Ulmus spp.; elm [Ulmaceae]  Small to large trees, often with deeply furrowed bark and slender branches.  Leaves simple, alternate, lop-sided at the base, with doubly-toothed margins, strictly parallel lateral veins, and short petioles.  Fruits are distinctively disk-shaped and winged.  Four species are common in Wisconsin.  U.americana (American elm) was once common throughout the state and widely planted as a street tree, but is particularly susceptible to Dutch elm disease and now primarily found only where protected by isolation or regular injections of fungicide.  U. rubra  (red or slippery elm) and U. thomasii (rock elm) are also native to Wisconsin.  U. pumila (Siberian elm) is an introduced and ecologically invasive species.  [080]