Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.
Genus Description - Fls perfect, in short racemes or fascicles; cal campanulate, 4–9-lobed or cleft; anthers extrorse; styles short; fr a flat, 1-seeded samara, usually shortly stipitate and often surmounted by the persistent and sometimes enlarged styles; embryo straight; trees with pinnately veined, inequilateral, usually doubly serrate lvs; the small fls appear from buds resembling but a little larger than the lf-buds. 20, mainly N. Hemisphere. Three cult. Old-World spp. that occasionally escape are included in the key but not described.