130 JOIRNAL OF THE ARNOLD ARBORETUM |vol. ii 



I^[)wards of fifteen hundred serial collections were made during the 

 season, aggregating about forty-five hundred herbarium specimens. Seeds 

 and nuts of more than one hundred species, chiefly of the trees and shrubs, 

 were taken, and about two hundred and fifty small photogra]>hs made, 

 illustrating the topography and vegetation of the region. 



The town of Cairo, located at the southernmost point of the state 

 of Illinois, at the juncture of the Ohio and Mississippi Rivers, is a few 

 miles north of the thirty-seventh parallel, or slightly south of the latitude 

 of Riclimond, Virginia; but owing to its interior continental position 

 the extremes of temperature and seasonal variation in i)recipitation are 

 much greater than in the corresponding latitude on the Atlantic sea- 

 board, resulting in conditions much less favorable generally in the central 

 Misslssip})i Valley for the nortliward extension of the rich and varied 



flora of the humid Lower Austral region of the Coastal plain than on 

 the eastern coast. 



Tlie topograj)hy of southern Illinois is remarkably varied; a number 

 of geological formations appear on the surface, giving rise to a wide range 

 of soils, drainage conditions and relief features, that afford habitat and 

 I)rotc(^tion to a diversified flora. In the vicinity of Cairo and for some 

 distance up the Mississippi River, and at intervals along the Ohio and 

 smaller intervening streams, are extensive swamps, lagoons and wide 

 alluvial bottoms, originally su1)jcct to annual overflow when the stream.*^ 

 WTre at flood. Leveeing and drainage have made considerable head- 

 w\ay against these conditions; and the woodman's axe and busy saw mills 

 have cleared away most of the splendid forest that formerly covered the 

 entire region; but still many small areas of swamps persist and remnants 

 of timber are preserved in the undrained and less accessible lands. 



The chief botanical interest of this region lies in the fact that here the 

 flora of the Gulf Coastal Plain makes a sharp salient to the northw\ard, 

 carrying a number of its species, in spite of the climatic handicap, to 

 some two or three degrees higher latitude than elsewhere in the conti- 

 nental interior, and in some cases even farther than on the temperate 

 Atlantic sea-board. The extreme northern point of this salient extends 

 up the valley of the Wabash River to above the mouth of the White 

 River, that forms the southern boundary of Knox County, Indiana, or 

 in the vicinity of Mt. Carmcl on the Illinois side; and is marked by the 

 outposts of such southern woody plants as the small Cane {Arundinaria 

 vuicrospcrma). Bald Cypress {Taxodium distlchum), Overcup Oak (Qucrcus 



iQ 



ijl 



Downy Swamp Maple {Acer ruhrum var. Drummondii), One-seeded 

 Honey Locust {Glcditsia aquaiica). Wisteria, Pumpkin Ash {Fraxinus 



>f 



Towards the northern limit most 



of these plants occur only locally in isolated swamps, but farther south 

 in Illinois they form with other associated species an important part of 

 the forest flora, and a little lower dow^n in the Mississinni Vallev the 



