This section is from the book "An Illustrated Flora Of The Northern United States, Canada And The British Possessions Vol1", by Nathaniel Lord Britton, Addison Brown. Also available from Amazon: An Illustrated Flora of the Northern United States, Canada and the British Possessions. 3 Volume Set..
Trees or shrubs, with alternate simple serrate petioled pinnately veined stipulate leaves, the stipules usually fugacious. Flowers small, monoecious, dioecious, perfect or polygamous, lateral or axillary, clustered, or the pistillate solitary. Perianth 3-9-parted or of 3-9 distinct sepals. Petals none. Stamens in our species as many as the perianth-lobes or sepals and opposite them; filaments straight; anthers ovate or oval, erect in the bud, longitudinally dehiscent. Ovary 1-celled (rarely 2-celled), mostly superior; ovule solitary, pendulous, anatropous or amphitropous; styles or stigmas 2. Fruit a samara, drupe or nut. Endosperm of the seed little or none. Embryo straight or curved; cotyledons mostly flat.
About 13 genera and 140 species, widely distributed in temperate and tropical regions.
Flowers borne in clusters on twigs of the preceding season; fruit a samara, or nut-like.
Flowers mostly expanding before the leaves; calyx 4-9-cleft; fruit a samara.
Flowers expanding with the leaves; calyx 4-5-cleft; fruit nut-like.
Flowers borne on twigs of the season, the pistillate mostly solitary; fruit a drupe.
I. ULMUS (Tourn.) L. Sp. PI. 225. 1753.
Trees, with 2-ranked straight-veined inequilateral serrate leaves, with thin caducous stipules. Flowers perfect or polygamous, fascicled or racemose, greenish, mostly axillary on the twigs of the preceding season. Calyx campanulate, 4-9-lobed, persistent, its lobes imbricated. Filaments erect, slender, exserted. Ovary sessile or stalked, compressed, 1-2-celled. Styles 2, divergent, stigmatic along the inner margin, ovule 1 in each cavity of the ovary, suspended, anatropous. Fruit a i-seeded flat orbicular or oval samara, its membranous wings continuous all around except at the apex, commonly as broad as or broader than the body. Embryo straight. [The ancient Latin name of the elm; Celtic, elm.]
About 16 species, natives of the northern hemisphere. Besides the following two others occur in the southern United States and one in Mexico. Type species: Ulmus campestris L.
Flowers appearing in the spring long before the leaves.
Leaves smooth or slightly rough above; samara densely ciliate.
None of the branches corky-winged; samara-faces glabrous.
Some or all of the branches corky-winged; samara-faces pubescent.
Leaves 2' - 5' long; flowers racemose; northern.
U. T homasi.
Leaves 1'-3' long; flowers fascicled; southern.
Leaves very rough above; samara not ciliate; twigs not corky-winged.
Flowers appearing in the autumn.