THE PEAIRIES 273 



4. The well-drained alluvial plains such as are shown at their 

 test along the Missouri, but which are more or less developed 

 along all the larger streams. The undrained portions of these 

 plains develop, of course, a hydrophytic flora. 



5. The prairie ridges which appear in all the forested rougher 

 parts of the state, but are most striking in the more heavily 

 timbered eastern parts where they have been known as "oak 

 openings" because the surrounding forest, consisting largely of 

 •oaks, encroached upon them. These prairie openings are some- 

 times mere tongues of greater prairie areas which extend into the 

 forest, but they are frequently surrounded by forest and may be 

 several miles from larger prairie tracts. They vary in area down 

 to a few square rods and are developed independently of geolog- 

 ical formations except in so far as these determine topography. 

 Detached areas of this kind are shown in plate II. 



6. The sand-dune areas (see plate XI. fig. 2). These are usu- 

 ally considered distinct from the prairie but a comparison of 

 the floras shows that they differ but little. 



In the following table a comparison is made between the floras 

 of these several types of prairie areas and the abundance and 

 relative di.stribution of the species is roughly indicated by letters : 

 a) indicating a dominant type or species; b) principal species 

 -which are common and widespread; c) rather common species, 

 also widely distributed, and sometimes locally common in fami- 

 lies, but on the whole less abundant than (b) ; d) species which 

 are locally common in families, but these often widely separated : 

 e) species represented by but few scattered individuals, but al- 

 most always present on the prairie; and f) rare species, often 

 absent. 



In addition to the letters symbolizing distribution other let- 

 ters are sometimes added to indicate that the species listed was 

 found at but one of the localities which represent a given type 

 area. The additional letter in each case is the initial of the 

 name of the county. Thus in the sand-dune column c^M means 

 that the species so marked is quite common but was obtained 

 only from the ^Muscatine county locality. The following locali- 

 ties were selected to represent the several type areas: 



1. The flat prairie: Emmet county, chiefly the northeastern 



