92 TREE ANCESTORS 



known from central and southern Europe. Southeastern North 

 America was still too tropical in its climate to permit the existence 

 of poplars and although we lack the proof it may be assumed that 

 the numerous Eocene forms lived on in Arctic lands until they were 

 gradually exterminated or driven southward by the more severe 

 climate that commenced to prevail in high latitudes before the 

 close of the OHgocene. 



The poplars are represented during the Miocene period by about 

 30 species, which are found from Greece westward to Spain in 

 Europe and throughout the western United States and Canada. 

 The Miocene lake of Florissant in the heart of the Colorado Rockies 

 has furnished 7 forms of poplar — one a splendidly preserved 

 Cottonwood that may well have been the ancestor of the existing 

 forms that are found at the present time in Colorado. Poplars 

 are found at this time along the Pacific coast, but none are known 

 from the Atlantic or Gulf coasts. 



The Pliocene period, which succeeded the Miocene and immedi- 

 ately preceded the Glacial period, has furnished about 16 species 



Fig. 18. Some Fossil Willows and Poplars (About | Natural Size) 



1. Leaf; 2, fruit of Salix varians Goeppert from the Miocene (Tortonian) of 

 Switzerland. 



3. Salix lavateri Heer from the Miocene (Tortonian) of Baden. 



4. Salix angusta Al. Braun from the Miocene (Tortonian) of Baden. 



5. Staminate catkin of Salix from the Miocene (Tortonian) of Silesia. 



6. Salix reticulata Linne from the Pleistocene of Germany. 



7. Salix herhacca Linne from the Pleistocene of Galicia. 



8. Salix polaris Wahl., from the Pleistocene of England. 



9. Salix myrtilloidcs Linne from the Interglacial of England. 



10. Leaf; 11, bracts; 12, fruit of Populus latior Al. Braun from the Miocene 

 (Tortonian) of Baden. 



13. Populus crassa (Lesq.) Ckl., from the Miocene of Florissant, Colorado. 



14. Populus balsamoidcs Goeppert from the Miocene (Tortonian) of Silesia. 



15. Populus aniblyrhyncha Ward from the Eocene (Fort Union) of Montana. 



16. Populus heliadum Unger from the Miocene (Tortonian ) of Baden. 



17. Populus attenuata Al. Braun from the Miocene (Tortonian) of Baden. 



18. Catkins; 19, leaf; 20, scales; 21, leafy twig of Populus mutahilis Heer from 

 the Miocene (Tortonian) of Baden. 



22. Populus halsamijera Linne from the late Pleistocene of Maine. 



