tor^', on Twin Mounds, at intervals along the western and south- 

 ern shores of the larger lakes, and along the west side of the Des 

 Moines and Little Sioux rivers, the presence of the groves is con- 

 sistent with the conclusions presented in the paper on "The 

 Prairies. ' ' Where the slopes leading down toward the north or 

 northward are gradual the currents of air follow the surface 

 closely, producing exposure to wind quite as complete as on wind- 

 ward slopes, and in such cases the prairie extends quite to the 

 water's edge. Illustrations may be observed at Terrace Park, in 

 the treeless intervals along the shore between Manhattan and the 

 head of the lake, and in the treeless intervals (where the slope 

 will uniformly be found to be gradual) on the south and west 

 sides of the larger lakes generally, and along the west side of the 

 Little Sioux (see Plate Y, ng. 2). 



However, at a niunber of points groves are also found on, or 

 at the base of, abrupt slopes which face the prevailing T\'inds. 

 Such groves are found along the west and north shores of the 

 larger lakes, only, and are e^ddently made possible by the bank- 

 ing up of vapors from the lake against the high banks. Where 

 such banks do not exist on the north and east sides of the lakes, 

 and the surface rises gradually toward the north and east, there 

 are no groves on the lake shore. Striking examples of this tree- 

 less condition are found at the head of West Okoboji lake, at 

 Emerson's Bay, on the north shore of the lower part of East 

 Okoboji lake, at the head of the same lake, and on the north and 

 east shores of Spirit lake. At such places the vapors are swept 

 away from the flatter shores and dissipated over the region ly- 

 ing beyond, and there is no accumulation of vapors under high 

 banks in which a mesophytic forest flora might develop. 



That the flatter leeward shores of the lakes, in which an ac- 

 cumulation of vapors is not possible, are comparatively xero- 

 phytic is shown by observations made in 1913 by ^Ir. D. H. Boot, 

 under the winter's direction, and which are here in part re- 

 corded. 



The obserA'ations were made at stations established along the 

 north and south half -section line in section 16, Tp. 99 X., R. 36 

 W.. north of East Okoboji lake. This is on one of the treeless 

 gradual slopes rising northward from the lake. The following 

 table gives the distances and elevations of the stations, the first 

 being nearest the lake : 



17 



