144 A TEXT-BOOK OF GRASSES 



system of rhizomes. These do not form a sod as the soil 

 is too poor in plant-food to support plants sufficiently 

 near together. Representative species are Ammophila 

 arenaria and Spg^mo-jpatens (Ait.) Muhl. along the sea- 

 coast, Calamovilfa longifolia (Hook.) Hack, in the Great 

 Plains, Elymus^flavescens Scribn. & Smith in the Colum- 

 bia River basin and Elymus arenarius^ L. of the Alaskan 

 seacoast. The first mentioned, Ammophila arenaria, 

 called beach- or mar ram-grass , is a typical sand-binder. 

 It not only produces widely extending rhizomes which 

 may reach great depth, but the culms push upward as the 

 sand drifts around them. (Par. 93). 



182. Pine-barrens. Sandy regions in which there is 

 a sparse forest-cover represent xerophytic conditions, 

 though less marked than those of dune areas. The pine- 

 barrens of the Atlantic coastal plain are typical of these 

 regions. They are mostly level areas covered with open 

 pine woods. Southward they include the turpentine 

 country, and in Florida they become the "high pine land" 

 and the still more xerophytic "scrub." These regions are 

 the home of the smaller species ^i_Bajiium and many 

 other peculiar grasses. 



183. Rocks. On account of the impervious sub- 

 stratum, plants growing upon rocks are insufficiently 

 provided with water unless near some source of supply, 

 such as spray from a waterfall, springs, melting snow and 

 the like. Hence xerophytic grasses may occur in a meso- 

 phytic region. Such grasses a.rgj}i i n rl>gmssesas_rhizornes 

 do not develop under these conditions. 



184. Deserts. Regions in which the deficiency in the 

 water-content of the soil is greater than in prairie and in 

 which the humidity of the atmosphere is very low, are 

 called deserts, or arid regions. Deserts owe their aridity 



