1922] WATERMAN—PLANT COMMUNITIES 29 
stage, and not the grass meadow. If we accept the hypothesis for 
the very recent formation of the large grass meadows, the absence 
of trees might be attributed to shortness of time, but the same 
condition is found in the smallest meadows, which from their posi- 
tion and general appearance must be regarded as contemporaneous 
with the wet depressions and with the sand ridges themselves, and 
here there should have been ample time for invasion. It does not 
follow necessarily that the grass meadow is an edaphic climax, but 
it is evident that that association will remain stable for a very long 
time. As already noted, the bog mat passes relatively rapidly into 
a tamarack cedar forest, which quickly receives hydrophytic decidu- 
ous elements, and thus passes into the climax deciduous forest. 
Summary 
1. Genetic synecology is that part of ecology which deals with 
the developmental relations of plant communities. In a limited 
region the development of successions (seres) is definitely related 
to the character of the substratum. In this region two such seres 
are found, the sand succession (psammosere) and the aquatic 
succession (hydrosere). The clay-gravel succession (geosere) has 
reached its climax on the surrounding moraines, and is observed 
only as it invades the sand ridge region. 
2. In this study the successional units, the concrete association 
and formation, are defined as follows. The association is a plant 
community of essentially uniform (or homogeneous) physiognomy 
and ecological structure, and of essentially uniform (or homo- 
geneous) floristic composition as regards dominant species. The 
formation is an association-complex characterized by a dominant 
association, but including all adjacent associations, whether mature 
or immature, and other more or less anomalous or unidentified com- 
munities associated with them. The unit above the formation is a 
formation complex, or aggregate, and is composed of the formations 
of a definite region which may be limited by climatic or geographic 
boundaries. The ground occupied by an association is called a 
locality, that occupied by a formation an area, and that occupied by 
a formation complex a region. 
3. The vegetation of the region studied is found to be a forma- 
tion complex consisting of a sand dune formation, a sand ridge 
