TEE GREAT AMERICAN DESERT 227 



{Prunus melanocafpa), several acres in extent are common in such 

 places. Green ash {Fraxinus lanceolata), cottonwood {Populus sar- 

 gentii) 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 favor- 

 able 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 

 gi-adations 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 

 {J uncus 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 aqiiatica). In the wet valleys along the Loup Eiver and in- 

 wet 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 (Epilohium lineare), St. John's wort 

 {Hypericum virginicum), goose grass {Galium trifidum) and marsh 

 bellflower {Campanula aparinoidcs) . Frequently the ferns are so dense 

 as to cause considerable diSiculty 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 prog- 

 ress that will probably throw considerable light upon the power of cer- 

 tain 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. 



