22 MISSISSIPPI STATE GEOLOGICAL SURVEY [Bull. 
b 
northern United States east of the Mississippi River and the 
srassy plains of the west, and extends south to Virginia and 
the mountains of Georgia and Alabama. 
The Arid Transition Area extends westward from the 
ereat plains across the Great Basin of Colorado, Utah and 
Idaho. 
The Carolinian Area embraces the regions south of the 
Alleghanian down to the Coastal plain of the south Atlantic 
and Gulf Coast. The Upper Sonozar is the arid western ex- 
tension of the Carolinian. The Austroriparian embraces the 
narrow strip corresponding to the Atlantic and Gulf Coastal 
plain, westward to and beyond the Mississippi River into 
Louisiana and Texas to 98° of longitude. Its arid counterpart 
stretching through western Texas, New Mexico, and southern 
California, and southward into Mexico, is known as the Lower 
Sonoran Zone. 
The Tropical Region reaches the United States only in 
the lower part of the peninsular of Florida. 
All of Misstssippi is embraced within the Austroriparian 
Area of the Lower Austral Zone, though a number of species, 
which properly belong in the Carolinian Area, reach this state 
in its extreme northeast corner. 
Light.—Light affects plant life very materially. All 
ereen plants require light to enable them to manufacture plant 
tissue out of the inorganic substances which they absorb from 
the soil and air. But some need more light than others. 
Grasses as a group are light-loving plants, thriving best and 
forming a close, carpet-like growth in open treeless areas. In 
forested areas, the taller trees reach often to great height in 
order that their leafy canopy may receive sufficient light. 
This is especially noticed in growths of pine forests where the 
struggle for light among the individual trees causes many to 
become overtopped and suppressed, so that they die out, while 
those remaining develop tall trunks, lifting their crown of 
feaves to a proper degree of exposure to light. 
Most forests in Mississippi are mixed growth of a great 
