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



crown is usually narrowly pyramidal, with slender, narrowly ascend- 

 ing branches. 



2. Populus acuminata Rydb., Torrey Bot. Club Bui. 20: 50. 1893. 

 Apache, Navajo, Yavapai, and Greenlee Counties, 5,000 to 7,000 



feet, along streams. Nebraska to Assiniboia, south to New Mexico 

 and Arizona. 



Lanceleaf cottonwood. This species reaches about the same size 

 as P. angustifolia, from which it is distinguished chiefly by the broader, 

 more spreading crown, stouter branches, and broader leaf blades. 

 Some of the Arizona specimens seem to be nearly intermediate. 

 A form with pubescent branchlets and bud scales (var. rehderi Sarg.) 

 is of frequent occurrence throughout the range of the species in 

 Arizona. 



3. Populus tremuloides Michx., Fl. Bor. Amer. 2: 243. 1803. 

 Apache to Mohave Counties, south to the mountains of Cochise 



and Pima Counties, 6,500 to 9,000 feet. Labrador to Alaska, south 

 to New Jersey, Missouri, Arizona, and northern Mexico. 



Quaking aspen. The species is represented in Arizona by the 

 golden aspen, var. aurea (Tidestrom) Daniels (P. aurea Tidestrom), 

 characterized by the more intense autumnal coloration of the foliage 

 (golden or orange). The trees reach a height of 24 m. (80 feet) 

 and a diameter of 75 cm. (30 inches) but usually are much smaller. 

 The slightest breeze causes movement of the leaves, hence the com- 

 mon name quaking aspen. 



4. Populus fremontii S. Wats., Amer. Acad. Arts and Sci. Proc. 10: 



350. 1875. 



Populus macdougalii Rose, Smithsn. Misc. Collect. 61 12 : 1. 1913. 

 Populus arizonica Sarg., Bot. Gaz. 67: 210. 1919. 



Throughout the State, along streams, 6,000 feet or lower. Western 

 Texas to Nevada, Arizona, California, and northern Mexico. 



Fremont cottonwood. The trees are frequently 15 m. (50 feet), 

 sometimes even 30 m. (100 feet) high, and the trunk diameter may 

 reach 1.2 m. (4 feet). The main branches are large, the crown wide, 

 flat topped, the older bark gray brown, thick, deeply furrowed. 

 Several segregate species have been published, but there is such 

 complete intergradation that it seems useless to maintain them. 

 Exceptionally small-fruited specimens are referred to var. arizonica 

 (Sarg.) Jepson. Specimens with persistently pubescent branchlets, 

 petioles, and leaf veins are referable to var. pubescens Sarg. and, 

 if thus characterized but having bluish-green foliage, to var. mac- 

 dougalii (Rose) Jepson. The last-mentioned form is common near 

 Yuma and in the delta of the Colorado River. Several specimens 

 from eastern Arizona approach P. wislizeni (Wats.) Sarg. in their 

 long pedicels, but they have the relatively broad capsules of P. 

 fremontii. 



2. SALIX. 29 Willow 



Trees or shrubs; bud scales with an inner membrane, this adherent, 

 or loose and then giving the appearance of 2 scales; leaves short- 



29 References: Sudworth, George B. poplars, principal tree -willows, and walnuts of the rocky 

 mountain region. U. S. Dept., Agr. Tech. Bui. 420. 1934. 

 Gooddlng, Leslie N. willows in region vin: notes on their classification, distri- 

 bution, AND PRESENT SIGNIFICANCE, WITH SUGGESTIONS FOR THEIR USE IN EROSION 



control. U. S. Dept. Agr. Soil Conserv. Serv. Regional Bui. 65. 1940. [Multigraphed.] 



