64 



The reason for its wide extent is obvious. The bunch-grasses 

 all belong to the same ecological type, and, with the unimportant 

 exception of Boiiteloua hirsuta, have approximately the same size. 

 Competition between them therefore is largely limited to a struggle 

 for ground space, and of that there is usually an abundance. There 

 is very little possibility of one species shutting off the light from 

 another, either by its size or by making- an earlier start in the 

 season. The dead leaves and culms with which each bunch is sur- 

 rounded make a good ground cover which holds the sand and ex- 

 cludes the g'rowth of seedlings of competing species. None of the 

 species is distingTiished by a particularly large seed production or 

 by special adaptations for seed dispersal. Few of them spread by 

 underground stems. Taking all these points into consideration, it 

 is clear that there are no particular adaptations which might lead 

 to a monopoly by one species in the consocies. The presence of so 

 many species indiscriminately mixed is caused by their uniform dis- 

 semination and continued by the evenness of their competition for 

 space. For a few species these statements do not hold. The small 

 bunches of Bouteloua hirsuta and the flat ones of Panicum pscudo- 

 pubsccns are easily overshadowed and killed by the growth of other 

 species. The loose, few-leaved bunches of Stipa spartea similarly 

 tend to be crowded out by species of denser habit. These three 

 species, accordingly, are not to be found throughout the consocies, 

 but tend to disappear as the surface becomes more completely covered. 



The numl)er of grasses which occur is large, and includes vir- 

 tually evei-y species of bunch-grass found in the region. Not all 

 of them occur tog-ether, or even in the same area, but in most places 

 three or four may be recognized as of chief importance, while the 

 others have more of the nature of secondan^ species. The following 

 bunch-grasses were observed : 



^Andropogoii scoparius Calamovilfa longifolia 



^Andropogoii f it r cat us *Koeleria cristata 



Sorghastniiit nutans '''Souteloua hirsuta 



^Lcptolouia coguatuiu ^Bouteloua curtipcndula 



Paspahiui sctaceiim Hragrostis t rich odes 



*Pauicuui virgatmn *Bragrostis pcctiuacca 



Panic u in perlonguin Poa pratensis 



Panicum Scribnerianuni "^Cyperus Schwcinitzii 



^Panicuin pseudopuhcsccns Care.v nnihcllata 



*Stipa spartea Care.v Muhlenbergii 



Of these twenty species, eleven, marked with an asterisk, have 

 been noted in some locality as dominant species, that is, so abundant 



