1. Bark not readily exfoliating; bracts of pistillate aments noticeably unequally 

 divided, broadly rounded at base; distribution in mountains of New 

 Mexico and Arizona 2. B. occidentalis. 



1. Betula nigra L. River birch. Fig. 393. 



Tree to 30 m. tall and a trunk to 8 dm. in diameter, with soft shaggy and 

 freely exfoliating salmon-pink bark and reddish-dotted twigs; leaves with a 

 tomentose petiole to 15 mm. long, rhombic-ovate, broadly cuneate to subtruncate 

 at base, acute at apex, to 1 dm. long and 8 cm. wide below middle, conspicuously 

 and often deeply doubly serrate, bright-green on upper surface, grayish-white on 

 lower surface and when young downy; aments formed in the fall and expanding 

 in early spring; staminate aments mostly 2 or 3 clustered, sessile, elongate, each 

 of the ovate to suborbicular bracts subtending 3 flowers; flowers consisting of 4 

 stamens adnate to a 4-parted calyx, with two bractlets; the pedunculate thick- 

 cylindric pistillate aments tomentose, solitary, 25-35 mm. long, the bracts 6-8 mm. 

 long, nearly equally divided into 3 oblong-linear lobes, subtending 2 or 3 flowers; 

 flowers naked, without bractlets or calyx; fruit an erect or pendent ovoid to 

 oblong-ellipsoid strobile, the scales deciduous from the persistent cone axis at 

 maturity to release the compressed laterally winged nutlets. 



Along streams and in bottomlands in e. Okla. (Creek Co.), e. Tex., Mar .-Apr.; 

 from Fla. and Tex., n. to N.E., Pa., W.Va., O., s. Mich., s. Wise, s. Minn, and 

 e. Kan. 



2. Betula occidentalis Hook. Water birch. Fig. 393. 



Shrub or small tree to about 15 m. tall, finely branched, with a trunk to 

 about 3 dm. in diameter; bark smooth, reddish or copper-colored, not readily 

 exfoliating; twigs glandular-dotted; leaves ovate, cuneate to truncate or somewhat 

 cordate at base, 3-5 cm. long, sharply singly or doubly serrate, resinous- 

 glandular above when young, glandular-dotted below; aments spreading or pendu- 

 lous; pistillate aments 2-4 cm. long, 4-10 mm. thick; bracts puberulent and 

 ciliate, the lateral lobes shorter than the middle one; wings of fruit usually 

 nearly or quite as broad as the puberulent nutlet. B. fontinalis Sarg. 



Along mt. streams and wet areas in forests, often forming thickets, in N.M. 

 (San Juan and Valencia cos.) and Ariz. (Apache and Coconino cos.); S.D. to 

 B.C., s. to Neb., N.M., Ariz, and Calif. 



3. Alnus Mill. Alder 



Trees or shrubs with 3-angled pith and few-scaled leaf buds; leaves ovate 

 to obovate, deciduous; aments racemose or cymose-clustered; staminate aments 

 with 4 or 5 bractlets and 3 (rarely 6) flowers upon each short-stalked peltate 

 bract; individual flowers usually with a 3- or 5-parted calyx and as many stamens; 

 filaments short and simple; anthers 2-celled; pistillate aments ovoid to ellipsoid; 

 bracts fleshy, each subtending 2 flowers and a group of 4 tiny scalelets adherent 

 to the bracts of the ament, in iruit woody, cuneate-obovate and truncate or 3- or 

 5-lobed; fruit a small nutlet, crowned with the short persistent styles and sur- 

 rounded by a membranous wing or a thin margin. 



About 35 species, mostly in cool temperate regions. 



1. Distribution eastern Texas and/ or southeastern Oklahoma (2) 



1. Distribution in mountains of New Mexico and Arizona (3) 



2(1). Principal leaves typically with 9 to 12 pairs of lateral veins; pistillate 



aments 3 to 10, mostly sessile, 1-1.5 cm. long, 5-15 mm. thick 



1. A. serrulata. 



2. Principal leaves with 5 to 8 pairs of lateral veins; pistillate aments usually 



2 or 3, rarely more or less, all peduncled, 1.5-3 cm. long, 15-20 

 mm. thick 2. A. maritima. 



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