A monograph of the genus Fagus Tourn. ex L. (Fagaceae)

Thirteen species, and seven additional subspecies of five species, are recognized and described. One species (F. okamotoi) and one subspecies (F. japonica subsp. angustifolia) are new to science. Five new combinations at the subspecific level are made. Two subgenera (Engleriana subgen. nov. and Fagus) are recognized. Within subgenus Fagus, four sections (Longipetiolata sect. nov., Lucida sect. nov., Grandifolia sect. nov., and Fagus) are described. The work was conducted in the New York Botanical Garden. Close to five thousand specimens were assembled from 25 herbaria in Northern Hemisphere: East Asia (seven), Europe (six), and North America (12).;Descriptions of the genus and all recognized infrageneric taxa are provided, and taxonomic keys are also provided. The features concerned in these works are gross features observable under a x30 lens, but micro-features of leaves are also mentioned briefly in discussions at higher taxonomic levels. Discussions and speculations on ecology, evolution, and phylogeny are also significant constituents of the present work. A specimen-citation list, 20 plates showing gross features, and 10 distribution maps appear as appendices.;The East Asian subgenus Engleriana is characterized by its "bush-like" tree-shape and many vegetative features. The differences among its four constituent (sub)species are chiefly in expressions of features of cupule/nut complex.;The East Asian section Longipetiolata (four species) is unique within the genus in its stout, blunt buds and papillate-glaucous lower leaf-surface. It shows a certain affinity to sections Lucida and Grandifolia. The four species of the other East Asian section Lucida have developed a clear pattern or tendency in their evolution of leaves. The three subspecies of each of F. sylvatica and F. grandifolia all have prominently winged nuts.;A clear evolutionary pathway of cupule-appendages from scale-like, through subulate, to thread-like, is seen in subgenus Engleriana and section Lucida. Specialized spathulate-foliaceous appendages occur in four of the five major subdivisions of the genus, except in section Longipetiolata. The supposed evolution in ecological adaptation is generally compatible with the taxonomic differentiation within subgenus or sections.