(Prunus melanocarpa), several acres in extent are common in such places. Green ash (Fraxinus lanceolata), cottonwood (Populus sargentii) and willow (Salix nigra, S. longifolia) also thrive on this soil. In the phim thickets near the river the vegetation often becomes so dense that it is almost impossible to penetrate to the river's edge. The soil in these places is very rich and moist, so that many species of the shade plants of moist rich woodlands find in such thickets very favorable conditions. On the open areas of the river flats buffalo grass and grama grass constitute the best forage known in the Sand Hills. These low, sod-forming grasses are especially valuable as winter forage.
Wet valleys are very common in the northern portion of the Sand Hill region, where the valleys are usually broad and long. The water table is near the surface in these valleys, so that the soil in many places is very wet and swampy. There are in this portion of the hills many gradations from the moderately dry hay valley through wet meadow valleys to valleys with large ponds or lakes. Hundreds of lakes occur in such situations throughout the northern half of the Sand Hill region. There have been two general kinds of wet meadows distinguished. The rush-meadow type is characterized by the presence of a number of rushes (Juncus tenuis and J. nodosus), and bulrush (Scirpus atrovirens and S. americanus). With these occur a few moisture-loving grasses, such as lowland rattlesnake grass (Panicularia nervata) and whorl grass (Catabrosa aquatica). In the wet valleys along the Loup River and inwet places on the river flats a second type of wet meadow is seen in the fern meadow. Shield fern (Dryopteris thelypteris) and the sensitive fern (Onoclea sensibilis) often occur in great quantities in such places with a mixture of willow herb (Epilobium lineare), St. John's wort (Hypericum virginicum), goose grass (Galium trifidum) and marsh bellflower (Campanula aparinoides). Frequently the ferns are so dense as to cause considerable difficulty in walking through this type of wet meadow.
There are two kinds of lakes in the Sand Hills, depending upon the amount of dissolved substances in the water, alkali lakes and fresh water lakes. It has been found that the alkalinity of the lakes varies between rather great extremes, even the freshest of the fresh-water lakes being somewhat saline. Whatever may be the cause of this gradient in alkalinity, it is an obvious fact that the degree of alkalinity exerts a very powerful influence upon the vegetation. Studies are now in progress that will probably throw considerable light upon the power of certain species of plants and animals to adjust themselves to this varying chemical relation. In many of the more strongly saline waters scarcely any vegetation appears, although the beach may be well clothed with
- Pound and Clements.