49 



habit is well illustrated by Lcptoloma cognatiim. The bunches of 

 each species are distinct in size, structure, and general appearance, 

 and when in a sterile condition can frequently be recognized by their 

 habit alone. Notes on the individual character of the launches will 

 follow. 



The living and dead leaves of the bunches cover the ground in 

 most cases so closely that other plants can not grow among them. 

 The two bunch-forming sedges, Carex Muhleiibcrgii and Cypcrus 

 Sclnvcmitzii, alone produce bunches so loose that various annuals 

 usually grow within them. Stipa spartca also produces loose bunches 

 through which Ambrosia psihstacliya or Tciicriuui occidentale may 

 grow. A number of small annuals may be found between the radiat- 

 ing culms of Panicuui psciidoptibcscciis at some distance from the cen- 

 ter, while the dense compact bunches of Koclcria cristafa and Lcpto- 

 loma cognatum are entirely free from other plants. 



Besides restricting the growth of other species, and thus retaining 

 the dominance in the association, they act efficiently in preventing-' the 

 blowing of the sand. The greater proportion of the surface is usually 

 entirely covered, and the small inteiwening spaces are so narrow that 

 the sand is not easily lifted Ijy the wind above the bunches. The tend- 

 ency to blow, if present, is usually shown by the slight elevation of the 

 grasses above the concave or trough-shaped interspaces. Neverthe- 

 less, blowing may sometimes take place to such an extent that the 

 whole association is destroyed, and succeeded by another in which 

 Panicuui pscudopubcsccns is the dominant grass, as will be described 

 later. It seems probable that in most of these cases the density of 

 the plant co^'ering has been reduced by pasturing or other recent 

 causes, or, con\Trsely, that under strictly natural conditions the Ininch- 

 grasses permanently prevent blowing. 



In some places the surface is entirely covered, either with bunch- 

 grasses alone or with mat-plants in addition, and there is every gra- 

 dation down to cases where l>ut little more than half the actual surface 

 is occupied. It may be arbitrarily assumed that the bunch-grass as- 

 sociation can not exist with more than half the sand exposed, and it 

 is certain that it may disappear with even more of the surface oc- 

 cupied. The proportion of the ground covered by the grasses varies 

 with the species, the habitat, and the stability of the sand. Of the 

 grasses which tend to cover a relatively small part of the surface 

 Koclcria cristata and Andropogon scoparius are good examples, while 

 the bunches of Lcptoloma cognatum show especially a tendency to 

 become confluent and to cover large unbroken areas. The consocies 

 which contain the largest number of species of bunch-formers are 

 also apt to occupy the space most completely. 



