PERKIN, A. G. AND EVEREST, A. E.--"The Natural Organic Colouring Matters," 655 pages, London, 1918. WAKEMEN, NELLIE A.--"Pigments of Flowering Plants," in _Transactions_ of the Wisconsin Academy of Sciences, Arts, and Letters, Vol. XIX, Part II, pages 767-906, Madison, Wisc., 1919. WATSON, E. R.--"Colour in Relation to Chemical Constitution," 197 pages, 65 figs., 4 plates, London, 1918. WHELDALE, M.--"The Anthocyan Pigments of Plants," 304 pages, Cambridge, 1916. WILLSTÄTTER, R. AND STOLL, A.--"Untersuchung über Chlorophyllen, Methoden und Ergebnisse," 432 pages, 16 figs., Berlin, 1913. CHAPTER IX ORGANIC ACIDS, ACID SALTS, AND ESTERS Organic acids, either in free form, or partially neutralized with calcium, potassium, or sodium, forming acid salts, or combined with various alcohols in the form of esters, are widely distributed in plants. They occur in largest proportions in the fleshy tissues of fruits and vegetables, where they are largely responsible for the flavors which make these products attractive as food for men and animals. But organic acids and their salts are also found in the sap of all plants, and undoubtedly play an important and definite part in the vital processes of metabolism and growth. CHEMICAL CONSTITUTION All organic acids contain one (or more) of the characteristic O // acid group, --COOH, or --C, known as "carboxyl." This \ OH group is monovalent, and in the simplest organic acid, formic acid (H_{2}CO_{2}), it is attached to a single hydrogen atom, thus, H·COOH. In all other monobasic acids, it is attached to some other monovalent group, usually an alkyl radical, i.e., a radical derived from an alcohol and containing only carbon and hydrogen (as methyl, CH_{3}, ethyl, C_{2}H_{5}, butyl, C_{4}H_{9}, acryl, C_{2}H_{3}, etc.). Hence, the general formula for all monobasic organic acids is R·COOH, the R representing any monovalent radical. In the simplest dibasic acid, oxalic (H_{2}C_{2}O_{4}), two carboxyl groups are united to each other, thus, HOOC·COOH; but in the higher members of the series, the two characteristic acid groups are united through one or more --CH_{2}-- groups, or their oxy-derivatives (as HOOC·CH_{2}·COOH, malonic acid; HOOC·CH_{2}·CH_{2}·CH_{2}·COOH, glutaric acid; HOOC·CHOH·CH_{2}·COOH, malic acid, etc.). Polybasic acids, containing three or more carboxyl groups, linked together through one or more alkyl carbon atoms, are also possible, and a few typical ones (as COOH | HOOC·CH_{2}·COH·CH_{2}·COOH, citric acid) are found in fruits and other plant tissues.