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change of level; and (4) the deposit, which is continually being 

 added to by sa-nd from the basin. The differences between these 

 physiographic divisions are reflected in the plants and animals which 

 inhabit them ; the groups are sufficiently distinct to be different as- 

 sociations, which take their names from the physical divisions of 

 the blowout. 



.Professor Gleason makes the point that the character of the 

 plant-covering is determined not merely by the kind of movement 

 of the sand (removal or deposition) but by the rate of movement 

 as well. Thus if deposition in a Ijare sand area is only one-fourth 

 inch in a certain season, seeds will be buried to that depth and will 

 not germinate, for the surface layer of the sand is very dry. If the 

 burial is to a depth of two inches, the seeds will probably germinate 

 (different species having different optimum depths for seed-burial). 

 If the burial should be to a depth of a foot or more, none of the seed- 

 lings will reach the surface. Different depths of burial favor differ- 

 ent species of plants. Where plants are already established, deposi- 

 tion of sand may favor those which can grow upward as fast as the 

 sand deepens. Where deposition is extremely rapid, however, even 

 the most rapid growers can not resist burial. As the degree of dep- 

 osition varies in any one place from year to year, conditions for 

 plant growth will be very unstable. In the same manner, removal 

 of the sand, if very slight, makes very little difference to most of 

 the plants. As the degree of removal increases, many of the plants 

 are killed, such species as can endure the undermining having the 

 advantage. When removal becomes very great, even the hardiest of 

 these plants are killed, and the result is bare sand. 



Plant growth in the blowout formation is thus extremely scanty, 

 as the plants are not adapted to the severe and continually changing 

 physical conditions. Most of the plants are slender annuals, the 

 species composition l>eing almost the same as that of the bunch-grass 

 interstitials. A few perennials sometimes persist as relics from 

 former bunch-grass. The animals are for the greater part the same 

 species as those of the bunch-grass interstitial animals, though a few 

 are distinctive of blowout associations, and are almost exclusively 

 species characteristic of open associations. Where the plant growth 

 is well developed, the animals are what may be termed resident 

 or endemic ; but in large expanses of bare sand the animals con- 

 sist largely of roving invaders from neighboring associations. Plant- 

 feeders in such situations are not so well represented as predaceous 

 animals, the former consisting in large part of more sedentary ani- 



