Subdivided plant geography into floristic plant geography and ecological plant geography.
The former is concerned with the division of the earths surface into major districts characterized by particular plants or taxonomic groups of plants, with the subdivision of these floristic districts, and with the geographical distribution (both past and present) of the various taxonomic units, such as species, genera, and families.
In a general way, floristic plant geography is concerned with species, ecological plant geography with vegetation.
This mattel- is bound up with the centres of origin and with the past migrations of species; and such questions are usually treated as a part of floristic plant geography.
Plant communities ay be classified as follows: A plant association is a cOmmunity of definite floristic corn)Sition it may be characterized by a single dominant species;, on the other hand, it may be characterized by a number of ~ominent species, one of which is abundant here, another there, hilst elsewhere two or more species may share dominance.
For vei Lmple, the sand dunes of North America and those of western am rope are widely separated in geographical position and there- ge~ e in floristic composition, yet they are related by common rai: