2 24 THE POPULAR SCIENCE MONTHLY 



and the plant of greatest importance in its reclamation, after struggling 

 against severe physical conditions for so many years is also the first to 

 disappear. It fails in the competition with the later arrivals and is 

 then forced to find other blow-outs in which it may continue its great 

 work. With the development of the bunch-grasses and the resulting 

 competition all of the blow-out grasses disappear, and then with the 

 incoming of the characteristic members of the bunch-grass association, 

 the change from blow-out to hillside is complete. The only indications 

 of the former history of the place are seen in the grassed-over crater 

 which frequently persists as a characteristic form, and perhaps a few 

 straggling clumps of the blow-out grasses lingering in the near vicinity. 



On the lower slopes of the hills and in the valleys many new species 

 are encountered as the bunch-grasses of the uplands are left behind. 

 Bather low down on the north-facing slopes one frequently finds con- 

 spicuous associations of willows {Salix humilis) and dogwoods 

 (Cornus stolonifera) . The prairie shoestring (Amorplia canescens) 

 also gives tone to the lower slopes in many places by its typical low- 

 branching, ashen-colored plants closely aggregated. Even the taller 

 shoestring (Amorplia fruticosa) occasionally wanders from its usual 

 habitat in the moist valley and is found on north slopes among the 

 willows and dogwoods. The presence of such plants always indicates 

 a higher percentage of soil moisture quite near the surface than is 

 found typically in the bunch-grass association. The explanation of this 

 phenomenon is not hard to find, because such associations and such soil 

 conditions almost always mark an outcrop of clay or other impermeable 

 rock strata which lead the ground water from under the hills in a hori- 

 zontal direction until it is brought near the surface. If the clay or 

 rock does not actually appear on the surface it is usually found a few 

 feet beneath, so that the effect is practically as has been given. The 

 water is frequently so abundant in such situations that it seeps out and 

 collects in cow tracks and other holes in the more tenacious soil. This 

 results in the development of a soggy soil where one finds such mois- 

 ture-loving plants as marsh mint (Stachys palustris), Venus's looking- 

 glass (Specularia perfoliata) , Solomon's seal (Vagnera stellata), heal 

 all {Prunella vulgaris) , long-bracted orchid (Cwloglossum bracteatum), 

 rush {Juncus halticus), liverwort (Marchantia polymorplia) , mosses 

 (Bryum sp.), etc. The cow tracks are frequently filled with filamen- 

 tous algge and free-swimming animals such as Euglena. 



The willow thickets, although quite striking structures on the lower 

 slopes, are still well within the bunch-grass association. But as one 

 gets down into the valleys proper the bunch-grasses, and also many of 

 their associates, are left behind. There are two quite distinct types of 

 valleys in the Sand Hills. The dry valleys are relatively short and 

 narrow and with a good covering of grasses which often form a close 



