C.D. Warner, et al., comp.
The Library of the World’s Best Literature. An Anthology in Thirty Volumes. 1917.
The Coloration of Flowers
By Grant Allen (1848–1899)
T
Some hints of a progressive law in the direction of a color-change from yellow to blue are sometimes afforded to us even by the successive stages of a single flower. For example, one of our common little English forget-me-nots, Myosotis versicolor, is pale yellow when it first opens; but as it grows older, it becomes faintly pinkish, and ends by being blue, like the others of its race. Now, this sort of color-change is by no means uncommon; and in almost all known cases it is always in the same direction, from yellow or white, through pink, orange, or red, to purple or blue. For example, one of the wall-flowers, Cheiranthus chamæleo, has at first a whitish flower, then a citron-yellow, and finally emerges into red or violet. The petals of Stytidium fructicosum are pale yellow to begin with, and afterward become light rose-colored. An evening primrose, Œnothera tetraptera, has white flowers in its first stage, and red ones at a later period of development. Cobea scandens goes from white to violet; Hibiscus mutabilis from white through flesh-colored to red. The common Virginia stock of our gardens (Malcolmia) often opens of a pale yellowish green, then becomes faintly pink; afterward deepens into bright red; and fades away at the last into mauve or blue. Fritz Müller’s Lantana is yellow on its first day, orange on its second, and purple on the third. The whole family of Boraginaceæ begin by being pink and end with being blue. The garden convolvulus opens a blushing white and passes into full purple. In all these and many other cases the general direction of the changes is the same. They are usually set down as due to varying degrees of oxidation in the pigmentary matter. If this be so, there is a good reason why bees should be specially fond of blue, and why blue flowers should be specially adapted for fertilization by their aid. For Mr. A. R. Wallace has shown that color is most apt to appear or to vary in those parts of plants or animals which have undergone the highest amount of modification. The markings of the peacock and the argus pheasant come out upon their immensely developed secondary tail-feathers or wing-plumes; the metallic hues of sun-birds, or humming-birds, show themselves upon their highly specialized crests, gorgets, or lappets. It is the same with the hackles of fowls, the head ornaments of fruit-pigeons, and the bills of toucans. The most exquisite colors in the insect world are those which are developed on the greatly expanded and delicately feathered wings of butterflies; and the eye-spots which adorn a few species are usually found on their very highly modified swallow-tail appendages. So too with flowers: those which have undergone most modification have their colors most profoundly altered. In this way, we may put it down as a general rule (to be tested hereafter) that the least developed flowers are usually yellow or white; those which have undergone a little more modification are usually pink or red; and those which have been most highly specialized of any are usually purple, lilac, or blue. Absolute deep ultramarine probably marks the highest level of all.
On the other hand, Mr. Wallace’s principle also explains why the bees and butterflies should prefer these specialized colors to all others, and should therefore select those flowers which display them by preference over any less developed types; for bees and butterflies are the most highly adapted of all insects to honey-seeking and flower-feeding. They have themselves on their side undergone the largest amount of specialization for that particular function. And if the more specialized and modified flowers, which gradually fitted their forms and the position of their honey-glands to the forms of the bees or butterflies, showed a natural tendency to pass from yellow through pink and red to purple and blue, it would follow that the insects which were being evolved side by side with them, and which were aiding at the same time in their evolution, would grow to recognize these developed colors as the visible symbols of those flowers from which they could obtain the largest amount of honey with the least possible trouble. Thus it would finally result that the ordinary unspecialized flowers, which depended upon small insect riff-raff, would be mostly left yellow or white; those which appealed to rather higher insects would become pink or red; and those which laid themselves out for bees or butterflies, the aristocrats of the arthropodous world, would grow for the most part to be purple or blue.
Now, this is very much what we actually find to be the case in nature. The simplest and earliest flowers are those with regular, symmetrical open cups, like the Ranunculus genus, the Potentillas, and the Alsine or chickweeds, which can be visited by any insects whatsoever; and these are in large part yellow or white. A little higher are flowers like the Campions or Sileneæ, and the stocks (Matthiola), with more or less closed cups, whose honey can only be reached by more specialized insects; and these are oftener pink or reddish. More profoundly modified are those irregular one-sided flowers, like the violets, peas, and orchids, which have assumed special shapes to accommodate bees and other specific honey-seekers; and these are often purple and not unfrequently blue. Highly specialized in another way are the flowers like harebells (Campanulaceæ), scabious (Dipsaceæ), and heaths (Ericaceæ), whose petals have all coalesced into a tubular corolla; and these might almost be said to be usually purple or blue. And finally, highest of all are the flowers like labiates (rosemary, Salvia, etc.) and speedwells (Veronica), whose tubular corolla has been turned to one side, thus combining the united petals with the irregular shape; and these are almost invariably purple or blue.