Popular Science Monthly/Volume 55/September 1899/The Colors of Northern Flowers
|THE COLORS OF NORTHERN FLOWERS.|
FOR profusion of bloom and brilliancy of coloring, the land of the tropics, with all its luxuriance of vegetation, can offer nothing to compare with a New England meadow in June. Along the great rivers of the South or in the islands of the East strange and beautiful flowers occur individually or in small groups, but the traveler looks in vain for myriads of blossoms giving a distinctive coloring to the landscape itself. It was long the popular notion that the colors of flowers were of no importance except as they gave human pleasure. This idea has been made familiar by a well known line of Gray's Elegy. It was a German pastor, Christian Conrad Sprengel, at the close of the last century, who first pointed out their true significance. So enthusiastically did he pursue his botanical studies that he neglected the duties of his office, and finally even omitted the Sunday sermon. The natural result followed, that he was deprived of his parish. In straitened circumstances he then sought unsuccessfully to maintain himself at Berlin by giving lessons in botany and Sunday excursions in search of plants. His book, now a botanical classic, attracted but little attention; his publisher did not even send him a copy of it, and in disgust he turned from the study of plants to that of languages. The title of the work, The Secret of Nature in the Form and Fertilization of Flowers Discovered, affords us the pleasure of knowing that he rightly estimated the importance of his observations, Sprengel clearly states that the bright hues of flowers, as is now well established, serve as signals to attract the attention of nectar-loving insects flying near by. He was led to this conclusion very fitly by the study of Myosotis, the "forget-me-not." He has not been forgotten. His name and theory were rescued from obscurity by Darwin; his book a few years ago was reprinted at Leipsic, and is now universally recognized, says H. Müller, as having "struck out a new path in botanical science."
A day's stroll through the fields and woodlands is sufficient to show that yellow and white blossoms are in Nature more common than red or blue. From an examination of 741 New England and Eastern species belonging to 48 families (see table) it appears that 164 are yellow, 283 white, 71 red, 136 blue and purple, and 87 green. Greenish flowers occur in 25 families, yellow in 29, white in 32, red in 16, purple and blue in 22.
Yellow appears to have been the first color developed, and flowers with this coloration are usually simple and regular in structure, as the buttercups and five-fingers. But why, it will be asked, should yellow have been the primitive color? The spores and spore-cases of the club mosses, and the pollen of all cone-bearing trees, and, in fact, of most plants, are yellow, and the yellow coloration of the first petals is doubtless correlated with this fact. Flowers of this tint are peculiarly attractive to yellow-banded flies, and when dull are avoided by beetles. Yellow flowers vary greatly in size, but pale yellow flowers are usually small, and bright or orange-yellow are large. Ranunculus ahortivus and R. sceleratus, which grow in wet places, are small and pale, while R. bulbosus and R. acris, the familiar buttercups of our meadows, are an inch broad. An apparent exception to the above rule is offered by the globe-flower (Trollius laxus), found in dense swamps, which has solitary, very large, pale greenish-yellow flowers. As the cultivated European and Asiatic species have bright yellow flowers, the coloring of the sepals of T. laxus, for the petals are wanting, has probably retrograded from growing in dense shade.
Yellow flowers in their natural state exhibit but little variation of color. They change most readily to white, and less often to red or blue. Under cultivation sudden variations from yellow to white have been observed. A double yellow hollyhock, according to win, suddenly turned one year into a single white kind, and a chrysanthemum has been seen to bear both yellow and white flowers. It will be observed in the accompanying table that in all families in which yellow flowers are common, white are also common, except in the Hyperaceæ, which contain no white-flowered species. Some species of mustard regularly fade to white, while many white flowers
The Predominant Colors of the Flowers of Ranunculaceæ to Cornaceæ in the Northern States.
|Magnoliaceæ||1||4||• •||• •||1||6|
|Anonaceæ||• •||• •||• •||1||• •||1|
|Menispermaceæ||• •||2||• •||• •||1||3|
|Berberaceæ||2||3||• •||• •||1||6|
|Nymphæeaceæ||3||2||• •||1||• •||6|
|Sarraceniaceæ||1||• •||• •||1||• •||2|
|Papveraceæ||4||2||2||• •||• •||8|
|Capparidaceæ||• •||1||• •||• •||• •||1|
|Resedaceæ||1||• •||• •||• •||• •||1|
|Violaceæ||4||6||• •||8||• •||18|
|Cistaceæ||4||• •||• •||• •||4||8|
|Droseraceæ||• •||3||1||• •||• •||4|
|Hyperaceæ||18||• •||2||• •||• •||20|
|Elatinaceæ||• •||• •||• •||• •||1||1|
|Caryophyllaceæ||• •||32||15||• •||6||53|
|Portulaceæ||1||• •||3||1||• •||5|
|Tiliaceæ||• •||2||• •||• •||• •||2|
|Camilliacæe||• •||2||• •||• •||• •||2|
|Linaceæ||3||• •||• •||1||• •||4|
|Rutaceæ||• •||2||• •||• •||1||3|
|Anarcardiaceæ||1||• •||• •||• •||5||6|
|Vitaceæ||• •||• •||• •||• •||7||7|
|Rhamnaceæ||• •||3||• •||• •||4||7|
|Celastraceæ||• •||• •||• •||2||1||3|
|Calycanthaceæ||• •||• •||• •||3||• •||3|
|Hammelaceæ||1||1||• •||• •||1||3|
|Halorageæ||• •||• •||• •||• •||9||9|
|Melastomaceæ||• •||• •||• •||3||• •||3|
|Lythraceæ||• •||1||• •||8||• •||9|
|Loasaceæ||1||• •||• •||• •||• •||1|
|Cactaceæ||3||• •||• •||• •||• •||3|
|Passifloraceæ||1||1||• •||• •||• •||2|
|Cucurbitaceæ||• •||2||• •||• •||1||3|
|Araliaceæ||• •||3||• •||• •||3||6|
|Cornaceæ||• •||11||• •||• •||2||13|
Darshow that they are descended from ancestral yellow forms by retaining vestiges of this color on the base of the petals, as in the water-crowfoot. The pale yellow flowers of Œnoiliera laciniata, of the cultivated Ribes aureum, and of Diervilla trifida in fading change to rose or red, exhibiting a tendency to develop red coloration. Aquilegia canadensis produces scarlet flowers, which are yellow inside and rarely all over. There are two other species in the Northern flora which exhibit similar coloring, Lonicera sempervirens and Spigelia marylandica, and the former is sometimes yellow throughout. Myosotis is at first pale yellow, and changes to sky-blue. But the best illustration of the transition from yellow to blue is exhibited by the violet family; the smallest and simplest species is yellow, the most highly specialized is blue, and all the intermediate stages are presented by Viola tricolor.
Honey-guides are exceedingly rare among yellow flowers. Cassia chamœcrista, which has nearly regular, showy yellow flowers, has two or three petals with a purple spot at base, while four of the anthers are yellow and six purple. It is interesting to compare with this flower the change of color presented by Arnebia. When the flower opens, each lobe of the yellow corolla is marked by a dark purple spot, which soon begins to fade, and by the next day has entirely disappeared. Saxifraga aizoides has golden flowers spotted with orange, and attracts a large number of insect visitors, and the yellow violets have their petals marked with dark-brown lines leading to the honey glands. Sulphur-yellow flowers are visited chiefly by bumblebees, and their coloration seems to have been developed by their selective influence from red or purple-flowered ancestors. Müller observed that the sulphur-yellow flowers of Sempervivum Wulfenii, wdiich are unlike the primitive yellow of the Crassulaceæ, are purple at base. This purple coloring he believed to be a remnant inherited from an earlier purple-flowered form. Hibiscus trionum, which is sulphur-yellow with a blackish eye, has perhaps been derived from a red-flowered ancestor, for the three other species of the genus are rose or flesh colored.
White flowers, in the opinion of the writer, are due to retrogression, and are derived from yellow, red, or blue, and in some instances from the primitive green, as in the involucre of Cornus. As a whole they present no advance in specialization over yellow flowers, and are often smaller and less conspicuous. When the petals of blossoms containing yellow, red, or blue pigments are placed in concentrated alcohol they turn to white. To produce these pigments is evidently more or less a tax upon the energies of the plant, which, whenever possible, is avoided. They are not present in the embryonic buds, and may not develop until they are well advanced in size. In Gentiana crinita the yellowish-white bud is nearly an inch long before the purple coloring appears, and the corolla always remains white at base. A stimulus to the growth of the plant makes itself apparent in the increased brilliancy of the flowers, as when they are exposed to clear sunlight or are treated with nitrate of soda, and may also be observed in the flushing of tulips, by which they lose their variegated colors when treated with strong manure. On the other hand, a check in nutrition and growth will cause a diminution of the perianth in size, accompanied by retrogression in color. When double red poppies are transplanted the whole plant is dwarfed, while the flowers are much smaller and pure white. This view of the origin of white flowers explains why they are the commonest in Nature, accounts for their being most numerous in families in which yellow flowers are likewise numerous, and why they are most true to name under cultivation. Many white flowers also exhibit other evidences of degeneration in their structure. Numerous species of Cruciferæ and Caryophyllaceæ have small white flowers, which regularly fertilize themselves; and in Lepidium, Stellaria, and Sagina the petals are sometimes present and in other instances are wanting.
White flowers often develop red or blue coloration. It is interesting to note that the red and white varieties of the hyacinth were derived from the wild blue form earlier than the yellow. Darwin gives an instance of a white and red rose produced on the same root, also of white and pink flowers on a single plant of Antirrhinum majus. Cratægus oxyacantha, a dark pink hawthorn, has been known to throw out a tuft of pure white blossoms. Every stage of the transition from white to red is placed before us by the rose family. The thorns are white, rarely tinged with rose; in the pear and apple the flowers are white, regularly shaded with red; and one of the Spiræas is rose, rarely white, while in the roses proper the six species are rose-colored, but the prairie rose changes to white. Under cultivation the wild geranium has been seen to produce upon the same plant both white and blue flowers. Good examples of the transition from white to blue and from blue to white may be met with in the Ranunculaceæ and Leguminosæ. Delphinium tricorne is bright blue, sometimes white, Viola canadensis has the petals white inside but the upper ones tinged with violet beneath, Astragalus has a part of the species white and a part purple, while it is common to find blue and white varieties of Hepatica growing on the same grassy bank. White flowers pass more readily into red, blue, or yellow than any one of these colors can be converted into any other, since it is easier to develop a new pigment than to transform one already existing. This is confirmed by the experience of florists, who always seek to obtain a white variety from which to develop the desired hue.
Red flowers are much rarer than blue, and both are seldom common in the same family. For instance, in the pink family red and white blossoms prevail, and there are no blue shades. The pinks are crimson and scarlet, often with elegant markings and a strong aromatic odor. The honey is deeply concealed, and they are visited almost exclusively by butterflies and millers. Twenty-eight species of diurnal Lepidoptera have been collected upon a single variety of Saponaria. Of the eighty species of Rosaceæ, thirteen are red and two purple, but the forty-four white flowers are very generally tinged or tipped with red. The two purplish-flowered species, Geum rivale and Potentilla palustris, belong to genera in which yellow predominates, and this primitive color is still evident in both their calyx and corolla. There are no blue or violet flowers. This family exhibits a marked tendency both in stem, leaf, bud, flower, and fruit to develop reddish coloration, a tendency which is probably due to the chemical constitution of the sap. There are no flowers in this family adapted to Lepidoptera, but they are visited by a mixed company of flies, beetles, and Hymenoptera. The smaller and less specialized Rosaceæ are yellow and white and are visited by a variety of short-lipped insects. With the increase of the flower in size and conspicuousness the number of insect visitors greatly increases, and the enlargement of the flower is attended by red coloration. Owing to the chemical constitution of the nutritive fluid, probably to its acidity (for when the petals of a rose are treated with ammonia they become blue), there has been no opportunity for the development of blue coloration by insects. With the enlargement of the perianth and the increased flow of sap, red tints have tended to appear by process of oxidation.
The correlation of red coloring with an increased flow of sap is well illustrated by the galls of the wild-rose tree, which are often "as rosy as the rosiest apple." An abnormal flow of sap is caused to the part stung by the insect, and red coloration is due to the action of light, for it is of no service to the plant. Again, when the flowers of Cratægus coccinea are stung by the gall-fly the different organs all become bright red, and the change in coloring is accompanied by an increase in size. In some instances red colors, according to Darwin, indicate greater vigor on the part of the plant, and I have also observed that the dwarfing of red flowers under cultivation may cause them to revert to white.
It was long, indeed, believed that the same species could not produce yellow, red, and blue flowers. But this doctrine, to use the words of Dr. Lindley, "must now be laid up in the limbo of pleasant dreams." This supposed law is contradicted by the hyacinth, pansy, Delphinium cardinale, and many other plants. Though red and blue coloring never occurs among the roses, a hyacinth has been seen to produce a perfectly pink and a perfectly blue blossom on the same truss, and the Borraginaceæ afford examples of flowers turning from red to blue in even a short space of time.
Blue is the highest color of the floral world, and is preferred by bees. Blue flowers are, as a rule, highly specialized both in form and color, and often possess marvelous mechanisms which aid in disseminating the pollen. This coloring is very common in the mint and pulse families, and in this district there are in the former forty-nine and in the latter sixty-one species of blue flowers. Their structure is such that few insects besides the long-tongued bees can gain access to the honey, and in some instances a single species of flower is visited by a single kind of bee, as one of the larkspurs by one of the bumblebees. While this high specialization of the flower may insure intercrossing, it is yet open to many objections, such as scarcity of proper guests, mechanical imperfections, perforation of the flowers by bees, and development of the perianth at the expense of the essential organs.
It is noteworthy that when genera occur containing three or more species they are seldom all blue or purple; one species at least, and frequently more than one, is yellow, white, or red. In Trifolium, T. pratense is rose-purple, T. repens white, and T. agrarium yellow. In the genus Astragalus a part of the species are violet or blue and a part white, and the same is true of Lespedeza and Vicia; in Lathyrus three species are blue-purple, one yellow, and one yellowish white. It is probably more advantageous in these genera for a part of the species to be of one color and a part of another than for all to be blue. When species are closely allied bees tend to visit them indiscriminately, as has been observed to be true of the buttercups, Spiræas, and golden-rods. During an afternoon the writer carefully collected the insect visitors to Solidago bicolor, our only cream-colored golden-rod. Both the number of species and of individuals taken was much larger than upon the yellow-flowered and more abundant varieties of this genus growing near by. There could be no doubt that the whitish coloration was beneficial in enabling insects to distinguish it more readily. Many purplish flowers are regular, often showing indications of degeneration, are devoid of honey, and are self-fertilized or adapted to Diptera, or, as in Hepatica, which is visited by bees for the pollen, open to a wide circle of visitors. In the sea purslane (Sesuvium maritimum), a prostrate maritime herb, there are no petals, but the five-parted calyx is purplish inside. The genus Ammannia of the Lythraceæ has the petals small, purplish, and in one species they are wanting; the axillary flowers of Bracenia purpurea are small and dull purple; in the common papaw the lurid purple flowers are large and adapted to Diptera, as are probably the lurid purple flowers of Calycanthus. Blue flowers may revert to red, white, or yellow. The fringed Polygala of Britain is usually bright blue, but often reverts to pink and white; there is a pure white variety of the blue-eyed grass; Mertensia virginica is purple-blue, rarely white; the larkspur is bright blue, sometimes white, and a white variety of the purple Trillium frequently occurs; there is, indeed, no improbability of a white-flowered form of every species being discovered. Viola calcarata is normally blue, but sometimes changes to the ancestral yellow.
The possession of a strong scent may, however, in many instances more than compensate for the absence of color. This is well illustrated in Lepidium sativum. The flowers are small and inconspicuous and in rainy weather do not fully open, yet, as it is odoriferous, Müller found it more abundantly visited by insects than any other crucifer. It is their strong odor, rather than their color, that renders so many umbellifers so attractive to a great variety of insects. Nocturnal flowers, which are visited by moths, are usually white and sweet-scented, though the evening primrose is yellow and Saponaria officinalis is rose-colored. Kohler and Schübeler have shown that a larger proportion of white flowers are fragrant than of any other color. Of 1,193 white flowers examined by them, 187 were odoriferous; of 951 yellow, 75; of 923 red, 85; of 594 blue, 31. But neither color nor odor will long alone serve to insure the visits of insects. The common elderberry exhibits the disadvantages which may attend the want of honey when there is but a limited supply of pollen. There are great masses of odoriferous flowers which convert the shrub into a huge bouquet, but it blooms at mid-summer, when it must contend with many nectar-yielding plants. As a result, it is almost wholly deserted by insects. Only four species of flies have been taken upon it, and repeatedly the blossoms were examined without discovering a single visitor, and yet upon the jewel-weed and the red-osier cornel, a few yards away, scores were at work.