Plant Migration Studies 9 



ravines along the Niobrara River in Keya Paha, Brown, and 

 Clierry counties (65). The occurrence of this tree in Nebraska 

 is a puzzle to the botanical geographers, for it is difficult to con- 

 ceive of an}^ means by which the seeds could be carried from the 

 nearest known stations. Even should we consider the possibility 

 of its dissemination from the Black Hills the difficulty is nearly 

 as great, for the distance is fully one hundred and fifty miles, a 

 part of it across the very rough country known as the "Bad 

 Lands." 



Jjlack Birch {Br.tula occidcntalis Hook.) occurs abundantly in 

 the Rocky Mountains west of Nebraska {66) and has extended 

 from thence eastward into the state in Sioux County. 



River Birch (Bctiila nigra L.) is found in the Missouri forests 

 southeastward, and has extended its range northward along the 

 eastern border of the state, being reported from Cass County 

 (67). 



HAIRS 



The Willows (Salix spp.). The bicarpellary seed-pods con- 

 tain two rows of inverted seeds (anatropous), each of which de- 

 velops a circular tuft of long straight ascending hairs on its 

 funicle. Upon the dehiscence of the mature fruit the seeds are 

 released, when the hairs 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 (5). 



Almond Willow (Salix aniygdaloidcs And.) is found abun- 

 dantly in the Missouri forests, and has followed the river valleys 

 across the Plains to the Rocky Mountains (6) and even to 

 Oregon. 



Shining Willow (Salix liicida Muehl.) occurs in the Missouri 

 forests and has moved up the river to Cass County (7). 



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

 Missouri forests, from which it has extended up the river val- 

 leys, across the Plains to the Rocky Mountains (8), California, 

 and Oregon. 



19 



