34 PLANT LIFE OF ALABAMA. 



The opportunities for the dispersion of species by these means are 

 more or less diminished by intervening extensive water and desert 

 areas or high mountain ranges. 



These primary and secondary factors, in their ever- vary ing combi- 

 nations, are the conditions by which the various associations of species 

 and their restriction within specific areas of greater or less extent are 

 determined. Such areas constitute the floral regions; and the system- 

 atic relationship of the different species, their numerical proportions, 

 and their various assemblages impart to each region its floral character. 



Suitable environment, that is, a proper combination of conditions of 

 moisture, sufficient room and light, proper exposure, etc., determines 

 the place in which a plant finds all the requirements for its existence 

 met, that is, its habitat. 



The conditions which outline its habitat, in combination with the 

 greater factors of latitude, altitude, rainfall, etc. , determine the distri- 

 bution of plants over wider areas, in which the particular plant (species) 

 may find few or many suitable localities, which areas constitute its 

 rcmge. Within this range the plant may be found in few or many 

 places, isolated or gregarious, but outside of this range it does not 

 occur. 



DISTRIBUTION OF SPECIES AS DEPENDING UPON GEOLOGICAL 



HISTORY. 



The distribution of plants can not always be explained on the ground 

 of their dependence upon the atmospheric and terrestrial factors. 

 The differences in climatic conditions become too insignificant to 

 explain the confinement of many species within extremely narrow 

 limits, and at the same time topographic and other conditions of 

 environment offer no satisfactorv account. Premising the theory 

 that the existing plants are the descendants of similar types which 

 flourished in past periods of the history of our globe, in most instances 

 an explanation is easily found on geological grounds. Viewed in this 

 light, the occurrence of the hemlock (Tsuga canadensis), for example, 

 with its northern companion, the sweet birch, on the extreme southern 

 extension of the Allegheny Mountains, in Winston County, Ala., in 

 a completely isolated spot hundreds of miles distant from the range 

 of its distribution, can be accounted for when they are regarded as 

 the sole remnants of the northern arboreal flora which during the 

 glacial period was pushed to lower latitudes and which on its recession 

 to cooler zones left these trees behind in the narrow valley of the 

 Sipsey River, where at present the former shades the cliff-bound 

 banks. The Torreya ( Tumion taxifoliuni) and the Florida yew (Taxus 

 floridana) of the valley of the Apalachicola River in western Florida, 

 the American smoketree or chittamwood (Ootinm cotinoide*} in its 

 isolated localities in north Alabama and southwestern Missouri, and 



