SUMMARY1The stems of more than 500 species of dicotyledonous plants from 175 families have been examined histologically, using sections fixed in formalin‐acetic‐alcohol and in which the tannins have been made conspicuous by iron salts. The leaves of the same or nearly related species have been examined chemically for leuco‐anthocyanins. In 83 % of the cases the presence or absence of tannins in the stems coincides with the presence or absence of leuco‐anthocyanins in the leaves. It is therefore suggested that the histological reaction for tannins is most commonly due to leuco‐anthocyanins.2In about one‐fifth of the cases where agreement has not been observed, ellagic acid is present in the leaves and it is presumed that ellagitannins are responsible for the staining reactions observed in the stems. In other instances, e.g.Cornus, Sambucus, Passiflora, leuco‐anthocyanins, although absent from the leaves, are present in the stems. The remaining cases are accounted for either by dissimilarity of the species examined (e.g. Santalaceae) or by the presence of constituents which might react with the iron salts although not of tannin nature (e.g. mucilage in Boraginaceae and aucubin‐like substances in Scrophulariaceae).3The tendency of leuco‐anthocyanins and other tannins to be more frequently present in woody plants is confirmed and its significance discussed.4The occurrence of tannins can be used as a taxonomic character, for although these substances are of very widespread occurrence amongst the Dicotyledons, some families are highly tanniniferous, in others little or no tannin has been detected, and a third group of families have given mixed results.5Some of the orders of Dicotyledons, as understood in the Englerian system, tend to produce tannins more freely than others, but there is no more than a limited agreement between the occurrence of tannins and the composition of the orders as understood in any of the well‐known systems of classification.6A very interesting parallelism has been noted between the occurrence of tannins and the phylogenetic status of the families in which they occur as revealed by the ‘advancement index’ assigned to them by the statistical technique developed by Spome. The capacity to synthesize tannins seems to decrease as the advancement index increases.The assistance of the Director and Staff of the University Botanic Garden, Cambridge, in supplying botanical material is gratefully acknowledged. Thanks are also due to the gardens staff at Kew for their collaboration in growing and collecting botanical material. So far as one of us (E. C. B‐S.) is concerned, the work described in this paper was carried out as part of the programme of the Food Investigation Organization of the Department of Scientifi