90 



as a mere expansion of the spaces between the bunches. The sand 

 thus exposed is but sHghtly concave, indicating that wind erosion 

 has only begim. The smah quantity of sand removed is piled up 

 in a scarcely perceptible heap along the lee side of the blowout. There 

 is then at the outset a differentiation of two of the physiographic 

 parts, the basin and the deposits, and each of these is soon occupied 

 by its characteristic association. The windward slope and the lee 

 slope may not appear at first, but may be consequent upon the greater 

 development of the excavation. 



THE BASIN ASSOCIATION 



The basin has always the most meager vegetation of the blowout, 

 and in the first stages is either absolutely bare or occupied by one or 

 two perennials left as relics from the Panicum pseudopuhescens as- 

 sociation. The annual interstitial plants, so abundant in that associ- 

 ation, do not grow here because of the removal of sand, which pre- 

 vents the proper planting of their seeds, as will be shown later. As 

 the blowout deepens and widens, a few hardy deep-rooted perennials 

 appear in the bottom, and these constitute the basin association 

 proper. Most notable among the few species is Acerates viridiflora 

 and more especially its varieties laiiccolata (Pi. VI, Fig. 2) and 

 linearis. The varieties reach here their largest size and best de- 

 velopment. The roots go down to a very great depth ; the stems are 

 one to five in number and lie prostrate on the sand. Acerates vir- 

 idiflora, var. lanceolata blooms and produces fruit in this precarious 

 situation; A. viridiflora, var. linearis has not been seen in fruit or 

 flower, is always smaller in size, and may possibly be a juvenile 

 form of the other variety. These two varieties are more widely 

 distributed and more frequent in the basins than in any other habitat. 

 Many blowouts are entirely bare in the basin except for a singde in- 

 dividual of the variety lanceolata. The plants are never numerous, 

 but are conspicuous because of the absence of other species. They 

 are known to occur in the Hanover, Oquawka, and Havana areas, 

 and doubtless occur in other regions where blowouts are developed. 

 They also grow in the bunch-grass and Panicum pseudopuhescens 

 associations, but are never common. It may be that in some blow- 

 outs they are merely relics, but their number and frequency in that 

 situation are incompatible with their distribution outside. Again, 

 they have not been seen on the windward slope, where relics mig'ht be 

 expected. Also, the only plant in a small secondary blowout, newly 

 excavated on the deposits of an older one, was a single plant, exactly 



