278 PRACTICAL COURSE IN BOTANY 
indicate. Local conditions in limited areas may lead to the 
segregation of smaller and more compact groups called socie- 
ties. This term, however, is used rather loosely, being treated 
in some works as synonymous with formations, in others as 
analogous with what have here been defined as colonies. 
317. Principles of subdivision. — The mixed associations 
described in the last paragraph are quite independent of 
Fia. 414.— A colony of Alabama primroses (Gnothera speciosa). 
botanical relationships, and any of the factors named in 
310, or others of a different kind, could be made the basis of 
their classification. They might be grouped, for instance, 
according to their economic uses, or according to origin, 
whether native or introduced, as best suited the purpose of 
the classification in each case. The moisture factor, however, 
has been generally agreed upon as the one most convenient 
for ordinary purposes. Upon this principle plants are divided 
into three great groups : — 
