1914] VESTAL— PRAIRIE VEGETATION 



393 



THE LICHEN ASSOCIATION 



Lichen formation. — Shantz 17, p. 188. 

 Lichen formation. — Shantz 18, pp. 22, 62. 



The lichen growth on rock surfaces contributes an element 

 to the vegetation of detrital soils of the higher mesa and mesa- 

 terrace levels within the region. Surface rocks are present on 

 practically all the mesa-tops. The dominant species, as given 

 by Shantz (17, 18) are Parmelia conspersa (Ehrh.) Ach., Rinodina 

 oreina (Ach.) Mass., Lecanora calcarea (L.) NyL, and Lecanora 

 subfusca allophana Ach. 



THE MAT ASSOCIATION 



Mat formation of buttes and cliffs. — Pound and Clements 13, p. 376. 

 Mat formation of buttes and cliffs. — Harshberger 7, 535 (after Pound 

 and Clements). 



The open growth of dwarfed perennials which occupies sterile 

 gravelly stations should perhaps not be regarded as a separate asso- 

 ciation; it is, however, distinctive in growth-form, and is frequent 

 and well defined on buttes and hogbacks of northeastern Colorado, 

 eastern Wyoming, and western Nebraska. It is physiologically 

 * similar to the Bouteloua hirsuta growth, and to the Artemisia frigida 

 consocies, sometimes being found with these (fig. 8), not, however, 

 in extreme conditions of coarse soil and exposure to sun and wind, 

 as on certain abrupt mesa-crests. A number of the species are 

 found also in gravel slide growths of the mountains. These growths 

 have been studied by Clements, and by Schneider (16), and are 

 akin to the mat growths of stony habitats of the plains, in several 

 respects. 



Distinctive plants of the mat association are: Townsendia 

 exscapa (Rich.) Porter, Orophaca tridactylica (Gray) Rydb. {Astraga- 

 lus tridactylicus Gray), Lesquerella montana (Gray) Wats., Parony- 

 chia Jamesii T. and G., Hymeno pappus filif otitis Hook., Gilia 

 pinnatifida Nutt. Several species of Ckrysopsis, Geranium 

 Parryi (Engelm.) Heller, Oreocarya virgata (Porter) Greene, and 

 Phacelia heterophylla Pursh are frequently present. 



