190 



proceeded from three principal centers or zones: the Austro- 

 riparian zone or Coastal plain, a U-shaped area lying along the 

 Atlantic and Gulf coasts and the lower Mississippi valley: 

 a southeastern center lying within the U, and a southwestern 

 one lying to the west of it. The last two centers have furnished 

 most of the plants of the sand region and, indeed, of Illinois as 

 a whole, and the floral elements from them will be spoken of 

 as the Atlantic and the Sonoran elements respectively. A 

 northern extension of the Sonoran element has occupied the 

 inland territory designated by Engler as the Prairie province, 

 and an eastern arm of this province reaches across northern 

 Illinois into Indiana. This is shown by Pound and Clements on 

 their map of the Prairie province i '98, '00). This arm is almost 

 surrounded on the north, east, and south by the Atlantic prov- 

 ince, and it is also intersected along the streams by broad or 

 narrow strips of w r oodland, representing the same province. 

 When we consider also the fact that in Illinois the forests are 

 climatic and the prairies edaphic. it is not surprising that the 

 Sonoran floral element in the Illinois prairies is obscured by 

 the presence of a large number of species of the Atlantic ele- 

 ment derived from the adjacent forests. 



Of the 117 species growing on the prairie formation of the 

 sand region 34, or 29 percent, are of typically western range, 

 that is, from Illinois west across Nebraska and thence south in- 

 to Texas, a distribution in many cases practically coincident 

 with the Prairie province and clearly indicative of their Sono- 

 ran origin. These are as follows: 



*Equisetum robustum Petalostemon purpureus 



st '/pa spartea * Euphorbia Geyeri 



*( , ahnn<>rilt'<i longifolia *Callirrhoe triangulata 



*Bouteloua hirsuta *Opuntia humifusa 



*Eragrostis trichodes *(Enothera rhombipetala 



• Cyperus Schioeinitzii Asclepias Sullivantii 



*Cyperus Bushii Lithospermum Gmelini 



Carex gravida Lithospermum linearifolium 

 *Cycloloma atripliei 'folium Verbena stricta 



*Froeliehia oampestris Verbena bracteosa 



