202 CLIMAX FORMATIONS OF NORTH AMERICA. 



order of succession to abundant fresh water, Spartina patens, Juncus gerardi, 

 Scirpus americanus, and Aspidium thelypteris. The environmental factors 

 which control distribution and succession were determined to be (1) sub- 

 stratum, (2) currents, (3) tidal changes in water-level, (4) salinity, and (5) water 

 temperatures, and were related to the occurrence of the various dominants. 



Sherff (1912) has reached the following conclusions in the study of the 

 marsh-plants of Skokie Marsh in Illinois: 



Readings by atmometer at different levels in Phragmites and Typha show 

 that the evaporation is proportionate to the height above the soil. In certain 

 species, Teucrium ocddentale, Polygonum muhlmbergii, etc., the depth of the 

 water-table is much the most potent controlling factor. Two or more species 

 may live together in harmony because (1) their underground stems may lie at 

 different depths, (2) their roots may be produced at deferent depths, (3) roots 

 at the same depth may make unlike demands upon the soil, (4) the aerial shoots 

 may have unlike growth-forms, or (5) similar growth-forms may vegetate 

 chiefly at different times of the year. If one or more of these conditions 

 control the composition of a community, the latter may be called comple- 

 mentary. The different root-systems may function in a competitive or com- 

 plementary manner, as the case may be, but even if the root-systems be com- 

 plementary, the community may be competitive because of the competition 

 between the aerial parts. Competitive root-systems may render competitive 

 some communities otherwise complementary. 



The same author (1913 : 154) has also studied the evaporation conditions in 

 different communities of Skokie Marsh, and foimd that evaporation was 

 usually lowest in the center of the reed-swamp, somewhat higher in the swamp 

 meadow, still higher in the outer part of the reed-swamp, and highest in the 

 Quercus-Fraxinus forest. 



X 1 I 



§ I jtP'ne barren formation 



n Prunus macitima Association 



ffi t 



S: Ammophila Association 



t 



Ass 

 Juncus gerardi Association 



Scirpus Association 

 Bacchars Association 



■2— Extreme tide line 



uj Xanthram-Atriplex Association % 



I \ '^ 



High tide line ^ 2 



I \ ' 



m 



u '^ Spartina cynosuroides Association 



q 



Spartina patens Association 



Low tide line ROCKWEED FORMATION. 



I 



roit^. 



g ^ ^^ — ^OSTERA FORMATION 



iij Cnondrus Association Mud and sand 



s Rocks 



S * > » 



\ ^^Dasya Association 



'Sand and gravel 



<n EXPOSED PROTECTED 



Fig. 5. — Haloaere at Cold Spring Harbor, New York. After Transeau. 



