88 TREE ANCESTORS 



Over twenty kinds of willows have been discovered in the Pleis- 

 tocene deposits and only two or three of these are extinct species. 

 The details of their present range and Pleistocene occurrences are 

 too extensive for the present sketch so that only a few will be 

 mentioned. One or the other of the four herbaceous small leafed 

 Arctic and Alpine species Salix herbacea, S. polaris, S. retusa and 

 S. reticulata are found in Pleistocene deposits as far south as New 

 York state in this country, and Switzerland and Galicia in Europe. 

 Three of them occur in Germany. The northern peat bog species 

 Salix repens and 5. myrtilloides are both found in England. The 

 sub-Arctic species ^a/z:!; aurita and -5. caprea occur respectively in 

 England and Denmark. The osier or basket willow is recorded 

 from France and Wiirttemburg, and a very similar form occurs in 

 the Pleistocene deposits of North Carolina and Kentucky. The 

 white willow {Salix alba) and the crack willow {Salix fragilis) both 

 occur in France and Wiirttemburg. With the amelioration of 

 conditions following . the last retreat of the ice the Arctic forms 

 withdrew to the far North with the sub-Arctic and cool temperate 

 species in their wake, and these far northern forms are circumpolar 

 at the present time, although those willows that attain to the 

 stature of trees and inhabit the Temperate Zone are different in 

 each of the three continents of the Northern Hemisphere. 



THE POPLARS (POPULUS) 



The poplars, although they show their community of origin 

 with the willows, differ from them sufficiently to be readily dis- 

 tinguishable. They are all trees and on the whole average larger 

 than the willows. The catkins are pendulous instead of erect; 

 there is a rudimentary perianth or flower envelope, and the bracts 

 of the flowers are toothed or cleft instead of entire as in the willows; 

 the leaves are usually broad instead of narrow, being ovate or 

 deltoid and often cordate, and the leaf-stalks are long and often 

 flattened — a feature well exemplified in the quaking aspen. 



The generic name Populus is of obscure etymology but was the 

 classical name of the poplar, of which there are several European 

 species. The most important of these is the white, silver poplar, 



