226 NATURAL HISTORY BULLETIN 



A further confirmation of this fact is furnished by the dis- 

 tribution of the introduced xerophytic weeds which are marked 

 with an asterisk in the list of phmts. These plants have become 

 almost equally well-established on both sandy areas and ordinary 

 prairie, and demonstrate that conditions in these two kinds of 

 areas are not dissimilar. 



Much emphasis has been placed recently on the succession of 

 floras in sand-dune areas, and a careful study of any such region 

 shows that there is a striking change in the flora as the dune be- 

 comes more and more fixed. But a comparison of different areas 

 shows that the emphasis which is usually placed upon certain 

 species in connection with local studies is misleading if any ap- 

 plication of the results is extended to wider areas. This is il- 

 lustrated by a comparative study of the dune areas of Muscatine 

 and Harrison counties at opposite extremities of the state of 

 Iowa. In Harrison county the first plants to appear in blowouts 

 or on new dunes are Cassia chamaccrista and Dalea enneandra, 

 which are followed very quickly by Crotalaria sagittalis, Stropho- 

 styles helvola, S. JJauciflora, and such well-marked xerophytes 

 as Cenchrus caroliniana, Cypcrus Schwcinitzii, Lygodesmia ros- 

 trata, and Salsola Jiali var. icmdfolia, and later by species of 

 Desmanthus, Desmodium, Lespedeza, Melilotus, and other xero- 

 phytic genera. 



In Muscatine and Louisa counties, on the other hand, the 

 pioneer and most effective hold-fast in blowouts and on new dunes 

 is Tephrosia virginiana, accompanied or followed by both the 

 common species of StrophosfyJcs, Ccnclirns, Carex cephalophora, 

 Cyperus ScKiveinitzii, Litliospermum Gmelini, Pentsiemon 

 granilifJorns, Polanisia trachyspcrma, and finally a long list of 

 other prairie xerophytes. 



In the earlier stages the preponderance of individuals of 

 leguminose species is striking, and in such situations they freely 

 develop nitrifying root-tubercules. This is especially true of 

 the Harrison county area where the leguminose plants greatly 

 predominate, and produce great numbers of root-tubercles. 



should also be remembered in this conuection that the general character 

 of a flora of any region is determined by the least favorable rather than 

 the most favorable periods of the year, especially during the growing 



