384 IOWA ACADEMY OF SCIENCE Vol. XXV, 1918 



Invasion of Rudfiials Accompanying Pasturage. 

 This region has recently been pastured, which condition is 

 affecting- tbe prairie type materially. In the lowland alluvial 

 areas and along the borders of ravines the prairie plants have 

 almost exclusively been driven out by such ruderaJs as Ambrosia 

 artemesifolm, Verbena hastata, Polygonum pennsi/lvankum, P. 

 persicaria, setaria viridis and Poa pratemis, though Baptisia 

 leucuntha, Aster novae-angliae, Aster iivnltifloru^ and Kulinia 

 still persist in spots. On the slope Poa prate nsis is creeping in, 

 giving a marbled appearance where its colonies are supplanting 

 Aster Drummondi, Solidago rigida,- nemorosa, Gentian puber- 



Fig. 15S. — Uliiius americana, Populus deltoides and Pyrus lowensis. — 

 pioneers of the forest advancing up the ravines. 



ula, Andropogon scoparius, A. furcatus, Sporobolus crijptandrus, 

 Boutelua curtipendida, B. Mrsuta and Koeleria criMata, Lichens, 

 Cnicus Hillii, Liatris, Cacalia tuberosa, Oenothera biennis and 

 Astragalus cargocarpus. These plants have hard, short, thick 

 subterranean stems with intensive root systems occupying smaL 

 space. They seem to thrive in the hard, gi*avelly, stony, drv soil 

 of these hill crests where blue grass and other ruderal invaders 

 have not penetrated. The most of tlie ruderals thrive bcsr with 

 more of moisture and a finer soil than is here foimd, hen^e these 

 hill crest types 'by their tolerance of this habitat can maintaiii 

 such a formation much longer after invasion processes have com- 

 menced than the hillside and alluvial associations can maintain 



