IOWA ACADEMY OF SCIENCES. 157 



with abundant slopes, are scarcely eroded by even the most 

 violent precipitations of moisture." 



Professor Udden's recent admirable report also bears on 

 this question, and sliould not be overlooked by the student 

 of loess-problems. 



"No distinction can be made between the origin of east- 

 ern and western loess. The finer quality and lesser thick- 

 ness of tlie former rather suggest that there had been more 

 moisture (/. c, a shorter dry period during each year) and, 

 hence, less dust; that the winds were less violent, and 

 that there were greater areas completely covered with 

 vegetation, this resulting in the necessity of transporting 

 dust much greater distances, which would therefore be 

 finer. 



CLIMATIC AND EDAPHIC FACTORS. 



While the physiographic and geological formations have 

 an important bearing on the distribution of plants there 

 are certain other factors such as climate and the edaphic 

 that must be taken into account when considering the 

 distribution of plants. Mr. ISIicholas Whitford*in a paper 

 on "The Genetic Development of the forests of northern 

 Michigan'' considers the ecological factors into edaphic, 

 atmospheric, hydrodynamic, and biotic. The hydrodynamic 

 may play some part in the distribution of seeds, and the 

 biotic determine the tension lines between forest and 

 prairie, the atmospheric influence the soil so as to make 

 it receptive for tree growth. 



Altitude. The altitude of the Missouri river basin is 

 not far from 1,000 feet. It is somewhat less on the im- 

 mediate shore lines of the Missouri and more than this 

 towards the interior and northward. The region here 

 considered lies between 39.5^ and 43.5 north latitude. 



The following altitudes are taken from reports of the 

 Iowa Geological Survey t and Gannett's table of altitudes. 



•Bot Gazette 31: 291. 



+Bain, H. F. Geology of Plymouth county. lo. Gi»l. Surv. 8: 320. 

 Bain, H. F. Geology of Woodbury county. lo. Geol. Surv. 5. 

 Bain, H. F. Geology of Carroll county. lo. Geol. Surv. 9 : 59. 



