Chemical markers, to be useful in macrosystematics, must be considered in order of decreasing generality. Thus the discovery of evolutionary relationships among the families of plants requires attention to be paid initially to the ubiquitous flavonoids. Within this biosynthetic class of metabolites the flavonol myri-cetin and the proanthocyanidins jointly classify the dicotyledons into three major blocks formed, if Takh-tajan's (1969) associations of families into subclasses is adopted in first approximation, by (1 ) Magnoliidae, Ranunculidae, and Caryophyllidae; (2) Hamamelidae, Dilleniidae, and Rosidae; and (3) Asteridae (Table 1). The present overview comprises comments on the three subclasses of block 1 and on the first subclass of block 2.

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