Deciduous shrubs with gray bark. Twigs somewhat 3-sided; pith small, 3-sided, continuous. Buds sessile or stalked, rather large, solitary, with 3 scales. Leaf-scars alternate, half-round, raised; bundle-traces 3 or compound. Both kinds of flowers in catkins, the staminate for next season usually conspicuous in winter, along with the persistent cone-like fruiting catkins of the preceding season and the small undeveloped pistillate catkins of the next season.
Buds sessile, with 3-6 imbricated scales
Buds stalked, with 2 or 3 scales
b. Stems with linear transverse lenticels; fruiting catkins bent downwards
b. Lenticels shorter, fewer, darker; fruiting catkins erect
1. A. crispa (Ait.) Pursh.Green Alder. Ascending and bushy shrub up to 3 m. high; young branches glabrous or sparsely pubescent; buds sessile. Labrador to Alaska, south to North Carolina, Michigan, Minnesota, and Alberta (Fig. 58).
2. A. rugosa (DuRoi) Spreng. Speckled Alder. Hoary Alder. (A. incana of authors, not (L.) Moench.). A shrub or small tree 2-8 m. high, the twigs glabrous in winter; trunk marked with whitish linear lenticels up to 7 mm. long (hence the name, speckled alder); fruiting catkins ovoid, 12 mm. long, bent downwards; nut orbicular. The flowers begin to appear in March. Low grounds, Labrador to Saskatchewan, south to West Virginia, Indiana, and Iowa; the common species northwards (Fig. 59).
Fig. 58. Alnus crispa.
Fig. 59. Alnus rugosa.
Fig. 60. Alnus serrulata.
3. A serrulata (Ait.) Willd. Smooth Alder. Brookside Alder. (A. rugosa of authors, not Spreng.). A shrub or small tree 2-7 m. high, the bark smooth; fruiting catkins erect, ovoid, 12-18 mm. long; nut ovate. The flowers begin to appear in February and March. Stream-margins, Florida to Louisiana, north to Nova Scotia, Ohio, Missouri, and Oklahoma; the common species southwards (Fig. 60).