ALTAMAHA GRIT REGION OF GEORGIA 31 



Both in dry and wet places, not only in the area under con- 

 sideration but in other pine-barren regions, we find plants with 

 well-known devices for protection against excessive transpiration, 

 such as reduced, coriaceous, glaucous, or vertical leaves. 



From one end of the region to the other, a distance of some 240 

 miles, the general aspects of soil and topography remain about 

 the same, and we find essentially the same types of vegetation 

 repeated in each county. The slight differences in the composition 

 of the flora of similar habitats in different parts of the region are 

 probably due more to distance than anything else. Differences in 

 climate doubtless have some effect, but probably not as much as 

 distance. It would be hard to find an area of equal extent in the 

 Eastern United States with a more uniformly distributed flora. 

 Probably at least three-fourths of the species known from the 

 whole region may be found in any one of its counties. In view of 

 these facts it seems safe enough to treat the whole region as a unit 

 in most of the discussions which follow. 



Causes of Local Diversity. 



The factors determining the composition of the vegetation of 

 any particular region or locality are extremely complex. The 

 location of each individual plant in the Altamaha Grit region 

 may be considered as due to the combined influence of some or all 

 the factors enumerated in the following synopsis (which has been 

 designed with special reference to the region here discussed, and 

 is therefore not to be regarded as of universal application) . 



A. Present environment. 



1 . Actinic. 



Average intensity of light (varying with the natu*re of the sur- 

 rounding vegetation, slope of ground, etc.). 

 Range of diurnal variation. 

 Seasonal variations (due to defoliation of deciduous trees, etc). 



2. Atmospheric (climatic). 



Temperature. 



Average, maximum, minimum, etc. 



Diurnal and seasonal variations. 

 Humidity. 

 Precipitation. 



Average annual amount. 



Seasonal variations. 

 Wind. 



