84 



companying- it is an increased development of Bouteloua hirsuta, 

 which, as already noted, may almost be regarded as a mat. Until the 

 mats become continuous there is an excessive growth of the usual 

 interstitial plants. Aristida tiiherciilosa, Ambrosia psilostachya, and 

 Monarda punctata are especially abundant, and the others of less 

 frequency are Talinuni riigospennuin, Spccnlaria perfoliata, Brig- 

 eron raiiiosus, and Arabis lyrata. The latter species frequently grows 

 by the hundred upon the mats of Selagindla. Opuntia RaHnesquii 

 also occurs rarely, and the only known Illinois station for Opuntia 

 fragilis is in one of these reversional stages and the adjoining bunch- 

 grass association. The perennials probably include both pioneers and 

 relics, but they can not be distinguished in the field. Those obsei^ved 

 are Aster sericciis, Aster linariifolius, Pentstemon hirsutus, Callirhoe 

 triangulata, and Lithospenmim Gmelini. Beside the nomial bunch- 

 grasses, scattered bunches of other species occur, which may be either 

 relics or pioneers. 



As the Selaginclla mats grow older they become dark and char- 

 coal-like in appearance and are frequently occupied by crusts of 

 Cladonia. At a later stage small mats of Antennaria come in. 



The order of entrance of the bunch-grasses was not observed, but 

 depends largely upon the nature of the neighboring areas of the bunch- 

 grass association. At the border of some of these reversional stages, 

 portions of extremely large mats of SclagiueUa w^ere found in the 

 bunch-grass, indicating the invasion of the grasses from the margin 

 toward the center. The dense mats of Selaginclla probably seiwe to 

 check their rapid development. 



The Blowout Formation 



Of all the features of the action of the wind upon sand, the saucer- 

 shaped or bowl-shaped excavations known as blowouts are the most 

 peculiar (PI. VI.I, Fig. 2; PI. VIII, Fig. i). Blowouts probably oc- 

 cur in every large un forested sand region. They reach a large size 

 and a considerable depth, and are frequently a prominent feature of 

 the landscape. The physical conditions and the movement of the sand 

 within them have apparently not been fully described, and the vege- 

 tation of American blowouts is still very imj^erfectly knowii. 



Cowles (iSpg: 195-197) mentioned the blowouts or 'Svind- 

 sweeps" of the south shore of Lake Michigan, but did not describe 

 the vegetation in detail. As usual, they stand in a direction parallel 

 to the wind, and may reach down almost to the water-level. One of 

 them, at Dune Park, Indiana, has steep sides from 30 to 60 feet 



