The Migration of Our Forest Trees. 193 



Where the fruit separates from its pedicel it drops with the heavier 

 seed end down, and is given a whirling motion by the wing, and when 

 caught by the wind is carried many metres before it reaches the 

 ground. 



White Ash (Fraxinus americana L. ) is common in the Missouri 

 forest area, from which it has extended up along the eastern border of 

 the state to Sarpy county. 



Green Ash (Fraxinus laiwcolata Bork.) is common in the Mis- 

 souri forest area, from which it has spread westward and northward 

 along the river valleys, across the state to the western counties. 



Red Ash (Fraxinus penusylvanica Marsh.) is found with the pre- 

 ceding and apparently has been disseminated with it. 



Red Bud (Cercis canadensis L. ). The bean-like pods are very 

 flat and thin, and are well adapted to be carried in the wind a few 

 metres. It is common in the Missouri forests and has extended north- 

 westward into Nebraska as far as Lancaster and Douglas counties. 



The Maples (Acer spp.). The bicarpellary fruit develops two thin 

 membranous curved and slightly twisted wings, one on the back of each 

 carpel. At maturity the carpels split apart, and each falls slowly with 

 a whirling motion, while the wind carries it to a greater or less distance 

 from the parent tree. In a high wind this distance may be twenty to 

 thirty metres, or even more. 



Mountain Maple (Acer glabrum Torrey) is abundant in the Rocky 

 mountains of Wyoming and southward, from which it has extended 

 eastward into the extreme western part of the state at two stations 

 (Sioux and Scott's Bluff counties.) 



Silver Maple (Acer saccharinum L. ) occurs abundantly in the 

 Missouri forest area from which it has extended up the Missouri river 

 nearly to the mouth of the Niobrara river and westward fifty to sixty 

 miles, in the moist lands along the streams. 



Box Elder or Ash-leaved Maple (Acer negundo L. ) grows abund- 

 antly in the Missouri forests, from which it has extended across the 

 state. As this species occurs in the Rocky mountains from New Mexico 

 northward it is possible that some of the trees in western Nebraska 

 have come down from the mountains and met those disseminated di- 

 rectly from the eastern forest areas. 



Ironwood (Ostrya virginica (Miller) Wild.). The small nut is 

 enclosed in a bladdery bag, which is so much larger that it serves the 

 purpose of a wing. A dozen or more of these are aggregated into a 

 loose strobilus. The obvious purpose of this structure is the easy 



