148 A TEXT-BOOK OF GRASSES 
188. Distribution of grasses.—As stated in a preceding 
paragraph (Par. 121), the grasses are represented in all 
parts of the earth’s land-surface where the conditions are 
suitable for the growth of flowering plants, from sea-level 
to the snow-line on the high mountains, from Greenland 
to the antarctic continent, from swamp to desert, and 
from the deep forest to the clefts of the boldest cliff. The 
great tribes Andropogonese and Panicez predominate in 
the warmer regions, while the Agrostidese and Festucese 
predominate in the cooler regions. Space will not permit 
of detailed references to the distribution of genera and 
species. Small genera are often much restricted in their 
area while large genera are usually distributed over a 
wide area. The great genera Andropogon, Panicum, 
Paspalum and Eragrostis are found throughout the tropics 
of both hemispheres. Muhlenbergia and Bouteloua, also 
_ large genera, are confined to the American continent and 
are especially well represented on the Mexican plateau. 
Poa and Festuca are found in all continents, but mostly 
in: the cooler regions, extending to the northern and 
southern limit of vegetation, and well represented in 
alpine regions, even of the high mountains of the tropics. 
189. Distribution of species——Species also vary greatly 
as to the extent of the area in which they are found. 
Certain agressive species known as weeds are now wide- 
spread over extensive areas of both hemispheres. Crab- 
grass (Syntherisma sangutinalis ) and goose-grass (EHleusine 
indica (L.) Gaertn.) are familiar examples. Heteropogon 
contortus (L.) Beauv. is an example of a similarly wide- 
spread species which is native throughout its area. Many 
species of the seashore and of marshes are likewise exten- 
sively distributed. Spartina glabra Muhl. and Ammophila 
arenaria are found on the seacoast of Europe and America, 
