views updated

Ulmus (elms; family Ulmaceae) A genus of mostly large trees, in which the leaves are asymmetrical at the base, and usually hairy, at least beneath. The hermaphrodite flowers appear in clusters before the leaves. The tiny nutlets have a broad, oval wing that aids wind dispersal, and is notched at the tip. The virulent strain of Dutch elm disease that was introduced about 1968 from N. America into Europe has destroyed most European elm trees, but the roots are not normally killed and many elms are re-establishing themselves from sucker shoots. The disease is caused by a fungus Ophiostoma ulmi) and is apparently always transmitted by the elm-bark beetle (Scolytus). Some eastern Asian elms appear to be immune to this disease. Elm wood is used sometimes for making coffins, and hollowed-out elm stems were formerly used for water pipes, as the wood does not decay readily when waterlogged. There are 18 species, occurring in the northern temperate zone, and in the mountains of the Asian tropics.