162 



four predominant species are all small, and produce low flat 

 bunches, giving the association an appearance widely different 

 from the original. The sedges, especially Cy penis Schweinitzii 

 and Carex gravida still persist, almost as abundantly as before. 

 Certain other species are much more abundant, such as Monar- 

 da punctata, Cassia chamwcrista, and Opuntia humifusa, as well 

 as the less important Polygonum tenue, Crotonopsis glandulosa, 

 and Polygala verticillata. Leptilon canadense and Ambrosia 

 psilostachya are usually very abundant, but on account of their 

 slender habit are not very conspicuous. Euphorbia Geyeri, more 

 characteristic of blow-sand, is in some places common, and 

 Chrysopsis camporum, Froelichia campestris, (Enothera rhombipel- 

 ala, Croton glandulosus, Lespedeza capitata, Lithospermum line- 

 arifolium, and Commelina virginica are well distributed, but 

 seldom plentiful. 



Table II. shows the composition of the vegetation of a pas- 

 tured bunch-grass prairie, which, however, has not been in pas- 

 ture for two years preceding. The first quadrat is near the foot 

 of a dune, and the survey ran up the dune, ending near its top. 



Table III. gives the results of a survey of a field which has 

 been used for pasture for several years. The bunch-grass has 

 been largely destroyed, and is replaced by Opuntia. Ambrosia, 

 and Leptilon. 



THE BLOW-SAND ASSOCIATION. 



The bunch-grass prairie, while of a more or less permanent 

 nature, is in no sense a climax association, but may be modified, 

 through the action of certain physical factors on the one hand 

 and of biotic factors on the other, along two definite and dis- 

 tinct lines, culminating in two widely different plant associa- 

 tions. In the first case the wind is the principal factor, and 

 primarily .through its influence the bunch-grass association is 

 changed into blow-sand and blowout associations and ulti- 

 mately into a prairie. In the second case, biotic factors are of 

 chief importance, and the prairie is finally succeeded by the 

 black-jack forest, representing an entirely distinct formation. 



Wherever considerable bodies of sand occur, the wind 



