382 GRASSES OF IOWA. 



of Dickinson, Emmet, Palo Alto, Buena Vista, Sac, Carroll, southeast 

 part of Audubon, Adair, Warren and Appanoose counties. The general 

 trend of the streams is in a southeasterly direction. The counties of 

 Lyon, Osceola, Sioux, O'Brien, Plymouth, Cherokee, Ida, Woodbury, 

 Monona, Crawford, Harrison, Shelby, Pottawattamie, Mills, Cass, 

 Montgomery, Fremont, Page, Taylor, Adams, Union, Ringgold, De- 

 catur and Wayne are drained entirely toward the Missouri, although 

 several streams of considerable size reach the Missouri in that state. 



GENERAL TOPOGRAPHY. 



General topography exerts a marked influence on vegetation. This 

 is much more pronounced in regions with considerable altitude, since 

 some plants are quite sensitive to slight changes in altitude and other cli- 

 matic conditions. It is, therefore, important to discuss altitude. Al- 

 though Iowa is a prairie state, and one in which but slight variation 

 might be expected, there are some strongly contrasted topographic fea- 

 tures which have been set forth in papers by Calvin, 1 Keyes, 2 Beyer, 3 

 Norton, 5 Tilton and Bain. 6 



ALTITUDE. 



The altitude varies throughout the state, but, on the whole it is 

 much more uniform than in many other sections of the United States. It 

 is far less variable than the altitudes of Nebraska and South Dakota. 

 Though the altitude of Dickinson, Emmet, Palo Alto and Carroll coun- 

 ties is greater than those occurring in eastern Iowa, especially northeast- 

 ern, the country presents a far less broken appearance than the rugged 

 bluffs along the Mississippi in the region about New Albin and Mc- 

 Gregor. While altitude is an important factor in the development of 

 plant life, especially in mountainous regions, as in the Rockies and 

 elsewhere, where different species often take on an entirely different char- 

 acter within a few hundred feet, showing marked zonal distribution. Al- 

 titude is not so important a factor in the distribution of grasses and other 

 plants in Iowa as might seem at first sight. The general aspect of the 



1. Geology of Allamakee county. la. Geol. Surv. , 4. 



2. Geology of Lee county. la. Geol. Surv. , 3. 



3. Geology of Marshall county. la. Geol. Surv. 7. 



4. Geology of Linn county. la. Geol. Surv. 7. 



6. Geology of Madison county. Ta. Geol. Surv. 7:494. 



