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 conspicuous 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 lowbranching, 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 horizontal 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 moisture-loving plants as marsh mint (Stachys palustris), Venus's lookingglass (Specularia perfoliata), Solomon's seal (Vagnera stellata), heal all (Prunella vulgaris), long-bracted orchid (Cæloglossum bracteatum), rush (Juncus halticus), liverwort (Marchantia polymorpha), mosses (Bryum sp.), etc. The cow tracks are frequently filled with filamentous algæ 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
