The Migration of Our Forest Trees. 195 



spread out almost spherically, and are caught by the winds and floated 

 away for long distances, often a mile or more, or in high winds, many 

 miles. 



Black Willow (Salix nigra Marsh.) is common in the Missouri 

 forests, from which it has spread up the streams, apparently across the 

 state. 



Almond Willow (Salix amygdaloides And.) is found abundantly 

 in the Missouri forests, and has followed the river valleys across the. 

 plains to the Rocky mountains and even to Oregon. 



Shining Willow (Salix lucida Muehl.) occurs in the Missouri for- 

 ests and has moved up the river to Cass county. 



Sand-bar Willow (Salix fluviatilis Nutt.) is abundant in the Mis- 

 souri forests, from which it has extended up the river valleys, across 

 the plains to the Rocky mountains, California and Oregon. 



Bebb's Willow (Salix bebbiana Sarg.) is found in the Black Hills 

 of South Dakota, and the Rocky mountains from Montana to Colorado, 

 from which it has extended eastward so as to enter the northwest cor- 

 ,ner of the state (Dawes and Sioux counties). 



Diamond Willow (Salix misso^l^iensis Bebb.) is common along the 

 Missouri river in western Missouri, from which region it has extended 

 its range northward along the river, and westward in the Republican, 

 Platte and Niobrara river valleys to the western border. 



The Poplars (Populus spp.). The bi- and tricarpellary seed-pods 

 develop two or three rows of seeds having the same general structure 

 as those of the Willows. On the escape of the. seeds they are buoyed 

 up by the attached mass of fluffy hairs, carried away by the winds, 

 sometimes for several miles. 



Quaking Aspen (Fopiihis tremuloides Michx.) is a Rocky moun- 

 tain tree whifch has extended eastward into Nebraska but a few miles 

 in the western counties from Banner to Sioux, Dawes and Sheridan 

 counties. 



Balsam Poplar (Populus balsaniifera L.) occurs in the Rocky 

 mountains of Wyoming, from which it has extended eastward into 

 Nebraska in Sioux county. 



Narrow-leaf Cottonwood (Populus angustifolia James) is also a 

 member of the forests of the Rocky mountains, from which it has come 

 eastward into Nebraska in Sioux and Scott's Bluff counties. 



