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..
Shrubs or trees, with dentate or serrulate leaves, few-scaled or naked buds, and flowers of both kinds in aments, expanding before, with or after the leaves, making in most species their first appearance during the preceding season, the staminate pendulous, the pistillate erect, clustered. Staminate flowers 3 or sometimes 6 in the axil of each bract, consisting of a mostly 4-parted perianth, 4 stamens and subtended by 1 or 2 bractlets; filaments short, simple; anther-sacs adnate. Pistillate flowers 2-3 in the axil of each bract, without a perianth, but subtended by 2-4 minute bractlets; ovary sessile, 2-celled; styles 2; bracts woody, persistent, 5-toothed or erose. Nut small, compressed, wingless or winged. [Ancient Latin name derived from the Celtic, in allusion to the growth of these trees along streams.]
About 14 species, natives of the northern hemisphere and the Andes of South America. Besides the following, some 4 others occur in western North America. Type species: Alnus vulgaris Hill.
Nut bordered by a membranous wing on each side.
Nut acute-margined, wingless.
Leaves obovate, broadly oval or suborbicular, dull; aments expanding long before the leaves.
Leaves green and glabrous or pubescent beneath, obovate to suborbicular.
Leaves finely serrulate; native.
Leaves dentate-serrate; twigs glutinous; introduced tree.
Leaves mostly oblong, bright green and shining above; aments expanding in late summer or
Betula Alnobetula Ehrh. Beitr. 2: 72. 1788.
Alnus viridis DC. Fl. Fr. 3: 304. 1805.
42: 42. 1842. Alnus Alnobetula K. Koch, Dendr. 21: 625. 1872. Alnus mollis Fernald, Rhodora 6: 162. 1904.
A shrub, 2°-10° high, the young leaves glutinous and more or less pubescent, the twigs glabrous or pubescent. Leaves oval or ovate, obtuse or acute, sharply and more or less irregularly serrulate or incised-serrulate, when mature dark green and glabrous above, light green and glabrous or pubescent beneath, 2'-5' long, 1 1/2' - 3' wide; petioles 4"-12" long; aments expanding with the leaves, the staminate slender, naked, 1 1/2'-2 1/2' long, the pistillate oblong or ovoid-oblong, slender-peduncled, becoming 4"-10" long and 4"-5" in diameter in fruit, their bracts irregularly 5-toothed; nut oblong, the thin wings about as broad as the body.
Newfoundland to Manitoba, Massachusetts, New York, Michigan, and in the higher Alleghanies to North Carolina. Also in Europe and Asia. June. Consists of many races, differing mainly in the amount of pubescence.