The Colors of Northern Polypetalous Flowers

Abstract

1. Throughout the Choripetalæ, with few exceptions, conspicuousness is correlated with fertilization by insects. In the grape family the flowers depend upon their strong scent rather than upon bright coloring. In many genera it is possible to arrange the species in a progressive series, in which there is an advance from inconspicuousness, few visitors, and self- fertilization, to many visitors, great conspicuousness, and the loss of the power of self-fertilization. Pigments may be developed in all the organs of the inflorescence, as bracts, stems, sepals, petals, stamens, and pistils. 2.The green flowers of the Polypetalæ are small, and the petals are frequently wanting. The white and yellow flowers vary from small to large, are the most common, and contrast more strongly with the foliage than purple or blue. Of the seventy-one polypetalous families, forty-three contain white, forty-one yellow, and twenty-nine both kinds of flowers. White flowers are most common in families or genera containing shrubs and trees, small flowers aggregated in a dense inflorescence, and nocturnal flowers. Dark nocturnal flowers are strongly scented. Yellow flowers are more commonly herbaceous and are most abundant in the same families as white flowers, unless the species are shrubs or trees. 3. There is no evidence of the preference of beetles for flowers of any particular color. They do not avoid dull yellow. They are most common on small, white-clustered flowers with easily accessible honey and pollen. Diptera visit most frequently white and yellow flowers, but as they become more specialized and restrict themselves to flowers the percentage of visits to red and blue flowers increases. They appear to find a parti-colored, mottled, or dotted inflorescence, as in the Cruciferæ and Saxifragaceæ, attractive. Carrion flies prefer malodorous luridpurple or flesh-colored flowers. 4. The changes of color and their sequence in individual flowers are noteworthy. Green changes to white (Cornus), to yellow (Thlaspi, Cardamine), to red (Hydrangea), to purple (Clematis), to violet (Cobæa); white changes to green (sepals of Helleborus niger), to yellow (Lantana), to red (Dianthus, Hibiscus mutabilis), to blue (many large blue flowers remain white until nearly ready to expand); yellow changes to white (Draba), to red (æsculus), to blue (Myosotis); red changes to blue (Venetus and many Boraginaceæ); violet and blue may turn purple, green, or white in fading. The tendency of green, white, and yellow to change to red or blue is much stronger than the reverse. 5. The floral colors are often correlated with the colors of the stems and leaves, as in Sedum. The foliage of the plants with white flowers is, as a rule, paler than when the flowers contain pigments. The development of bright colors in autumn leaves presents a series of color changes, which are in part parallel to those which occur in flowers. With the disappearance of the chlorophyll the leaves become whitish, yellow, or red, according as the cells contain no pigment, or solid yellow granules, or red pigment dissolved in the cell sap. The leaves of many plants are yellowish green, due to the presence of a yellow pigment. Green, yellowish-green, and greenish-yellow flowers contain chlorophyll, and though usually small are occasionally of large size. Many white and yellow flowers are derived directly from the primitive green. White is usually a structural or optical color due to the unequal reflection and refraction of light by the intercellular air spaces and the cells devoid of pigment. White flowers are a less tax upon the energies of the plant. Flowers of all colors may revert to white, which is commonest in nature and most true to name under cultivation. If with the disappearance of the chlorophyll there is an insoluble yellow pigment in the cells, the petals are a pale yellow, and with its increase change to bright yellow or orange. The development of anthocyanin, or red pigment, dissolved in the cell sap, changes white flowers to red and yellow flowers to scarlet. With a decrease in the acidity of the cell sap the red flowers become blue. Müller's observations led him to the conclusion that the honeybee prefers blue, violet, various shades of purple and red, to white and yellow, and avoids scarlet and lurid colors. 6. The formation of pigments is effected by the chemical composition of the soil, by altitude or the intensity of light, by latitude, and by the absence or presence of moisture, as well as other ecological forces. The particular coloration of flowers is largely a chemical problem.