220 NATUEAL HISTOEY BULLETIN 



These prairie openings differ much in size. Some are mere 

 tongues or extensions of the greater prairie (see plate III, fig. 1), 

 while others are small areas, entirely detached, and often quite 

 remote from the greater prairies. (See plate II, figs. 1 and 2). 



They differ also in the character of the soil. They may occur 

 on geest, or drift, or loess, or sand. But whatever differences 

 they may present in these respects they agree in appearing uni- 

 formly in rough areas and they have the same flora, a flora 

 identical with that of the drier prairie as is shown in column IV 

 of the table of plants. 



The forested areas of this region are located in the rougher 

 sections and these prairie openings are so situated on the tops 

 of the ridges or on the slopes that they are exposed to the two 

 great agents of evaporation herein discussed, namely the "two- 

 o'clock sun" and the prevailing southwesterly summer winds. 

 The character of the topography therefore determines whether 

 the line between the prairie opening and the forest is sharp 

 or whether there is a mingling of the flora of the two types show- 

 ing a scattering of trees over an area in which prairie plants 

 are also abundant, resulting in the so-called "oak-openings", or 

 "oak-barrens". 



The sharpness with which the prairie openings are sometimes 

 defined is illustrated in plate II, figure 2. This is only one 

 of several openings of this kind which cap the narrow ridges 

 which extend to the west and southwest on the convex side of a 

 great bend of the Iowa river six miles above Iowa City. These 

 openings are removed several miles from open prairies yet their 

 flora is that of the dry prairie. Excepting where otherwise 

 stated the plants listed in column IV of the plant list were col- 

 lected in this locality. In the locality represented in flgure 

 2 the open area, here scarcely more than a rod wide, is 

 covered with a typical unmixed prairie flora. On either side, 

 but especially to the north (the right in the picture) the writer 

 year after year, found an abundance of deep-woods species, such 

 as Osmunda Claytoniana, Adiantum pedatum, Cypripedium pu- 

 hescens, Orchis spectabilis, and others, beginning within a dis- 

 tance of not more than ten or fifteen feet from the margin of the 

 prairie opening. Beyond points from one to four rods from 



