174 IOWA ACADEMY OF SCIENCES. 



sippi basin occur in the second or older alluvial flood plain, 

 namely, 



Morus rubra, Gytnnocladus Canadensis, 



Juglans nigra, Celtis occidentalis, 



Ultnus Americana. 



These plants occur at an altitude of nearl}^ one hundred 

 feet above the flood plain of the Missouri. The same 

 species also occur on the western slope of the hills where 

 sufficient a^e has been attained. Their occurrence under 

 these conditions is due to lines of least tension. In the 

 Mississippi basin and its tributaries such places would be 

 occupied by — 



Acer nigrum. Querctis rubra 



Juglans cinerea. Quercus tinctoria. 



Querciis alba. Cratcsgus species . 



When we compare the trees we find but few prominent 

 species of western Iowa that occupy the uplands of east- 

 ern Iowa, namely, Crataegus mollis Quercus macrocarpa Q. 

 rubra, Ulmus fulva and a few others It seems to be a 

 general law that closely related species orenerally have 

 different habitats. Juglans cinerea along the Mississippi 

 occupies the higher stony hills and this is more and more 

 evident as the region of its greatest prominence is reached. 

 It is easy therefore for Juglans nigra and its other south 

 ern types to become important ravine and bluff plants 

 along the Missouri. 



Towards the east the xerophytic area becomes increas- 

 ingly less, the ravines being filled to a considerable extent. 

 These older ravines contain larger amounts of humus. 

 These soils being well aerated permit decomposition and 

 nitrification much more readily than in older soils, hence 

 the appearance here of such mesophytic plants as 



Cystopceris fragilis . Viola puhescens . 



Dicentra cucuUaria. Uviilaria ^randiflora. 



Stnilacina racernosa. Aniphicarpcea monoica. 



These basins filled with humus also are more subject to 

 washing owing to changes brought about by cultivation at 

 the base of a ravine or the making of roads. These banks 

 contain no plants though there is enough moisture present. 



