igo UNIVERSITY OF COLORADO STUDIES 



narrowleaf cottonwood has a leaf as narrow as is found in some species 

 of willow. A glance at the medium-sized branches will show, however, 

 a whitish-gray bark in the poplar and a dark gray in the willow. Both 

 the willows and poplars have the flowers in catkins. The pollen-bearing 

 and seed-bearing catkins are borne on different trees. Those of the 

 willows are erect and those of poplars hang down. Both willows and 

 poplars produce "cotton," but this is more abundant with some species 

 than others. The "cotton" is made up of hairs surrounding the seeds. 

 Since the seed-bearing trees occur in about the same number as the 

 pollen-bearing trees it happens that, in nature, one-half of the trees are 

 cotton-bearers. About twenty-five species of Populus are known to 

 science; all belong to the northern hemisphere, but a few species extend 

 well south into Mexico, central Asia and northern Africa. Northward 

 the range is to the Arctic Circle. 



Poplars in Colorado during former geologic times. — It is evident 

 from the present wide geographical distribution of the poplars that they 

 are an old group of plants. This is also well shown by the fossil leaves 

 of these trees found in various parts of the world. In Colorado speci- 

 mens of fossil poplars have been found at Florissant and at Golden. 

 There were at least five species of poplars growing at Florissant during 

 Tertiary times.* The fossil remains are very perfect as to the leaves 

 and we can tell the shape, size and veining. Unfortunately nothing is 

 known about the height of the trees and their general habit of growth. 

 However, we may suppose that they looked very much like our cotton- 

 woods of today. Some of the leaves were large, as much as 14 cm. 

 (5I in.) broad. Other trees had long and narrow leaves, like those of 

 our present-day narrowleaf cottonwood. One species was much like 

 the quaking aspen in the form and size of leaf. It is interesting to know 

 that these poplars of a bygone day are so similar to living species. The 

 climate of Florissant is cooler than it used to be. In those days the 

 poplars grew along with such plants as holly and live oak and per- 

 simmon, for fossil remains of these trees occur also at Florissant. At 

 the present time the poplars of Florissant grow with willows, birches, 

 pines and spruces. In fact the species of Populus are now nearly all 



■ According to Professor T. D. A. Cockerell, who has kindly placed his notes at my disposal. 



