198 



CLIMAX FORMATIONS OF NORTH AMERICA. 



xerophytic conditions will shortly disappear. The presence of definite groups 

 of plants in each zone is due chiefly to soil conditions in that zone; also to 

 the position of the water-table and secondary changes dependent thereon, as 

 aeration, temperature, composition of the peat, etc. The absence of certain 

 plants from certain zones is due to decrease in the amount of light. Chamae- 

 daphne is apparently imable to grow in this area in a light of 0.003. 



Coons (1911 : 36, 54) has recognized the following series of habitats in the 

 study of the water sere at Saginaw Bay, Michigan: 



1. Lake with the formation of a series of sand-bars becomes — 



2. Bay, changed by a barrier beach or sand-spit into — 



3. Lagoon or swale, changed by vegetation and inwash, and wind-blown sand also, 



into — 



4. Undrained swamp or marsh. Drainage and humus formation change this to — 



5. Thicket and wooded swamp, and — 



6. Drained situations. 



The sequence of formations and associations is indicated by a tabular 

 outline : 



Transeau (1911) finds that the bogs and lake basins of the Huron River are 

 associated with deposits of glacial drift. 



Such basins are caused (1) by the melting of stagnant bodies of ice in old 

 glacial drainage-channels after their abandonment; (2) by the differential 

 settling of fluvio-glacial deposits; and (3) by the unequal deposition of glacial 

 material in moraines and till-plains. In most cases, marl and peat deposits 

 are associated. In one lake studied, the aquatic stage consisted chiefly of 

 Sdrpus lacustris, Castalia tvberosa, and Sagittaria rigida. The next stage con- 

 tained sedges and grasses, with Carex, Panicularia, Heleocharis, Dulichium, 

 Dryopteris, and ScvieUaria dominant. The third stage consists of the willow- 

 maple society represented by Salix, Cornus, Acer rvbrum, and Ulmus ameri- 

 cana as dominants. Along the western or bog margin exists a heterogeneous 

 bog, sedge, and shrub society which connects the aquatic stsige with the final 

 bog stage of the tamarack. In another lake the sequence is indicated by the 

 following stages: (1) aquatic, Potamogeton, Nymphaea;.{2) bog-sedge society, 

 Carex, Heleocharis, Eriophorum; (3) Cassandra^Sphagnum society; (4) tama^ 

 rack society; (5) poplar-willow-maple society. 



Dachnowski (1911 : 1) has discussed the vegetation of Cranberry Island, 

 Ohio, imder the following captions: 



(1) Habitat. (2) Chemical analysis of the substratum. (3) Reducing action of peat soil. 

 (4) Physiological properties of bog water. (5) Bacterial flora of the peat substratum. 

 (6) Origin of the habitat. (7) Flora. (8) Atmospheric influences as ecological conditions 

 for growth: (a) climatic conditions, (b) r61e of substratum temperature in bog habitats, 

 (c) differences between air and soil temperatures, (d) r61e of the evaporating power of the air. 



