Colours Vegetable

Colours, Vegetable, fall into four classes - (i) those in the cell-walls of bark; (ii) those in woods, as in logwood; (iii) those dissolved in cell-sap, as in the blues, violets, and some reds in flowers; and (iv) those connected with protoplasmic granules, such as chlorophyll (q.v.), etiolin, xanthophyll, anthoxanthin, phycoxanthin, and phycoerythrin. Those of the three first-named classes are probably derivatives of tannin (q.v.). Etiolin, a pale yellow, seems to be an antecedent or constituent of chlorophyll. Xanthophyll in fading leaves, and anthoxanthin in yellow flowers, seem to be derivatives of chlorophyll, probably degradation-products. Phycoxanthin and phycoerythrin, characteristic of the olive and of the red sea-weeds respectively, are more soluble substances. The chemistry of these substances is almost unknown, nor is there any evidence of their having in many cases acquired a physiological importance beyond that of mere excreta. It is otherwise with colours in the floral region. Colours occurring here fall into two series: the cyanic, blue, violet, and red in solution, and the xanthic, yellow, orange, and red in chromoplastids. The two series seldom co-exist in one flower. Many large groups of plants have a limited colour-range, e.g. the Umbelliferae, nearly all white- or yellow-flowered; the Cruciferae, the same, with a few reds; the Caryophyllaceae, mostly red and white. Colour and perfume are frequently correlated, many white flowers being sweet-scented, especially in the evening; whilst many brownish flowers have a scent of carrion. This seems to be a close adaptation to cross-pollination by dusk-flying moths in the one case, and by carrion-flies in the other. Wasps seem partial to orange flowers, and humming-birds to red ones. A connection of predominating flower-colours with latitude - reds being most tropical, blues less so, and yellows and whites more polar - coupled with the opening of flowers in spring and summer in order of colour - blues on an average first, then whites, purples, yellows, reds - suggests a close connection between floral colour and the intensity of the sun's light. The whole subject has, however, been the theme of more hasty guesses than sober investigations.