Attractive Odours in Flowers.—The sweet odours of flowers, like their colours, seem often to have been developed as an attraction or guide to insect fertilizers, and the two phenomena are often complementary to each other. Thus, many inconspicuous flowers—like the mignonette and the sweet-violet, can be distinguished by their odours before they attract the eye, and this may often prevent their being passed unnoticed; while very showy flowers, and especially those with variegated or spotted petals, are seldom sweet. White, or very pale flowers, on the other hand, are often excessively sweet, as exemplified by the jasmine and clematis; and many of these are only scented at night, as is strikingly the case with the night-smelling stock, our butterfly orchids (Habenaria chlorantha), the greenish-yellow Daphne pontica, and many others. These white flowers are mostly fertilized by night-flying moths; and those which reserve their odours for the evening probably escape the visits of diurnal insects, which would consume their nectar without effecting fertilization. The absence of odour in showy flowers, and its preponderance among those that are white, may be shown to be a fact by an examination of the lists in Mr. Mongredien’s work on hardy trees and shrubs.[22] He gives a list of about 160 species with showy flowers, and another list of sixty species with fragrant flowers: but only twenty of these latter are included among the showy species, and these are almost all white flowered. Of the sixty species with fragrant flowers, more than forty are white, and a number of others have greenish, yellowish, or dusky and inconspicuous flowers. The relation of white flowers to nocturnal insects is also well shown by those which, like the evening primroses, only open their large white blossoms after sunset. The red Martagon lily has been observed by Mr. Hermann Müller to be fertilized by the humming-bird hawk moth, which flies in the morning and afternoon when the colours of this flower, exposed to the nearly horizontal rays of the sun, glow with brilliancy, and when it also becomes very sweet-scented.
[22] Trees and Shrubs for English Plantations, by Augustus Mongredien. Murray, 1870.
Attractive grouping of Flowers.—To the same need of conspicuousness the combination of so many individually small flowers into heads and bunches is probably due, producing such broad masses as those of the elder, the guelder-rose, and most of the Umbelliferæ, or such elegant bunches as those of the lilac, laburnum, horse chestnut, and wistaria. In other cases minute flowers are gathered into dense heads, as with Globularia, Jasione, clover, and all the Compositæ; and among the latter the outer flowers are often developed into a ray, as in the sunflowers, the daisies, and the asters, forming a star-like compound flower, which is itself often produced in immense profusion.
Why Alpine Flowers are so Beautiful.—The beauty of alpine flowers is almost proverbial. It consists either in the increased size of the individual flowers as compared with the whole plant, in increased intensity of colour, or in the massing of small flowers into dense cushions of bright colour; and it is only in the higher Alps, above the limit of forests and upwards towards the perpetual snow-line that these characteristics are fully exhibited. This effort at conspicuousness under adverse circumstances may be traced to the comparative scarcity of winged insects in the higher regions, and to the necessity for attracting them from a distance. Amid the vast slopes of debris and the huge masses of rock so prevalent in higher mountain regions, patches of intense colour can alone make themselves visible and serve to attract the wandering butterfly from the valleys. Mr. Hermann Müller’s careful observations have shown, that in the higher Alps bees and most other groups of winged insects are almost wanting, while butterflies are tolerably abundant; and he has discovered, that in a number of cases where a lowland flower is adapted to be fertilized by bees, its alpine ally has had its structure so modified as to be adapted for fertilization only by butterflies.[23] But bees are always (in the temperate zone) far more abundant than butterflies, and this will be another reason why flowers specially adapted to be fertilized by the latter should be rendered unusually conspicuous. We find, accordingly, the yellow primrose of the plains replaced by pink and magenta-coloured alpine species; the straggling wild pinks of the lowlands by the masses of large flowers in such mountain species as Dianthus alpinus and D. glacialis; the saxifrages of the high Alps with bunches of flowers a foot long as in Saxifraga longifolia and S. cotyledon, or forming spreading masses of flowers as in S. oppositifolia; while the soapworts, silenes, and louseworts are equally superior to the allied species of the plains.
[23] Nature, vol. xi. pp. 32, 110.
Why Allied Species of Flowers Differ in Size and Beauty.—Again, Dr. Müller has discovered that when there are showy and inconspicuous species in the same genus of plants, there is often a corresponding difference of structure, those with large and showy flowers being quite incapable of self-fertilization, and thus depending for their very existence on the visits of insects; while the others are able to fertilize themselves should insects fail to visit them. We have examples of this difference in Malva sylvestris, Epilobium augustifolium, Polygonum bistorta, and Geranium pratense—which have all large or showy flowers, and must be fertilized by insects—as compared with Malva rotundifolia, Epilobium parviflorum, Polygonum aviculare, and Geranium pusillum, which have small or inconspicuous flowers, and are so constructed that if insects should not visit them they are able to fertilize themselves.[24]
[24] Nature, vol. ix. p. 164.
Absence of Colour in Wind-fertilized Flowers.—As supplementing these curious facts showing the relation of colour in flowers to the need of the visits of insects to fertilize them, we have the remarkable, and on any other theory, utterly inexplicable circumstance, that in all the numerous cases in which plants are fertilized by the agency of the wind they never have specially coloured floral envelopes. Such are our pines, oaks, poplars, willows, beeches, and hazel; our nettles, grasses, sedges, and many others. In some of these the male flowers are, it is true, conspicuous, as in the catkins of the willows and the hazel, but this arises incidentally from the masses of pollen necessary to secure fertilization, as shown by the entire absence of a corolla or of those coloured bracts which so often add to the beauty and conspicuousness of true flowers.
The Same Theory of Colour Applicable to Animals and Plants.—It may be thought that this absence of colour where it is not wanted is opposed to the view maintained in the earlier part of the [preceding chapter], that colour is normal and is constantly tending to appear in natural objects. It must be remembered, however, that the green colour of foliage, due to chlorophyll, prevails throughout the greater part of the vegetable kingdom, and has, almost certainly, persisted through long geological periods. It has thus acquired a fixity of character which cannot be readily disturbed; and, as a matter of fact, we find that colour rarely appears in plants except in association with a considerable modification of leaf-texture, such as occurs in the petals and coloured sepals of flowers. Wind-fertilized plants never have such specially organized floral envelopes and, in most cases, are entirely without a calyx or corolla. The connection between modification of leaf-structure and colour is further seen in the greater amount and variety of colour in irregular than in regular flowers. The latter, which are least modified, have generally uniform or but slightly varied colours; while the former which have undergone great modification, present an immense range of colour and marking, culminating in the spotted and variegated flowers of such groups as the Scrophularineæ and Orchideæ. The same laws as to the conditions of a maximum production of colour are thus found to obtain both in plants and animals.
Relation of the Colours of Flowers and their Geographical Distribution.—The adaptation of flowers to be fertilized by insects—often to such an extent that the very existence of the species depends upon it—has had wide-spread influence on the distribution of plants and the general aspects of vegetation. The seeds of a particular species may be carried to another country, may find there a suitable soil and climate, may grow and produce flowers; but if the insect which alone can fertilize it should not inhabit that country, the plant cannot maintain itself, however frequently it may be introduced or however vigorously it may grow. Thus may probably be explained the poverty in flowering-plants and the great preponderance of ferns that distinguishes many oceanic islands, as well as the deficiency of gaily-coloured flowers in others. This branch of the subject is discussed at some length in my Address to the Biological Section of the British Association,[25] but I may here just allude to two of the most striking cases. New Zealand is, in proportion to its total number of flowering-plants, exceedingly poor in handsome flowers, and it is correspondingly poor in insects, especially in bees and butterflies, the two groups which so greatly aid in fertilization. In both these aspects it contrasts strongly with Southern Australia and Tasmania in the same latitudes, where there is a profusion of gaily-coloured flowers and an exceeding rich insect-fauna. The other case is presented by the Galapagos islands, which, though situated on the equator off the west coast of South America, and with a tolerably luxuriant vegetation in the damp mountain zone, yet produce hardly a single conspicuously-coloured flower; and this is correlated with, and no doubt dependent on, an extreme poverty of insect life, not one bee and only a single butterfly having been found there.