1 9 19] SARGENT— NORTH AMERICAN TREES 213 



This is the common and probably the only cottonwood of the valley of 

 the lower Colorado River. It is common on both banks of the river at Yuma, 

 and is planted in some of the towns of the Colorado Desert region like Yuma, 

 Mecca, and Indio. It has also been planted at the Needles on the Colorado 

 River in San Bernardino County, Cahfornia. 



PopuLUS Fremontii S. Watson. — This is the common and only 

 cottonwood of the valleys of northern and central California west 

 of the Sierra Nevada. The leaves are slightly cordate at the 

 broad base and coarsely serrate often with few teeth. The fruit 

 is ovate with a disk about 5 mm. in diameter, on pedicels 3-5 mm. 

 in length. 



In San Bernardino County, California, Nevada, Utah, and Arizona poplar 

 trees occur which, although the disk of the fruit is smaller or larger than that 

 of the typical F. Fremontii, until better known are best considered perhaps 

 varieties of that species. Three of these forms may be distinguished as follows : 



PopuLUS Fremontii var. Thornberii, n. var. — Leaves broadly 

 ovate, abruptly contracted into acuminate points, slightly cordate 

 at the wide base, coarsely crenately serrate with numerous teeth, 

 glabrous, 6-8 cm. long and broad; petioles 3.5-4 cm. in length. 

 Flowers not seen. Fruiting aments 5-6 cm. long, the capsules 

 elhpsoidal, 3-valved, deeply pitted, 8-9 mm. long; disk 3 mm. in 

 diameter; pedicels 2-3 mm. in length. 



A large tree with pale deeply furrowed bark and pale gray glabrous 

 branchlets. 



Low ground near Tucson, Pima County, Arizona, C. S. Sargent, March 27, 

 1916. 



From the typical F. Fremontii this variety differs in the more numerous 

 serratures of the leaves, in the ellipsoidal, not ovate, fruit with a smaller disk, 

 and in the much shorter pedicels. This tree was shown to me by Professor 

 J. J. Thornber of the University of Arizona, whose name I venture to asso- 

 ciate with it. 



PopuLUS Fremontii var. pubescens, n. var. — Differing from the 



type in its more pubescent branchlets. 



This is a common tree in San Bernardino and San Diego counties, Cali- 

 fornia, and extends into Nevada and southern Utah. The branchlets of the 

 type specimen of F. Fremontii, which was collected by Fremont in the upper 

 Sacramento Valley, are described as slightly pubescent, but on the other 

 specimens of this tree \vhich I have seen from CaHfornia north of San Ber- 

 nardino County they are glabrous, and as the range of the trees with the 



