SUMMARY1The commoner phenolic constituents of dicotyledonous plants are eight in number (Fig. 1): two leuco‐anthocyanins, three flavonols, two hydroxycinnamic acids, and hexahydroxydiphenic acid, present after acid hydrolysis as its dilactone, ellagic acid.2Most of the other phenolic constituents found in the higher plants are derivable from the above by a limited number of unit ‘transformations’, which are specified.A summary account is given of the recorded occurrence of these constituents.3Results are given of a chromatographic survey of the phenolic constituents of the leaves of dicotyledonous plants. The systematic significance of these results and of the recorded constituents is discussed.4The probability is considered that in the course of evolution the ability to form (a) leuco‐anthocyanins, and (b) trihydroxy‐substituted derivatives, if lost, is not thereafter regained. The implications ofab→a0b0being an irreversible process are discussed from the phylogenetic viewpoint.5Especial taxonomic significance is attached to the restriction of certain unit transformations to particular familie