106 OUTLINE OF PLANT GEOGRAPHY 



The following species of oaks occur in the oak openings, viz., 

 Quercus alba, Q. rubra, Q. coccinea, Q. velutina, Q. macrocarpa, 

 and Q. Maryland ica. The common hickory is Carya microcarpa. 



The forest of the northern transition region according to Harsh- 

 berger l is of three kinds. On the ridges and hill-slopes oaks are 

 the predominant trees, but with these are associated hickories 

 (Carya alba), aspens, cotton- woods, ash (Fraxinus sambucifolia) , 

 hornbeams (Ostrya, Carpinus), canoe-birch, butternut (Juglans 

 cinerea), and occasionally bass-wood (Tilia Americana). In the 

 moister alluvial soils the silver maple, green ash, elm, bass-wood, 

 black walnut, hickory and cotton-wood are found. 



At the northern limit of the region, there are sandy areas where 

 the white pine and yew occur, with birches and alders, this forma- 

 tion being decidedly boreal in character. 



Further south, in Iowa, there occur in the alluvial formations 

 a number of more southern species, like the honey-locust (Gle- 

 ditschia), Sycamore (Platanus occidentalis) , and mulberry (Morns 

 rubra). 



The grass-lands become more and more predominant westward, 

 until the unbroken prairie is reached, which at the north extends 

 unbroken to the foot of the Rockies. 



Toward the south the grass-lands are less extensive, and in 

 western Kansas and Nebraska the region is rather a dry steppe, 

 than true prairie, and this is still more marked in western Texas 

 and New Mexico, where the country is an arid steppe or desert, a 

 northward extension of the dry plateau of Mexico. 



With the diminution in rainfall toward the west, and the in- 

 creased elevation, the close sod of the eastern prairie is replaced 

 by an open formation of bunch grasses, among which occur many 

 species of plants like the yuccas, and occasional cacti, characteris- 

 tic of the arid regions further south. 



The eastern prairie region has a fair rainfall, and the absence 

 of trees has been the subject of some speculation, as conditions 

 would seem to be favorable for their growth. 2 It has been con- 

 jectured that the prairies owe their origin to the burning of the 

 forests by the Indians to furnish feed for the herds of buffalo, but 

 the latter themselves may very well have played an important 



1 Loc. cit., p. 521. 



2 Haishberger discusses this question st some length, see loc. cit., p. 516. 



