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.