1922] WATERMAN—PLANT COMMUNITIES 3 
latter refer especially to the beginning of the succession, they are 
more suitable in this connection than such terms as hydrosere, etc., 
which apply to the moisture content of the whole succession, because 
in most cases the initial moisture condition does not persist, and the 
substratum generally approaches a mesophytic condition. While 
this classification may be logically defensible, CLEMENTS does not 
give sufficient consideration to the fact that the actual lines of 
development are determined by the nature of the substratum, that 
the floristic content of the pioneer stages is absolutely different in 
clay, sand, or rock, and that it is only as the seres approach the 
climax stage that they begin to converge and to resemble each other. 
Furthermore, standing water should be regarded as a type of sub- 
stratum, because its pioneer stages are practically identical in ponds 
on rock, sand, or clay, and are quite different from the pioneer 
stages of wet sand or clay, to which stages the term hydrarch 
should be restricted. The subdivisions of the primary succession 
(prisere), therefore, should be sand succession (psammosere), clay 
succession (geosere), rock succession (lithosere), and aquatic suc- 
cession (hydrosere). The first three successions have wet and dry 
initial stages (hydrarch, xerarch). It is evident that this classifi- 
cation does not distinguish the many types of substratum containing 
mixed sand, clay, and gravel. It does not seem, however, that these 
are sufficiently well marked or sufficiently different as to vegetation 
to warrant establishing one or more additional seres for them at 
present. 
The terminology of the units of genetic synecology is being much 
discussed at present. It is generally agreed that the fundamental 
unit in the developmental classification of communities is the associ- 
ation. At first this was defined in terms of the habitat, but in 1921 
NICHOLS (7), as a result of several questionnaires sent to eighty-five 
ecologists, reported at the recent meeting of the Ecological Society 
of America at Toronto that a large majority of the ecologists con- 
sulted favored the following statements: (1) That the term plant 
association be recognized as applicable both to the abstract vegeta- 
tion concept and to the concrete individual pieces of vegetation 
upon which this concept is based; (2) that plant association in the 
abstract be defined somewhat as follows: an ecological vegetation- 
