Most of us lump plants together in one big family, and when pressed can only explain their grouping by what they're not--not an animal, not a mineral, and so just a plant. In reality, there are hundreds of different plant families, each grouped logically by a unique family history and genealogy. This brings sense and order to the more than a quarter of a million different plant species covering a diverse spectrum that includes soaring sequoias (Cupressaceae), squat prickly pear (Cactaceae), and luxuriant roses (Rosaceae).

Plant Families is an easy-to-use,...