Pigment transactions between animals and plants.

Summary 1. Animals, in common with plants, are capable of elaborating their own supplies of tetrapyrrolic pigments, i.e. the porphyrins and bilichromes, as well as pterins and certain indolic biochromes, including melanins and indigoids. But they must depend upon plants for the primary synthesis of other vitally important biochromes including flavins, flavonoids, quinones, and notably the carotenoids. These must be assimilated by animals, directly or ultimately from the plant kingdom. 2. Colour is expressed in certain large organic molecules as a consequence of chemical resonance, evoked by the presence of unsaturated intra-molecular bonds. This condition, allied closely with chemical instability and metabolic reactivity, thus underlies certain biocatalytic functions, fulfilled for example by some biochromic molecules, such as oxidative enzymes, and some vitamins and hormone-like regulators. 3. Commonest examples of such biocatalysts include the several classes named above, conspicuously such porphyrinic tetrapyrroles as chlorophyll, haemoglobin, and the oxidases and peroxidases, including cytochromes and catalase. Among the open-chained members or bilichromes, we find some of these in red algae; phyto-chrome initiates numerous vital biochemical processes in green plants; and bilirubin and biliverdin are found in blood and bile of animals. 4. Among the pterins are members which simulate closely some physiological functions of vitamins B1 and B2. Some promote sexual activity in aphids, and in the royal jelly of honeybees, they determine whether a hatchling shall develop into a queen or a neuter worker. 5. Riboflavin, acquired in minute but fundamentally necessary amounts by animals' consumption of plants, whether directly or indirectly, is a unit of the vitamin B2 complex, and is stored either unmodified or conjugated with the animal's protein. 6. Flavonoids and quinones are similarly acquired from ultimate plant sources. Such compounds may or may not undergo minor chemical modifications within the bodies of consumers. Quercitin, a flavonoid, favours normality of the eye-lens, skin and blood capillaries, while, among the naphthoquinones, the K vitamins ensure blood coagulability. The benzoquinones include the Q-enzymes or ubiquinones, which serve as metabolic oxidative catalysts. 7. Integumentary melanins, derived from oxidative degeneration of tyrosine, chiefly by animals as well as by certain plants, may serve usefully in screening underlying tissues against injurious light rays or, in insects, as well as in some cold-blooded vertebrates, may be capable of effecting either concealment or advertisement. Related to these so-called indole pigments are the indigoids which are breakdown products of tryptophan, and are encountered chiefly in excretory materials, some especially under pathological conditions. Dibromindigo, an ancient dye recovered from certain marine gastropods, is something of an enigmatic exception. 8. Among the carotenoids, manufactured de novo only by plants, are found the known precursors of the A vitamins. It is these compounds that represent by far the most prominent members of the world's pigment crop. In their handling of ingested carotenoids, animals emphasize any of several metabolic alternatives, e.g. (a) non-selective assimilation of all types; (b) rejection of all classes from any storage; (c) selective uptake of the hydrocarbon or carotene kind; or (d) solely of the alcoholic or other oxygen-containing members (xanthophylls); or finally (e) oxidative conversion of carotenes or xanthophylls into innovated red or other richly coloured derivatives. Some animals are without A vitamins among any carotenoids they may store. Numbers of arthropods so characterized nevertheless exhibit photokinetic responses. Plants are without vitamin A per se, carrying only carotenoid precursors thereof; however, single-celled phototactic phytoplankton, e.g. dinoflagellates, respond to light by their diurnal vertical migrations. It seems reasonable to suppose that animals, evolving from the primitive plant world, must have inherited therefrom many similarities in their cytoplasmic constitution and basic metabolic needs, but not in all instances the means of fully supplying them. They must, accordingly, continue to rely upon the plant world for the synthesis, de novo, of materials such as carbohydrates (for fuel, inter alia) and many amino acids for their protein upkeep, as well as certain biocatalysts, notably of the bio-chromic type, such as vitamins, e.g. B2 (riboflavin), B12 (cyanocobalamin), thenaphthoquinone K vitamins, and the A provitamin pigments, including carotenes and close chemical relatives thereof, although the animals have developed the capacity to split such pigments, deriving thus for themselves the A vitamins proper.