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..
Betula Alnus var. incana L. Sp. PI. Ed. 2, 1394. 1763. Alnus incana Willd. Sp. PI. 4: 335- 1805.
A shrub, or rarely a small tree, 8°-40° high, the twigs glabrous, the young shoots pubescent. Leaves oval or ovate, acute or sometimes obtuse at the apex, finely serrulate or dentate, with the teeth serrulate, obtuse or some of them acute at the base, dark green above, pale or glaucous and pubescent, at least on the veins beneath, 2-5' long, 1 1/2-4' wide, the veins prominent on the lower surface; stipules oblong-lanceolate, deciduous; petioles 4"-12" long; aments unfolding much before the leaves, the staminate 1 1/2'-3' long, the pistillate ovoid, about 1/2' long and 3"-5" in diameter in fruit, their bracts 5-toothed; nut orbicular, coriaceous-margined.
A shrub 5°-20° tall, or sometimes a small tree, attaining a maximum height of 40O and a trunk diameter of 6', the bark smooth, the younger shoots somewhat pubescent. Leaves green on both sides, obovate or oval, mostly obtuse and rounded at the apex, narrowed or rounded at the base, sharply and minutely serrulate, when mature glabrous above, usually pubescent beneath, at least on the veins, 3'-5' long, stipules oval, deciduous; petioles 4"-12" long; aments unfolding much before the leaves (or in the South after the leaves), the staminate 2'-4' long, the pistillate ovoid, 6"-9" long in fruit; nut ovate, narrowly coriaceous-margined.
Alnus noveboracensis Britton, has leaves acute at both ends and densely pubescent on the veins beneath. It may be a race of this species. It is known certainly only from a Staten Island tree, now destroyed.
A tree, reaching a maximum height of about 75° and a trunk diameter of 2 1/2°, the bark smooth, the branches nearly horizontal, the foliage glutinous. Leaves broadly oval, orbicular or obovate, thick, dark green, dull, often obtuse at both ends, coarsely dentate and the teeth denticulate, glabrous above, pubescent on the veins beneath, 2-5' long; petioles 1/2'-l' long; aments appearing from naked buds, expanding much before the leaves, the staminate 3'-4' long, the pistillate ovoid-oblong, .6"-9" long in fruit; nut wingless, coriaceous-margined.
In wet places, Newfoundland to New Jersey and Illinois, escaped from cultivation. Native of Europe. Wood soft, brown; weight per cubic foot 35 lbs. Irish mahogany. Hollard. Ooler. Black alder. April.
Betula Alnus maritima Marsh. Arb. Am. 20. 1785. Alnus maritima Muhl.; Nutt. Sylva 1: 34. t. 102. 1865.
A small tree, sometimes 300 tall and the trunk 6' in diameter, glabrous or very nearly so throughout. Leaves oblong, ovate-oblong or obovate, firm, acute at both ends, bright green and shining above, pale green and dull beneath, sharply serrulate, 2"-4'^ long, 1-2' wide; petioles 3"-10" long; aments unfolding long after the leaves, their buds developing during the season, the staminate l'-2 1/2' long, the pistillate oblong, 9'-12" long, 5"~7" in diameter in fruit; ,nut oblong-obovate, wingless, coriaceous-margined.
In wet soil, southern Delaware and eastern Maryland; also in Oklahoma. Closely related to A. japonica of northeastern Asia. Wood soft, light brown; weight per cubic foot 31 lbs. Aug.-Sept.