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, with close or shaggy bark, odd-pinnate leaves and serrate or serrulate leaflets. Staminate flowers in slender drooping aments, borne mostly in 3's on a common peduncle at the base of the shoots of the season, or clustered and sessile or nearly so in the axils of leaf-scars at the summit of twigs of the preceding year; calyx adnate to the bract, 2-3-lobed or 2-3-cleft; stamens 3-10; filaments short. Pistillate flowers 2-6, together on a terminal peduncle; bract fugacious or none; calyx 4-toothed; petals none; style 2 or 4, papillose or fimbriate, short. Fruit subglobose, oblong or obovoid, the husk separating more or less completely into 4 valves; nut bony, smooth or angled, incompletely 2-4-celled; seed sweet and delicious or very bitter and astringent. [From the aboriginal name Hicori.]
About 15 species of eastern North America, one in Mexico. Type species: Juglans alba L.
Bud-scales few, valvate; lateral leaflets lanceolate or oblong-lanceolate, falcate.
Nut not compressed or angled; seed sweet.
Nut somewhat compressed or angled; seed intensely bitter.
Leaflets 5-9; nut smooth.
Leaflets 9-13; nut angled.
Bud-scales imbricate; lateral leaflets not falcate.
Husk of the fruit freely splitting to the base; bract of the staminate calyx at least twice as long as the lobes.
Bark shaggy, separating in long plates; foliage glabrous or puberulent.
Leaflets 3-5 (rarely 7); nut rounded at the base, 6"-10" long.
Leaflets oval to oblong-lanceolate, puberulent.
Leaflets lanceolate, glabrous or glaucous beneath.
Leaflets 7-9; nut usually pointed at both ends, 1'- 1 1/2' long.
Bark close, rough; foliage very pubescent and fragrant.
Rachis of the leaves and staminate catkins densely hirsute.
Rachis and staminate aments scurfy, at least when young.
Husk of fruit not freely splitting to the base; bract of the staminate calyx about as'long as the lobes.
Fruit nearly globular; nut thin-shelled; bark shaggy, at least when old.
Fruit little flattened; bract of staminate calyx short.
Fruit much flattened; bract of staminate calyx long.
Fruit obovoid; nut thick-shelled; bark close.
Foliage glabrous, or little pubescent; anther-sacs acute.
Foliage pubescent or scurfy; anther-sacs obtuse.