310 IOWA ACADEMY OF SCIENCE Vol. XXVI, 1919 



transpiration and hence they may be expected to hold their own in 

 such habitats. 



The most xerophytic species is R. pratincola and it further pro- 

 tects itself by not reaching a greater average height than three deci- 

 meters. It thereby receives protection from the surrounding vege- 

 tation and is less exposed to the greater evaporation due to in- 

 creased height. 



No list of prairie plants would be complete without R. pratincola. 

 It is not limited by any type of soil or topography, being absent only 

 from bogs. It grows on every type of prairie ; flat, rolling, alluvial, 

 exposed ridges, sandy areas, and even sand dunes. The habitat of 

 maximum exposure expresses itself in stunted plants, shorter and 

 finer-leaved than usual. 



R. pratincola is a troublesome weed in the prairie grainfields. It 

 is difficult to eradicate due to its rather deep-growing, perennial 

 rhizome. , 



Department of Botany, 

 The State University. 



