222 MISC. PUBLICATION 4 2 3, U. S. DEPT. OF AGRICULTURE 



Knowlton hophornbeam. Plant often treelike but not more than 

 3.6 m. (12 feet) high, the bark ashy gray. The fruiting "cones" 

 resemble those of hops. 



2. BETULA. Birch 



Winter buds enveloped by several scales; bark smooth, copper- 

 colored (in the Arizona species) with conspicuous lenticels; herbage 

 resinous ; flowers usually 2 or 3 in the axil of each bract, the staminate 

 ones with a small perianth; styles 2; stigmas terminal; nutlets winged. 



1. BetulafontinalisSarg.,Bot. Gaz. 31: 239. 1901. 



Tunitcha and Lukachukai Mountains (Apache County), Betatakin 

 (Navajo County), also reported from the Kaibab Plateau and the 

 Grand Canyon (Coconino County), 7,000 to 8,000 feet, mostly along 

 streams, often forming thickets. South Dakota to British Columbia, 

 south to Nebraska, New Mexico, northern Arizona, and California. 



Water birch. A small, finely branched tree, reaching a height of 

 9 m. (30 feet) and a stem diameter of 25 cm. (10 inches) but perhaps 

 never so large in Arizona. The plant affords good browse for sheep 

 and goats. 



3. ALNUS. Alder 



Winter buds enveloped by a pair of stipular scales; bark becoming 

 dark gray or brown, and scaly ; flowers 2 or more in the axil of each 

 bract, the staminate ones with a small perianth ; nutlets with a narrow 

 border. 



Alders are browsed to some extent by livestock and tend to check 

 erosion along watercourses in the mountains, where they often form 

 thickets. 



Key to the species 



1. Leaf blades ovate or oblong-ovate, rounded, truncate, or subcordate at base, 

 deeply and doubly serrate-dentate, often somewhat lobed; stamens 4. 



1. A. TENUIFOLIA. 



1. Leaf blades elliptic or ovate-oblong (exceptionally ovate), acutish or short- 

 cuneate at base, shallowly and doubly serrate-dentate, seldom lobed; 

 stamens 1 to 3 (usually 2) 2. A. oblongifolia. 



1. Alnus tenuifolia Nutt., North Amer. Sylva 1: 32. 1842. 

 Tunitcha Mountains and White Mountains (Apache County), 



Pinaleno Mountains (Graham County), 7,000 to 9,000 feet, mostly 

 along streams. Yukon to New Mexico, Arizona, and California. 



Thinleaf alder. A large shrub or small tree up to 7.5 m. (25 feet) 

 high, with reddish brown bark on the older trunks. 



2. Alnus oblongifolia Torr., U. S. and Mex. Bound. Bot. 204. 1859. 

 Apache and Coconino Counties south to Graham and Pima Coun- 

 ties, 5,000 to 7,500 feet, mostly along streams in the mountains. New 

 Mexico, Arizona, and northern Mexico. 



New Mexican alder. A tree, up to 18 m. (60 feet) high and 1 m. 

 (3 feet) in trunk diameter, with grayish-brown bark on the older 

 trunks. More widely distributed and abundant in Arizona than A. 

 tenuifolia. 



