ALTAMAHA GRIT REGION OF GEORGIA 33 



These variable factors depend almost entirely on local conditions 

 of topography and soil, which are indicated for each habitat dis- 

 cussed below. The extent of many of the factors has never been 

 ascertained, while of some others it can only be roughly indicated. 

 Local differences of temperature for instance have not been 

 measured, but these depend mostly on the amount of shade and 

 therefore on the vegetation itself. 



Historically the whole flora without exception is believed to 

 have come into the region since the Pleistocene period (which may 

 have been not more than ten or fifteen thousand years ago). 

 Many of the species have probably come into existence since 

 that time, as already suggested, while' others which are older 

 have found their way in from more or less distant regions. 

 Little is known about the actual facts of migration in the coastal 

 plain, but the study of ranges throws some light on the 

 subject, and in the discussions which follow the probable 

 origin of some of the habitat- groups is suggested by this 

 means. 



Some of the factors under the third head , such as adaptations 

 to environment, are briefly indicated for each habitat-group 

 but it is not expedient to discuss the properties of each species 

 separately where so many are enumerated and so little space can 

 be given to each. This will be a fruitful field for future investi- 

 gation, especially since so many of these plants are of such 

 restricted range that they have not yet come to the notice of 

 morphologists. In the taxonomic list will be found references to 

 anatomical studies which have been made of some of the same 

 species elsewhere. 



Classification According to Habitat. 



A plant-community is generally understood as an association 

 of plants growing in proximity and subject to the same conditions 

 of soil, temperature, moisture, illumination, and other factors 

 which go to make up environment. An assemblage of similar 

 plant-communities, not too widely separated to differ essentially 

 in environment, constitutes a habitat -group. 



There are many analogies between habitat-groups and tax- 



