BETULACEAE 



The Alders 



Characteristics of the Genus Alnus B. Ehrh. 



HABIT. Deciduous shrubs or small to medium-sized trees. 



LEAVES. Alternate; simple; usually serrate or dentate; pin- 

 nately veined; falling without change of color; petioled; stip- 

 ules fugacious, ovate, acute, scarious. 



FLOWERS. Regular; monoecious; apetalous; mostly appear- 

 ing before or with leaves (rarely opening in autumn); in 1—3 

 flowered cymes; formed during previous season; staminate in 

 long, pendulous aments, every scale bearing 3-6 flowers, each 

 flower subtended by 3-5 bractlets and composed of 4-parted 

 calyx and 4 (rarely 1-3) stamens; pistillate in erect, stalked, 

 ovoid or oblong aments, appearing below staminate flowers, 

 individual flowers in pairs, composed of a naked ovary sur- 

 mounted by 2 stigmas and subtended by 2-4 bractlets. 



FRUIT. Small, flat, chestnut-brown, wingless or laterally 

 winged nutlet, bearing remnants of style at apex; in persistent, 

 semi-woody strobiles, each scale bearing 2-4 nutlets. 



TWIGS. Slender to moderately stout; round; reddish or 

 tinged with red ; marked by raised leaf scars and lenticles ; pith 

 homogeneous, triangular in cross section. Winter buds: ter- 

 minal absent; lateral-stalked, 2-3-scaled, usually red; twig 

 lengthening by one of upper lateral buds. 



BARK. Astringent; mostly gray; smooth, except at the base 

 of trunks of large trees. 



WOOD. Light; soft; straight-grained; diffuse-porous; dur- 

 able in water; heartwood red-brown; sapwood very thick and 

 whitish. 



SILVICAL CHARACTERS. Tolerant to intolerant; rather 

 short-lived; shallow, spreading roots; on moist or wet sites, 

 commonly along streams or on mountain slopes. 



GENERAL. This genus contains about 30 species scattered 

 through the cooler portions of the Northern Hemisphere and 

 extending into the mountains of South America ; 8 species attain- 

 ing tree size are native to North America, although 7 of these 

 are typically shrubs and only Alnus rubra is of commercial im- 

 portance. The European alder, Alnus glutinosa (L.) Gaertn., 

 has become naturalized in the eastern United States. 



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