STANDLEY — TREES AND SHRUBS OF MEXICO. 157 



18. SAIICACEAE. Wiilow Family. 



Trees or shrubs; leaves alternate, stipulate, entire, dentate, or lobate, de- 

 ciduous ; flowers dioecious, in catkins ; fruit a small capsule, the seeds bearing 

 long white hairs. 



The following genera are the only ones of the family : 

 Stamens numerous ; bracts incised ; disk cup-shaped ; winter buds with several 



scales 1. POPULUS. 



Stamens usually less than .5 ; bracts entire ; disk represented by one or two 

 small glands ; winter buds with a single scale 2. SALIX. 



1. POPULUS L. Sp. PI. 1034. 1753. 



The species of Populus are generally distinguished from those of Salix by 

 their broad leaves, but one Mexican representative, P. angustifolia, has leaves 

 as narrow as those of some willows. 



The native species are widely used in Mexico as shade trees, for which 

 purpose they are very satisfactory, since they start readily from cuttings or 

 from large branches placed in the ground, and grow rapidly. They are not 

 very long-lived and the trees are frequently killed by mistletoe {Phoraden- 

 dron). The pistillate trees are not desirable as shade trees, for in the spring 

 when the fruit is ripe the seeds fly everywhere through the air, filling people's 

 eyes and nostrils and becoming a general nuisance. This trouble may be 

 avoided by planting only cuttings taken from staminate trees. 



Besides tlie native species, the white poplar, P. alba L., of the Old World, 

 and its various forms ("alamo bianco") is cultivated in central and southern 

 Mexico. It is distinguished by having the lower surface of the leaves covered 

 with a dense white tomentum. P. nigra L.. the black poplar, another Old 

 World species, is said to be cultivated in Mexico.^ 



The most common name for the species of the genus is " aiamo." 

 Petioles rounded, not flattened laterally ; leaves with very minute teeth. 

 Leaf blades ovate, dark green above, very pale beneath, rounded at the base. 



1. P. trichocarpa. 

 Leaf blades lanceolate, pale green on both sides, obtuse or acute at the base. 



2. P. angustifolia. 

 Petioles laterally compressed ; leaves usually with large teeth. 

 Leaf blades not deltoid in outline, orbicular, oval, oblong, or broadly ovate, 

 pale beneath. 

 Petioles densely tomentose ; leaf blades tomentose beneath when young, the 



teeth large 3. P. monticola. 



Petioles and leaves glabrous; leaf blades with small teeth. 



4. P. tremuloides. 

 Leaf blades more or less deltoid, not pale beneath. 

 Pedicels as long as the capsules or longer. 



Leaf blades mostly broader than long, the tip short, entire ; capsules 10 



to 13 mm. long 5. P. wisllzeni. 



Leaf blades much longer than broad, the tip verj' long, crenate-serrulate ; 

 capsules 7 to 8 mm. long " 6. P. dimorpha. 



^Apparently this name has been applied by some Mexican writers to the 

 native P. mexicana. 



