally demand large masses of colour as an attraction; or, again, the presence of scent, honey, or other special allurements, may make up in particular cases for the lack of bright corollas. Yet, on the whole, it may be said that, other things equal, high modification in form is accompanied by a decreased expenditure on coloured adjuncts.
Nor is it only in the shape and colour of individual flowers that plants vie with one another for the favours of their insect guests. Like varieties are also to be found in the mode of massing the blossoms so as to attract from a great distance the eyes of passing bees or butterflies. We must remember that the facets of the articulate visual organ are not adapted for perceiving small objects except at a comparatively close range. Hence those plants which can group their several blossoms into large and conspicuous bunches may derive special advantages from the extra attractiveness thus attained. Such species as the peony or the tulip bear a single terminal blossom at the end of their stalk. Others, like the pimpernel or the veronica, have a few tiny flowers half hidden at the axes of the leaves. But the hyacinth, the laburnum, and the lilac, group their bloom into large upright or hanging masses; while the cowslip, the carrot, and the calceolaria produce flattened heads which strike the eye from a considerable distance. The dog-rose, with its scattered flowers, does not catch our passing glance so readily as the apple-tree or the may; and the great tropical flowering forest trees may often be discerned by human sight at almost incredible distances for the stay-at-home European.
But the composite plants offer by far the most instructive example of the effect produced after many generations of unconscious selection by the visits of insects. The first approach toward their mode of aggregation may be seen in the head of clover, where a number of separate little pea-blossoms are collected into a compact assemblage by the shortening of their several stalks. In the scabious we find the like tendency carried still further by the addition of a broad receptacle and a bunch of surrounding leaves, known as an involucre, which fulfils the protective functions of a calyx for the compound group. The real calyx, however, on each single blossom, still retains its original form, and doubtless assists in the performance of its proper office. But in the true composites, like daisies or dandelions, the separate flowers have almost merged their distinct individualities in that of the complex whole. The calyx has become degraded into a mere bundle of hairs (known as a pappus), which serves as a float for the mature seed, and forms the “clock,” blown away by village children from the withered dandelion head, as well as the gossamer-like wings that carry the thistle seeds among the farmer’s corn. The involucre here usurps the whole protective function: and the head of flowers is mistaken by the ordinary human observer for a single blossom. But if we look close into the daisy, we see that its centre comprises a whole mass of little yellow bells, each of which consists of corolla, stamens, and pistil. The insect who alights on the head can take his fill in a leisurely way without moving from his standing-place; and meanwhile he is proving himself a good ally to the plant by fertilising one after another of its numerous ovaries. Each tiny bell by itself would prove too inconspicuous to attract much attention from the passing bee; but union is strength for the daisy as for the state, and the little composites have found their co-operative system answer so well, that late as was their appearance upon the earth, they are generally considered at the present day to be the most numerous family both in species and individuals of all flowering plants.
Nor has the process of differentiation stopped even here. Amongst the composites themselves great variety may be observed in the means adopted for the attraction of insects. The simplest form of composite head, which we see in the thistle and the artichoke, consists of uniform flowers, none differing in shape or colour from their neighbours. The common English centaury shows an intermediate stage, in which the outer florets are longer and larger than those in the centre of the head. The sunflower and the ragwort advance a step farther in the same direction, their outer florets having become ray-shaped or ligulate, but still preserving the yellow hue of the central mass. The ray florets, in these cases, practically fulfil the functions of petals, while the inner blossoms continue to act as true floral organs. Finally, in the daisy and in many chrysanthemums, the outer florets, besides being prolonged into petal-like rays, are coloured white, pink, mauve, or blue, while the central mass retains its original colouration. Here we find the external row of flowers quite diverted from its true purpose, and devoted almost exclusively to the attractive function.
Even now we have not yet arrived at the last stages of the differentiating process. The complex heads of flowers thus formed again unite into still more complex masses. The daisy and the sunflower bear only one composite head on each stalk, but the common thistle produces a whole mass of heads in a kind of umbel, and the ragwort has bunches of such umbels growing together side by side. In the groundsel, each head of flowers looks like a single blossom; in milfoil, the umbellate form is almost exactly reproduced in still wilder profusion; while the pretty waving golden-rod caps the climax by collecting compound bundles of heads into a many-branched and multitudinous plume. Flowers too small to succeed individually thus succeed in serried masses; and masses, again, too small for success in single complexity, achieve attention in their turn by reuniting into yet more complex groups.
As to the special colouring matter employed in each case, but little can be said as yet about its determining causes. In a few cases, indeed, we can conclude with some probability that the existing hue has been developed because it subserved, as such, some special function. Thus night-flowering plants are usually pure white or pale yellow, the very colours best adapted for scattering the scanty moonbeams or the dying twilight, and so attracting the eyes of moths and other crepuscular insects. Again, Rafflesia, Hydnora, Stapclia, and many other fetid flowers, which obtain fertilisation by deceiving flies through their resemblance to putrid meat, imitate the lurid appearance as well as the noisome smell of carrion. Many orchids are believed to be coloured in mimicry of insects, either for the sake of attraction or of protection from hurtful creatures. Other flowers appear to cater specially for the peculiar tastes of certain insects, which exhibit a preference for red, blue, yellow, or orange, as the case may be, and these will receive more extended treatment in the succeeding chapter. Sir John Lubbock thinks that the lines or spots on many flowers act as guides for the bees, pointing out the exact spot where the honey may be found; and Fritz Müller suggests that their changing hues serve as timepieces to show the right
moment for effecting fertilisation. But in the majority of cases we cannot point to any such special determining cause for the particular hue which we find in nature. It is known that the colouring matters of flowers may be divided into two classes, the xanthic and the cyanic, whose types are respectively yellow and blue; and these two classes do not readily pass into one another. Thus, we cannot have a blue rose or a blue dahlia, though we may vary the hues of either blossom by proper treatment almost indefinitely within the prescribed limit. Hence, it might appear that each flower produced as a rule those colours which most readily result from the chemical properties of its constituents, varying the tint, so far as possible, under the influence of insect selection, in accordance with the nature of the percipient eye, of the surrounding foliage, and of other adventitious circumstances in the environment. It might well happen, however, in the majority of cases, that any bright colour would equally answer the attractive purpose, supposing only it contrasted sufficiently with the green leaves or other objects in the natural background. Such, at least, we know to be the fact with the eye of man, who is struck indifferently by the golden orange, the ruddy strawberry, the rosy-cheeked mango, or the purple grape.
