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Deciduous or semideciduous trees or shrubs. Leaves pinnately nerved, variously serrate to crenate, thin- to thick-coriaceous and rigid, glabrous or variously sparsely hairy at least beneath. Stipules extrapetiolar, caducous. Flowers but of two kinds, one functionally ♂ and the other functionally ♀, variously stalked and spirally arranged in fascicles of 3-15 on short lateral shoots. Ovary compressed, sometimes stipitate; Ovule 1, anatropous to amphitropous. Fruit a dry and compressed nutlet surrounded by a membranous reticulate-venose wing. Seed:


Asia-Temperate:, Korea present Asia-Tropical: Burma: present Central & S. Celebes: present Europe: present Flores: present Japan: present Lesser Sunda Is: present N. & NE. India: present N. Mexico: present N. Sumatra: present N. Thailand: present North America: present Southern America:, Argentina Northeast (Formosa present) U.S.A. east of the Rocky Mts as far north as 60°: present W. & SW. Russia: present
About 20-25 spp., distributed in Europe (as far north as 68°), W. & SW. Russia, N. & NE. India, Burma, China, Korea, Japan, Formosa, Indo-China, N. Thailand, and in North America from N. Mexico to the U.S.A. east of the Rocky Mts as far north as 60°. In Malesia: 1 sp. so far known from a few localities in N. Sumatra, the Lesser Sunda Is. (Flores), and Central & S. Celebes.
As has been indicated by SCHNEIDER, l.c., there seem to be three centres of distribution, l.c. the European centre (5-6 spp.), the Indian-E. Asian centre (10-15 spp.), and the North American centre (4-5 spp.). .


Currently there is not a single worldwide monograph of the genus available for reference. The latest and perhaps the most comprehensive revision since PLANCHON'S work (1873) is that by SCHNEIDER (1916). He distinguished 26 spp. and recognized 5 distinct sections in the genus based on morphological characters derived from inflorescence, flowers, and fruits.
When more specimens from China become available for further studies, I believe the number of species occurring in the Indian-E. Asian centre will have to be reduced considerably.


GAGNEP. - in Fl. Gen. I.-C. 1927: 674
ENGL - in E. & P., Nat. Pfl. Fam., ed. 3, 1. 1888: 62
TOUW & STEEN. - in Blumea. 1968: 84
LINNÉ - in DC., Prod. 17. 1873: 154
TUTIN - in Fl. Europ. 1964: 65
BERNARD - in Bull. Herb. Boiss. 1905: 1097
BAILL. - in Hist. Pl. 1877: 137
SCHNEIDER - in Oest. Bot. Z. 1916: 21, 65
ENDL., Gen. Pl. 1837: 276
BERNARD - in Bull. Herb. Boiss. 1906: 23
LINNÉ - in Sargent, Pl. Wils. 3. 1917: 238
PLANCH. - in Ann. Sc. Nat. 1848: 259
ENDL., Gen. Pl. 1842: 29
HUTCH. - in Gen. Fl. Pl. 1967: 147
B. & H. - in Gen. Pl. 1880: 351
HOOK.F. - in Fl. Br. Ind. 1888: 480