I(i2 



four predominant species are all small, and produce low flat 

 bunches, giving the association an appearance widely different 

 from the original. The sedges, especially <'i/ijerus Sehiveuittzil 

 and ('<tre.r (/)-(ni(la still persist, almost as abundantly as before. 

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

 </(i pHiidafd, Cd^sia cJianurcrisfii, and ()p)(ut'ai liiiiiiifi(S((. as well 

 as the less important Pn/i/t^oiiniii tnnie. ('rofanopsis f//(iii(lnio!ia. 

 and P(>li/(/(i/a rciiicilhifu. Lcjifl/oii ni/iddi'iisc and Aiiilifosia 

 psilosf(iclii/<i are usually very abundant, but on account of their 

 slender habit are not very conspicuous. Eupliorhia Gei/cri, more 

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

 ChrifSopsis fdiii/iiif/nit. Froehc/na fintijiesfns. (Knofhrrn rliomhi pd- 

 (i/(i, Crofoii iiliiii(lnl()Siiii. Lfspeih'Zd cKjiifafti. Lifhaxjimiiniii /iiic- 

 drifoliKin, and Coiniitellna oiryiHica 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 de.stroyed, and is replaced by Opiinfia, AiDhrosia. 

 and Leptllon. 



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 flrst 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 



