This volume - the first of this series dealing with angiosperms - comprises the treatments of 73 families, representing three major blocks of the dicotyledons: magnoliids, centrosperms, and hamamelids. These blocks are generally recognized as subclasses in modern textbooks and works of reference. We consider them a convenient means for structuring the hundreds of di­ cotyledon families, but are far from taking them at face value for biological, let alone mono­ phyletic entities. Angiosperm taxa above the rank of family are little consolidated, as is easily seen when comparing various modern classifications. Genera and families, in contrast, are comparatively stable units -and they are important in practical terms. The genus is the taxon most frequently recognized as a distinct entity even by the layman, and generic names provide the key to all in­ formation available about plants. The family is, as a rule, homogeneous enough to conve­ niently summarize biological information, yet comprehensive enough to avoid excessive re­ dundance. The emphasis in this series is, therefore, primarily on families and genera.