The natural classification of plants is based on the external (or morphological) features and internal features (ultrastructure, anatomy, embryology and biochemistry). The classification of angiosperms is given by George Bentham and Joseph Dalton Hooker. These two English botanists classified plants based on original studies of specimens. They published their well-known scheme of classification in a book Genera plantarum. Especially from the practical point of view, this system of classification is still regarded as the best classification system.