This southern "rock-elm" (or "September elm") is largely restricted to dry calcareous sites of the Ozark region, southern Interior Low Plateaus and Gulf Coastal Plain (K). Colls. from Fla. and perhaps Ohio (NY) appear to be from planted trees (Braun 1961). The species was used as a street tree in Ala. and Ga., as early as the 1890s (NY ex Biltmore), and it was used to produce several hybrids for potential use in horticulture (Burns & Honkala 1990). Despite susceptibility to Dutch Elm Disease (FNA 3), serotina-like trees may still be distributed under the name "americana" (e.g. trees 2000-75 & 80 at the Univ. of Ky Arboretum). There has been frequent confusion in vegetative colls. with other species, especially thomasii. Also, B's records of serotina from BREA, MADI, PIKE and perhaps BELL were based on misidentified americana (GH, US). Compared to thomasii (F, FNA 3, W), well-developed leaves at the ends of fertile shoots are mostly 6-10 x 3-4.5 cm (versus 8-12 x 3-5 cm); however, a cultivated tree at Vanderbilt had leaves 8-14 x 4-7 cm on fertile shoots (NY). Also, leaves tend to have narrower shape, more attenuated to acuminate apex, and more asymmetric base; surfaces are dull yellowish-green above (versus glossy dark green), and covered with soft yellowish or stramineous (versus whitish) hairs below; veins are fewer on each side (averaging 15-18 versus 21-24) and tend to be less closely spaced; tertiary veins are more distinct; petioles, young stems and buds are usually glabrous except for short hairs on scale margins (versus more generally pubescent). Some vegetative colls. from CHRI, EDMO, LYON, TRIG and central Tenn. (APSC) remain uncertain in identification, and may represent an undescribed variant of serotina or thomasii, perhaps with hybrid origin (M. Brock, pers. comm.); they may be similar to the mid-western variant of thomasii. These anomalous trees. are generally sympatric with serotina but leaves tend to be larger, with more widely spaced veins; fresh stems are distinctly pubescent, and occasionally stipitate-glandular; bud scales are also more pubescent and similar to thomasii.