AQUATIC ASSOCIATIONS 29 



of the recent precipitation has disappeared. Relative humidity 

 As low as that of the steppe. Temperature Above the average 

 for this area. Light intensity considerable, but many small areas 

 shaded. Wind Except in the protected depressions, average 

 velocity and hot. 



(b) Abundant plants in sand areas: The chief grass is the 

 sand grass (Calamovilfa), the needle grass (Stipa) is very 

 plentiful especially on the more loamy sandy soil, the bunch grass 

 ( Anclropogon) occurs in the rougher areas. The sand worm- 

 weed (Ambrosia), the bee plant (Cleome), Clammy weed (Pola- 

 nisia), annual erogonum (Eriogonum), and wormwood sage 

 (Artemesia) and lupine (Lupinus) are perhaps generally the most 

 conspicuous plants. Other abundant plants are Abronia, Allionia, 

 Franseria, the brome rape. (Orobanche), the groundcherry 

 (Physalis) spiderwort (Tradescantia) and spurge (Euphorbia). 

 The most common shrub is the wild rose (Rosa). 



We found no birds or mammals characteristic of the sand 

 areas here. Because of the small extent of the sandhills such 

 would hardly be expected to be found. 



(c) Characteristics of sandhill vegetation. With few ex- 

 ceptions the plants which grow on sandy soil have long tap roots 

 much longer than even very closely related plants which grow 

 on firmer soils. Plants with long tap roots would probably be 

 less likely to be killed by exposure caused by the drifting of the 

 sand than would those with short roots. The vegetation is de- 

 cidedly coarser than that of the steppe. Coarse plants would be 

 less apt to be covered by drifting sand. Blowing sand undoubtedly 

 prevents plants unable to resist it from becoming abundant. Pos- 

 sibly the secretion of oil from the leaves of various plants such as 

 the clammy weed, Abronia, Orobanche, may be a method of 

 resisting abrasion. 



7. STREAMS, PONDS AXD MARSHES 

 The chief permanent streams are the Little Missouri river, 

 the north and south forks of the Grand river and the north fork 

 of the Moreau river. Most of the creeks are but a string of 

 w r aterholes connected except during rains only by seepage flows. 

 The ponds are of two sorts (a) waterholes along the creeks and 

 (b) "blow outs" most of which contain water only during wet 

 spells. Marshes are very restricted. We found small ones at a 



