1899 ] DUNE FLORAS OF LAKE MICHIGAN III 
important function which dune plants perform for other plants 
is in the contribution of organic food materials to the soil. 
The oxidation or removal of decaying vegetation is so complete 
on the newer dunes that the accumulation of humus is not 
important. On the more established dunes the mold becomes 
deeper and deeper, and, after the lapse of centuries, the sandy 
soil beneath may become buried so deeply that a mesophytic 
flora is able to establish itself where once there lived the 
tenants of an active dune. The advance of a wandering dune 
often results in the burial of a large amount of organic matter; 
when this matter becomes unburied years afterward it may 
again furnish a soil for plants. Many fossil soil lines have thus 
been uncovered on the Sleeping Bear dunes at Glen Haven, 
ich, a 
IV. The plant societies. 
A plant society is defined as a group of plants living 
together in a common habitat and subjected to similar life con- 
ditions. The term is taken to be the English equivalent of 
Warming’s Plantesamfund, translated into the German as Pflanzen- 
verein. The term formation, as used by Drude and others, is 
more comprehensive, in so far as it is not synonymous. It may 
be well to consider the individual habitat groups in a given 
locality as plant societies, while all of these groups taken 
together Comprise a formation of that type, thus giving to the 
~— formation a value similar to its familiar geological applica- 
pe For example, one might refer to particular sedge swamp 
wees Beas Chicago, or, on the other hand, to the sedge 
Swamp formation as a whole; by this application formation 
Bic term of generic value, plant society of specific value. 
or ee may be still further subdivided into patches 
less radiall € former more or less irregular, the latter more or 
plant SOci = Symmetrical. Patches are to be found in any 
dotiiagar Y, where one OF another constituent becomes locally 
in slike, are conspicuously developed on the beach and 
that of vari Swamps. The term patch or zone has a value like 
“ty In taxonomy. Authors disagree, here as every- 
