ECOLOGY 141 
resist evaporation. They include most grasses of swamps, 
bogs, moist land along water-courses, and the inhabitants 
of forest and woodland. Grasses that become weeds in 
cultivated and waste soil usually belong to the meso- 
phytes. In general they have flat blades and will endure 
considerable alternation of conditions between a large 
amount of soil moisture and a moderate amount of 
drought. 
Familiar examples of mesophytes are the common cultivated 
grasses, such as corn, the small grains, sorghum, sugar-cane, the 
meadow grasses, common annual weeds, such as crab-grass and fox- 
tail, and the shade grasses of the tropical forests. 
Certain areas of open grass land include a mesophytic 
flora. Natural meadow land contains too much moisture 
to be classed as prairie. Grass land which contains an 
excess of water, but not enough to support strictly water 
plants, may be classed as bog, swamp, marsh or slough. 
The tundra of northern regions includes a large grass 
element. It is open wet land—wet because the subsoil is 
frozen and there is poor drainage. At high altitudes are 
found mountain meadows that support a mesophytic 
flora, even though the soil be dry, the low temperature 
being the determining factor. 
178. Xerophytes.—These are grasses that are fitted 
to endure soil conditions in which the moisture content 
is deficient. They are, in consequence of this deficiency 
provided with especial adaptations to resist evaporation. 
In xerophytes belonging to other families of plants, water- 
storing organs are common, but among grasses this 
adaptation is rare. 
Panicum bulbosum H. B. K., of New Mexico, is provided with 
a corm which probably acts as a storehouse of moisture. The corms 
at the base of some species of Melica, and the chain of corms in 
