18 ECONOMIC BOTANY OF ALABAMA. 



etc.) ; friends and enemies in the animal kingdom, and 

 competition of other plants of the same or different 

 species; as well as some characters of the trees them- 

 selves, such as the history of their migrations, and their 

 adaptations for dissemination. 



We can readily believe that if the force of gravitation 

 or the composition of the atmosphere varied much in 

 different parts of the world these variations would give 

 rise to important differences in vegetation; but as it is, 

 these factors are so uniform over the whole earth that 

 they have no appreciable geographical significance. 

 Terrestrial magnetism, atmospheric electricity, radioac- 

 tivity, and the movements of the moon and other celes- 

 tial bodies may have some influence on vegetation, but 

 these influences are as yet unknown, so that we need not 

 consider them further at present. 



Geological history is doubtless a very important geo- 

 graphical factor, but we do not yet know enough about 

 its details to separate its effects satisfactorily from 

 those of present environment. The density of the air, 

 which varies with altitude, probably affects plants some- 

 what (as it certainly does animals), but such effects, if 

 any, are obscured by corresponding altitudinal varia- 

 tions in light, temperature, and atmospheric humidity, 

 whose effects are much better known, for they can be 

 more easily isolated by experimental control. Altitude, 

 although very easy to measure and map with accuracy, 

 hardly needs to be taken into consideration in Alabama, 

 for within our limits there is about as much difference of 

 average annual temperature due to latitude (with a 

 range of nearly five degrees) as to altitude, with a range 

 of only 2,400 feet. And nearly all the trees growing 

 on the highest mountains of Alabama can be found 

 flourishing at much lower altitudes in the immediate 

 vicinity or even considerably farther south. 



Temperature is a very important factor in differen- 

 tiating the vegetation of tropical, temperate and arctic 

 regions, and the great differences betwen the vegetation 

 of humid and arid regions can safely be ascribed mainly 

 to differences in the yearly amount of precipitation; but 

 within the limits of a single state like ours neither of 



