216 MINNESOTA BOTANICAL STUDIES 



fact that the study in question covered so restricted an area that 

 the student failed to grasp the true significance of the habitats be- 

 fore him. The same careful analysis of the plant cover of a much 

 wider territory might in some such cases reveal the illogical nature 

 of such classifications. 



As to the relation between plant formations and associations 

 and succession I am somewhat uncertain, but at present I prefer to 

 speak of the stages of a succession as associations. There is the 

 further possible desirability of establishing the terms "temporary 

 climax," or even "sub-formation," to cover those successional stages 

 that appear to dominate a rather abnormally long period in the 

 development of a given formation. 



My own tendency has been to apply the term "formation" to 

 as broad units as possible. I have designated some formations and 

 associations by names that indicate some striking physical character 

 of the habitat, while in others the delimitation is made more easily 

 possible by reference to some dominant vegetational characteristic. 

 A combination of these two phases will be seen in a few instances. 

 Recourse to this method has resulted in the differentiation of plant 

 formations and associations that are reasonably definite throughout 

 the sandhills of north central Neljraska. 



I shall endeavor to maintain the now relatively common con- 

 ception and relation of formations and associations. When I use 

 the term plant society I mean to refer to a well-defined area or 

 subdivision of tertiary rank usually characterized by species which 

 become only locally dominant. These species are not included 

 among those which are formationally or associationally dominant. 

 Furthermore the plant society is sometimes characterized by a con- 

 spicuous seasonal appearance. 



From the geographical position of the sandhills of Nebraska 

 we might expect to find that the floras of the east and of the west 

 are to be found here existing under more or less equal terms. 

 Lying as they do between great stretches of Prairie Province to the 

 east and the short-grasses of the Great Plains to the west, the sand- 

 hills exhibit a remarkable commingling of plants from distant biotic 

 centers whose relationships are mirrored in the vegetation of this 

 inland dune region. Pound and Clements (56, 57) long ago brought 

 out some of these striking characteristics. These writers pointed 

 out that four floral "elements" are represented in the Nebraska 

 sandhills. These are (1) a prairie element invading from the east, 



