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 Spartina patens (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 marram-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 of Panicum 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 are bunch-grasses as rhizomes 
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 
