On the colors of spring flowers

The Living age 1952, 19.11.1881

From The Popular Science Review.

By Alfred W. Bennett, M.A., B. Sc., F.L. S.

Lectures on Botany, St. Thomas's Hospital.

Every one must have noticed the variations in the predominant color of our wild flowers as the season advances from spring to summer and autumn. In our hedge-banks the pure white of the larger stitchwort and "Jack-by-the-hedge" gives way to the bright blue of the speedwell, and then to the reddish purple of the black horehound and the various shades of the mallows. In our meadows the golden-yellow buttercups are gradually replaced by the pink of the sorrels and ragged robins, and then by the yellow ragwort and purple knapweed. Our riversides are gay in the early spring with the golden marsh-marigold, in the early summer with the yellow flag, in the later summer with the purple loosestrife. The bright scarlet of the poppies and the pimpernel only appears with the ripening corn. The blue campanulas, the bright-yellow St. John's wort, the purple heather, do not brighten the landscape till the summer is in its prime, when the green or inconspicuous flowers of the hazel, the elm, the oak, and nearly all our timber trees, have long since passed away. I do not know, however, that these facts have ever been tabulated, or any attempt made to reduce them to a general law; the present article is intended as a contribution to this object as far as our early spring flowers are concerned.

Under the title of early spring flowers I include all those named in Hooker's "Student's Flora" as beginning to blossom not later than April, with a very few additions which I think ought also to be included, at all events in our southern counties, viz., Ranunculus bulbosus, Lamium album, and Myosotis collina. As my object is to ascertain the prevalent color of the spring flora, I have confined the list to common plants, excluding those of less general distribution which might obviously introduce an element of error into the average. For this purpose I have taken as my guide the last edition of the "London Catalogue of British Plants," and have struck out all which do not bear at least as high a number as fifty. Though this mode of limitation is not altogether satisfactory — as plants of wide distribution may nevertheless not be common — it is, I think, the best available. In the limitation of species, I have followed Hooker's "Student's Flora." The classification of colors, where so many shades are represented, is not easy. I finally decided on arranging them under five heads, viz., (1) white, (2) green, (3) yellow, (4) red and pink, (5) blue and violet. Very slight shades of color, as in Anemone nemorosa, Cardamine pratensis, and Oxalis acetosella, are neglected. The sweet violet is placed under two heads. Finally, several large natural orders in which the flowers are very inconspicuous are entirely passed over, viz., the Amentiferæ, Juncaceæ, Gramineæ, Cyperaceæ, and Coniferæ.

Out of a total of sixty-four species, there are 26 white, amounting to 40.5 per cent; 9 green, or 14.1 per cent; 13 yellow, or 20.3 per cent; 5 red or pink, or 7.3 per cent; and 11 blue or violet, or 17.4 per cent. I have not been able to prepare a similar list of our common summer and autumn flowers; but even without this there are a few points which strike one at once. Firstly, there is the very great preponderance of white flowers, which is certainly not the case at any other time of the year. Yellow is also greatly in excess as compared with other seasons; and the number of red and pink flowers is extremely small. It is obvious that if the excluded natural orders named above were restored, the plants belonging to them having mostly inconspicuous green or brown flowers, while some have bright yellow anthers, the proportion of red and blue in particular would he greatly diminished. The common cuckoopint is reckoned a green flower, from the color of the spathe which encloses the whole inflorescence.

Before attempting to draw any conclusions from these figures, it may be useful to compare them with those relating to some other spring flora, say that of Switzerland. The difficulties in the way of any exact enumeration are here very great. The best materials at my hand are the two volumes already published of Schoth's "Alpine Plants;" but these include only two hundred species selected for a special purpose. Although not entirely satisfactory, such a list may yield some trustworthy results. I have here taken May, instead of April, as the latest early spring month; and have no data from which to exclude any species on account of their rarity.

Out of fifty species in this list (one A ndrocace chamajasme, being again reckoned twice over among the whites and pinks), 18, or 36 per cent. are white; 1, or 2 per cent. green; 10, or 20 per cent. red or pink; and 8, or 16 per cent. blue or violet.

Several points of contrast between these two lists will at once suggest themselves. The very small number of green flowers in the second may no doubt be due partly to the fact that Seboth's work includes a selection only of Alpine flowers suitable for cultivation. But this will hardly account for the other differences; the smaller proportion of white flowers, and especially the very much larger proportion — about three and a half times as many of ted and pink flowers, a fact which will be in accordance with every one's recollection of the early spring flora of Switzerland. Now let us see whether we can arrive at any general conclusions from these data.

* Dull. Soc. Bot. France, xxvi. (1879), p. 249.In the first place, it must be borne in mind that the two colors white and green stand on a different footing from all the rest, and may be regarded as, more cm, reedy speaking, an indication of the absence of color. The color of green petals is not due to a mixture of blue and yellow pigments, but to the presence of chlorophyll, the ordinary green coloring matter of leaves. The bright colors of petals are not usually assumed till immediately before their emergence from the bud; and not a few — as for example those of Cobæa scandens, are still green when they first open, acquiring their proper color only on full exposure to the light and warmth. A white flower again does not owe its color to a milk-white fluid in the cells of the petals. but to the presence of air. Seeing, therefore, that the bright-colored fluid pigments of petals are formed only under the influence of a sufficient supply of light and heat, the large proportion of green and white in our early spring flowers is easily accounted for. Then with regard to yellow, I find an exceedingly interesting observation by M. Flahaut* that "a solid, insoluble pigment, the xanthine of Frémy and Cloëz, is in the first place to be distinguished from all the soluble coloring matters, blue, yellow, red, and their mixtures, all of which are acted on very readily by reagents, and which are usually formed only in the epidermal cells." This xanthine Frémy states to occur always in "the form of clearly defined grains, occasionally in the epidermal, much more often in the deeper-lying cells, slowly soluble in alcohol and potassa. It is in all probability a modification of chlorophyll." The following is a list of the plants in which he has detected it: Ranunculus, Primula, Cheiranthusm Galeobdolon luteum, Doronium plantagineum, Alyssum saxatile, Cypripedium Calceolus, Azalea chinensis, Uvularia grandiflora, Eranthis hyemalis, Forsythia viridissima, Tussilago Farfara.

* Acta hort. Petrop. VI. ii., p. 279.

** Bull. Soc. Bot. France, xxvii. p. 103. It is worth noting that these are without exception early, and some of them very early, spring-flowering plants. The colors, therefore, which pre-eminently distinguish our summer and autumn flora, the red, pinks, blues, and some yellows (not due to xanthine, but to a soluble yellow pigment), are caused by the presence of substances which require both a strong light and a high temperature for their production, and Professor Batalin has shown this to be especially the case with the red coloring substance.* That the same species of flower frequently assumes a more intense color with increasing altitude in the Alps is a matter of ordinary notice, confirmed by the exact observations of M. Bonnier,** who states that this change is due to an actual increase in the amount of coloring matter in the cells. The difference already pointed out between the prevailing colors of the spring flora in England and in Switzerland, seems to me to be due to the same cause. Owing partly to the spring being a month later, partly to the more southern latitude, and consequent greater elevation of the sun, partly to the clearer air of a high altitude, the light which opens the earliest spring flowers is much stronger in Switzerland than in England, causing the appearance of those brilliant roses and pinks of the silenes, ericas, and primulas, and blues of the gentianas, soldanellas, and phyteumas, with which we have, with the exception of our bluebells, scarcely anything to compare in our spring flora. In the list given above, the most striking feature of the early spring flora of Switzerland is seen to be the very large ingredient of red and pink: but I believe a more complete analysis would show an almost equal preponderance of blue.

* Alpenblumen; ihre Befruchtung durch insekten and ihre Anpassungen an dieselben. Leipzig, 1881.

** See Nature, 1880, vol. xxi., No. 535.I have not in this paper touched on the interesting subject of the adaptation of the various colors of flowers to fertilization by insect agency, about which much has been, and very much might be, written. As Hermann Mü11er points out in his most recent publication,* changes in the color or form of flowers which are serviceable to them for purposes of fertilization, can only be the result of external physical causes, and must be perpetuated by natural selection acting on heredity. This writer, who has made the subject specially his own, fully confirms the statement of the greater brightness of color of the flora of the Alps as compared with that of the plains, a result not only of the occurrence of brighter-flowered species, but also of the greater intensity of color in the same species. This he attributes to the greater transparency of the mountain air, and consequent more intense light, an explanation which is confirmed by the experiments of Siemens with the electric light.** The observations of Muller with regard to the prevalent colors of Alpine flowers are completely in accord with those stated above, viz., the comparative scarcity of white, and the remarkable prevalence of red and blue flowers; he further states that those flowers only are red or blue which are visited chiefly or exclusively by bees and "hover-flies" (Syrphidæ). It would be interesting to compare this fact with the time of year at which these groups of insects are most abundant.

* Proceedings of Edinburgh Botanical Society, 1876. The conclusions arrived at in this paper are somewhat at variance with those of Mr. Buchan,* who states that the blues, on the average, flower considerably the earliest; then, in order, the whites, purples, and lastly the yellows and reds. It is possible that the discrepancy may arise from Mr. Buchan having based his result on the entire English flora, while I have taken only the commonest flowers.