i66 



The Poplars 



additional to those here described. Populus alba Linnaeus, of Europe and Asia, 

 is the type species of the genus. 



The Poplars are mostly trees of very rapid growth, with straight-grained, 

 easily splitting wood. The bark contains some tannin. The young branches are 

 round or angled, marked by the large leaf scars; the buds are usually resinous, 

 covered with several scales, the lower scales opposite, the upper overlapping. 

 The leaves are alternate and stalked, involute in the bud, entire-margined or 

 variously toothed; their stipules usually fall away as soon as the leaves begin to 

 unfold. The veiy small, imperfect flowers are borne in long, drooping catkins, 

 which appear in early spring from scaly buds separate from the leaf-buds, before 

 the leaves unfold, the staminate catkins on one tree, pistillate on another, 

 though very rarely both pistillate and staminate occur on the same tree; 

 each scale of the catkin subtends a single flower and is variously cut-lobed 

 or fringed ; -these scales fall away very early, leaving the cup-shaped, obHque 

 disk supporting the flower; the staminate flowers have several or many stamens 

 with smooth, separate filaments and short, red or purple anthers; the pistillate 

 flowers have a single one-celled ovary containing many ovules, a very short style, 

 and 2, 3, or 4 stigmas, which are dilated or slender and variously lobed 

 or parted. The fruit is a capsule which, when ripe, splits into as many recurved 

 valves as there were stigmas, releasing the very small seeds, which are provided 

 with a tuft of copious soft white hairs; these are widely disseminated by the wind 

 and have given the trees the popular name, Cottonwoods. 



Petioles round or channeled, scarcely or not at all flattened laterally. 

 Leaves densely and persistently white-tomentose beneath, lobed or 



coarsely toothed; introduced European tree. 

 Leaves glabrous or nearly so, vv^hen mature, crenate or crenulate; 

 native trees. 

 Foliage densely tomentose when young; capsules long-pedicelled. 

 Foliage not tomentose; fruits short-pedicelled. 



Leaves broadly ovate, truncate, or cordate at the base, the petioles 

 and nerves usually pubescent or ciliate. 

 Ovary tomentose; western tree. 

 Ovary glabrous; northern tree. 

 Leaves ovate to lanceolate, acute or obtuse at the base (cordate 

 in No. 8); petioles glabrous. 

 Petioles fully one half as long as the blades. 



Leaves green on both sides, abruptly acuminate. 

 Leaves pale or brownish beneath, acute or acuminate. 

 Petioles one third as long as the blades or less. 

 Leaves not cordate. 

 Leaves cordate. 

 Petioles strongly flattened laterally. 

 Leaves broadly deltoid, abruptly acuminate; stigma-lobes dilated. 

 Bract at the base of the pistillate flower large, dilated. 

 Pedicels as long as the fruits or longer. 

 Pedicels shorter than the fruits, often very short. 



I. P. alba. 



2. P. heterophylla. 



3. P. trichocarpa. 



4. P. candicans. 



5. P. acuminata. 



6. P. balsamijera. -^ 



7. P. angustijolia. 



8. P. Tweedyi. 



9. P. Wislizeni. 



