TEMPERATE EASTERN NORTH AMERICA AND SOME OF 



THE SYSTEMS WHICH HAVE BEEN USED IN 



SUBDIVIDING IT. 



In discussing the distribution of the plants mentioned in this 

 sketch it will rarely be necessary to go beyond the limits of tem- 

 perate Eastern North America, a region which may be regarded 

 as one of the primary phytogeographic provinces of the earth, 

 since its flora is mostly endemic. Out of some 6000 known species 

 of vascular plants indigenous to this part of the continent, 

 probably all but a few hundred are confined to it, being hemmed 

 in on all sides by barriers of various kinds : the arctic climate on 

 the north, the tropical climate on the south, the ocean on the 

 east and partly on the south, and the arid region on the west. 



For subdividing the flora of temperate Eastern North Amer- 

 ica on geographical lines several different methods have been 

 employed. Among them are those based on political boundaries, 

 parallels of latitude, altitude, temperature, drainage basins, and 

 geological formations. The first two are probably the most 

 common, but have little natural significance and are used mainly 

 for convenience. Altitude and temperature vary gradually 

 from place to place, so that the boundaries of zones based 

 on these factors are purely arbitrary, except in a few special 

 cases such as the seashore and the frost line. Drainage basins 

 have little to recommend them, from a phytogeographical 

 standpoint, except definiteness of boundaries; but the areas of 

 different geological formations — in temperate Eastern North 

 America but not necessarily in all other parts of the world — seem 

 to answer the purpose best of all, as will be shown below. 



The intimate relations between vegetation on the one hand, 

 and soil and topography (which are obviously correlated with 

 geology) on the other, are evident to every observer of geographi- 

 cal phenomena. Certain types of soil and topography are often 

 fairly constant throughout considerable areas, and usually ter- 

 minate more or less abruptly at their edges, so a classification 



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