THE SILVA OF COLORADO I9I 



confined to the cooler and temperate regions of the world. Only a few 

 occur in the warmer parts. Fossil remains at Golden, Colo., show that 

 five or six species of poplar grew there at a still earlier time than those 

 at Florissant. Thus we may be sure that for thousands and perhaps 

 millions of years the cottonwoods and poplars have been among the 

 more conspicuous trees of this part of the world. 



Genus POPULUS 



Trees with alternate, undivided leaves, lanceolate to broadly triangular or heart- 

 shaped; the buds with resinous, aromatic scales. Flowers in aments (catkins) 

 without floral leaves. Staminate and carpellate aments on different trees; the 

 aments made up of bracts, each bearing a single flower in its axil; staminate flowers 

 with 8 to 30, or more, stamens; carpellate flowers with i pistil, bearing 2 to 4 elon- 

 gated styles. Fruit a 2- to 4-valved capsule. Seeds smaU, provided with long, 

 densely matted, silky hairs, the "cotton." 



Key to Species of the Genus Populus' 



A. Leaves white-hairy underneath, more or less lobed. (Cultivated from Europe.) 



I. Populus alba 



B. Leaves smooth, not white-hairy, at least when full grown. 



a. Trees tall, spire-shaped, the branches extending upward close to the trunk. 

 (Cultivated from Europe.) 2. Populus nigra italica 



b. Trees not spire-shaped; branches variously placed. 



a. Leaf-stalk much flattened laterally where attached to blade of leaf. 

 Leaves easily rustled by the wind; blades broad. 



a. Leaf-blades somewhat circular in outline with short-pointed apex. 

 Bark very white. Tree of foothills and mountains. The "quaking 

 aspen." 3. Populus tremuloides 



b. Leaves rather broadly triangular or somewhat heart-shaped. Native 

 cottonwoods; but often planted for shade. 



a'. Teeth of the leaf-blades less than 10 on each side. Tree of southern 

 Colorado and southward. 4. Populus wislizeni 



b'. Teeth of the leaf-blades more than 10 on each side. 



aa. Leaf-blades very broad, often broader than long; toothed all 

 around except at base; tip of blade narrow-pointed. (Our 

 common western cottonwood.) 5. Populus sargentii 



bb. Leaf-blade generally not as broad as long; toothed all around. 

 Young twigs angular, due to development of cork in ridges. 

 (Cultivated from central U. S.) 6. Populus deltoides 



b. Leaf-stalk nearly cylindrical or slightly flattened. 



a. Leaf-blades ovate, abruptly angled at apex; rounded or cordate at 

 base; under surface whitish. 7. Populus balsamifera 



b. Leaf-blades lanceolate or ovate, generally narrowed at base. 



a'. Leaf-blades rather narrow; often abruptly pointed. Leaf-stalk 

 one-third as long as the blade, or less. 8. Populus angustifolia 



b'. Leaf-blades broader than in the preceding species; ovate or some- 

 what triangular; mostly long-pointed. Leaf-stalks slender, gen- 

 erally more than half the length of the blade. 



9. Populus acuminata 



1 This key includes the native Colorado poplars and three other species frequently planted for shade. 



