142 A TEXT-BOOK OF GRASSES 
Arrhenatherum elatius bulbosum (Par. 241) may serve for storage, 
although the plant last mentioned is not a xerophyte. 
In general, xerophytic grasses have become adapted 
to their surroundings by the production of impervious 
epidermis or of mechanical tissue in leaves and stems and 
by fine foliage. Roll-leaves, described in a preceding 
paragraph (Par. 139), are common. The foliage of xero- 
phytic grasses is nearly always firm and hard from the 
excessive development of sclerenchyma fibers and other 
mechanical tissue and the relative lack of soft parenchy- 
matous tissue. The stomata are in protected places, in 
the longitudinal furrows of the blades or on the inside of 
rolled blades. All these structures tend to retard evapo- 
ration and prevent the loss of water which cannot readily 
be obtained from the dry soil. 
There are four chief habitats where xerophytic grasses 
may be found,—prairie, sandy soil, rocks and desert. 
179. Prairie is open grass land where the soil is deficient 
in moisture. If open grass land occurs upon soil in which 
there is no deficiency of moisture it may be swamp, tundra 
or mountain meadow as indicated under a preceding 
paragraph (Par. 177). Prairies are found as isolated 
areas interspersed through regions that are chiefly occupied 
by a mesophytic flora, as the eastern united States. In 
Iowa and Missouri, they occupy large areas, with wood- 
land interspersed. A vast prairie extending from Texas 
northward far into Canada is called the Great Plains. 
Similar regions in western Asia are called steppes and in 
South America are called pampas and llanos. In Central 
America and in some other countries, they are known as 
savannahs (or savannas). The dominant plants of these 
prairies and plains are grasses. In general, there are 
many species producing rhizomes or stolons so that much 
