FLOWERING PLANTS AND FERNS OF ARIZONA 215 



The familiar cottonwood (P. fremontii) is conspicuous along streams 

 throughout the State, except in the higher mountains, and is planted 

 everywhere as a shade tree. The trees grow rapidly and propagate 

 readily from branch cuttings. The great quantities of pollen produced 

 by the staminate trees and the copious quantities of down}' seeds shed 

 by the pistillate ones are distinct disadvantages to the use of the 

 cottonwood in dooryards. Another drawback is the frequent mutila- 

 tion that results from windstorms. The wood is light and tough and 

 not durable, but has been used to some extent for fence posts. The 

 Pima Indians use the twigs of this species for basket material and 

 formerly ate the uncooked catkins. The inner bark was esteemed 

 by the Indians as an antiscorbutic. Cattle browse on branches within 

 their reach. Beavers cut the stems of poplars for dams and the bark 

 is their principal food. 



The aspen (P. tremidoides) occurs on the higher mountains, mingled 

 with conifers or in pure stands of considerable extent, conspicuous 

 because of the nearly white bark of the trunks and the brilliant yellow 

 of the foliage in autumn (pi. 8). Stands of aspen usually quickly 

 replace forest areas denuded by fire, in this way retarding soil erosion 

 and conserving soil moisture. Aspen stands are usually transient, 

 eventually being replaced by conifers. Aspen has limited value as a 

 timber tree and as browse for livestock. The wood is used chiefly for 

 paper-pulp and in making containers for butter, etc., as it imparts no 

 flavor. An infusion of the inner bark was formerly used in treatment 

 of intermittent fever, and is regarded by the Indians as an anti- 

 scorbutic 



Key to the species 



1. Leaf blades considerably longer than wide, finely crenulate or serrulate (except 



sometimes on vigorous shoots), gradually or not very abruptly acuminate; 



petioles terete, shorter (commonly much shorter) than the blades (2). 



2. Petioles usually less than one-third as long as the blades: blades lanceolate or 



ovate-lanceolate, more than twice (commonly 3 to 7 times) as long as 



wide, gradually acuminate at apex and often at base, the marginal teeth 



extending very nearly to the apex 1. P. angustifolia. 



2. Petioles commonly more than one-third (often one-half or more) as long as 



the blades: blades ovate or lance-ovate, commonly less than twice as 

 long as wide, somewhat abruptly acuminate at apex, rounded or broadly 

 short-cuneate at base, the marginal teeth not extending very nearly to 



the apex 2. P. acuminata. 



1. Leaf blades nearly as wide as, to considerably wider than long; petioles flat- 

 tened laterally (.3). 



3. Stigma lobes slender: leaf blades broadly ovate to rhombic-suborbicular, 



crenulate or serrulate with numerous teeth, abruptly very short-acu- 

 minate or apiculate at apex, short-cuneate, truncate, or rounded at base, 



usually conspicuously paler beneath 3. P. tremtloides. 



3. Stigma lobes broad, more or less flattened, crenate: leaf blades broadly 

 deltoid or subrhombic-deltoid, crenate or crenate-dentate (usually 

 coarsely so), abruptly and commonly broadly acuminate at apex, 

 truncate or very short-cuneate at base, not conspicuously paler beneath. 



4. P. FREMONTII. 



1. Populus angustifolia James in Long, Exped.: 1. 497. 1S23. 



Apache. Coconino, and Yavapai Counties, south to Cochise and 

 Pima Counties, 5.000 to 7.000 feet, along streams. South Dakota 

 to Alberta, south to Nebraska. New Mexico. Arizona, and Chi- 

 huahua. 



Narrowleaf cottonwood. A small tree, reaching a height of about 

 15 m. (50 feet) and a trunk diameter of 45 cm. (18 inches). The 



