204 MINNESOTA BOTANICAL STUDIES 



at any spot to exceed fifteen feet, although Bkie Lake in Garden 

 County is said to be very much deeper than this. 



The lakes of the sandhills occupy narrow valleys or portions of 

 broader valleys. Well-defined groups of lakes, characterized by 

 the presence of a number of larger sized lakes and numerous smaller 

 ponds and marshes of various ages, occur in eastern Cherry County ; 

 southwestern Cherry County at the headwaters of the Middle and 

 North Loup rivers ; central Brown County ; southwestern Holt 

 County, feeding streams whose union gives rise to the Elkhorn 

 River ; at the headwaters of the Dismal River in Grant and McPher- 

 son counties ; in north central Garden County above Blue Creek ; 

 and a group of quite alkaline lakes and ponds in southern Sheridan 

 County. The largest lakes are to be found in the Cherry County 

 and Garden County groups among which Dad's, Hackberry, Wil- 

 low, Big Alkali, Red Deer, Dewey, Crescent, and Blue lakes are the 

 best known. While in a number of cases streams arise from a lake 

 center, yet it appears that in no case does a lake or a lake group 

 possess a direct surface outlet by way of such streams. The drain- 

 age is typically subterranean and usually through an extensive 

 series of springs and marshes. There are often many large surface 

 springs along the upper courses of such streams. 



There is abundant topographic and other evidence to indicate 

 that in some of the lake centers a number of the lakes that are now 

 rather widely separated were at one time united into one large body 

 of water which occupied adjacent valleys. The old shore line of 

 such lakes is indicated by the margins of the present closely asso- 

 ciated or tributary valleys and wet meadows. 



GENERAL PLANT-LIFE CONDITIONS 

 Climatic and Soil Factors 

 As various writers have pointed out (24, 25, 26, 27, 30, 31, 36, 

 Z7 , 50, 51, 57, 60) the flora of a sand dune area consists partly of 

 plants especially adjusted to meet the rigors of sandhill conditions, 

 and partly of species found sometimes with greater abundance in 

 other associations, but whose successful responses under the new 

 conditions have enabled them to tolerate the vicissitudes of a dune 

 environment. This quality that I may term dune-tolerance is pos- 

 sessed to a greatly varying degree by different species. This differ- 

 ence enables us to classify certain species of plants as pioneers in 



