192 UNIVERSITY OF COLORADO STUDIES 



Technical descriptions. — The following descriptions of the different 

 species of aspen and cottonwoods give not only the technical characters 

 but also certain information in regard to appearance, habit, wood- 

 structure and uses. Citations are made to the more accessible books 

 of reference, as in the writer's article on the Pine Family published in 

 the preceding number of these Studies. The reader is referred to 

 that article for a fuller list of works dealing with the trees of Colorado. 

 In the key and description of species no mention is made of flowers and 

 fruit. These characters are fully described in the books cited. The 

 text figures, except Fig. 2, are all original, from specimens in the 

 University herbarium. Fig. 2 is copied from Sargent's Trees of North 

 America. Attention is called to Fig. i, which shows the true shape of 

 the leaves much better than the figures given by Sargent or by Britton 

 and Brown. 



I. Populus alba Linn. White Poplar, Silverleaf Poplar 



Leaves broadly ovate in outline, variously toothed, sometimes deeply cleft and 

 resembling in shape those of the soft maple. Young foliage densely white-hairy, 

 the lower surface persistently wooly. Leaf-stalks nearly cylindrical, considerably 

 shorter than the blades. 



A medium-sized tree with smooth, light-gray bark and spreading branches. 

 Wood soft, nearly white, somewhat heavier than that of most of the poplars. Numer- 

 ous varieties of this species are known in cultivation. The roots extend horizontally 

 a short distance below the soil and send up numerous suckers by which propagation 

 is effected. 



Native of Europe and Asia. Naturalized by cultivation in the New England and 

 Middle Atlantic States. Planted as a shade and ornamental tree in the towns and 

 cities of Colorado. The trees are usually bent in the direction of the prevailing 

 winds. In Colorado and elsewhere the variety with sharply lobed leaves is often 

 wrongly called "silverleaf maple." 



2. Populus nigra italica DuRoi. Lombardy Poplar 



Leaf -blades rhombic or deltoid, wider than long, crenulate all around; smooth 

 from the first; leaf-stalks equaling the blades in length. 



A tall, narrow, spire-like tree with prominent main trunk; the branches ascending 

 and very numerous; bark thick and furrowed. This is a horticultural variety of 

 the black poplar of Europe. 



Known only in cultivation. Formerly much planted throughout the United 

 States but now less common. The trees do well in Colorado but may be expected 

 to die at the top as they grow old. 



