59 



some cases it is evidently the result of the stabilization of the Pani- 

 ciiin pseudopnhcscens association, in which event it is characterized 

 by the greater abundance of that species, the better development of 

 mats, ^vhich are composed of Selaginella rather than Antennaria, and 

 the greater abundance of Scutellaria parvula and Arabis lyrata. In 

 other cases it is entirely independent of any former blov^ conditions, 

 and then contains less Panicimi pseudopiibescens, mats of Antennaria 

 rather than Selaginella, and a larger proportion of perennials, in- 

 cluding Aster sericeus and Auwrpha canesccns, \vhich are absent on 

 blowing sand. The two types have the same structure and represent 

 the same consocies, notwithstanding their difference in species. The 

 only ecological difference between them, aside from their origin, is 

 their age, and it may veiy properly be considered that the floral differ- 

 ence w^ill gradually disappear as the various perennials succeed in es- 

 tablishing themselves in the younger type. The order of appearance 

 of the species in this process of stabilization will be considered later. 

 There is usually a gradual change in the appearance of the con- 

 socies at its margin as other grasses appear or as Koeleria disappears. 

 The secondaiy species vary but little specifically, but the number of 

 individuals naturally increases or decreases according to the density 

 of the grasses. 



TJie Leptolonia cognatuui Consocies 



This consocies is extensively developed in the Hanover, the 

 Dixon, and the Oquawka areas, and in the last two is by far the most 

 important consocies characterized by a sing'le species (PI. I, Fig. 2). 

 In the Hanover area it is exceeded in extent by the Koeleria cristata 

 and the mixed consocies. It has a wider topographic range than the 

 Koeleria cristata consocies, and is found in the interdunal depressions 

 as well as on the hilltops. 



Leptolonia cognatuni is the principal bunch-fonning grass, and 

 its flat-topped bunches are usuallv so confluent that nine tenths of the 

 surface or more is occupied. The bunches are of such unifonn 

 height and density that the consocies appears as if artificially trimmed, 

 and has a generally g-ray-green color because of the numerous dead 

 leaves mixed with the living". The other g-rasses, which are usually of 

 larger size and bright green in color, stand out very prominently 

 against the background. In the serotinal aspect the plants are in 

 bloom, and the large panicles with the red spikelets are so numerous 

 that the whole consocies has a reddish hue. A few other grasses 

 may at some places occupy enough of the surface to afi^ect the gen- 

 eral appearance of the consocies. They are Koeleria cristata at Han- 



