T1901] GENETIC DEVELOPMENT OF FORESTS 315 
presence of nitrifying bacteria. These probably bring about the 
xerophytic structures of plants so commonly seen in hydrophytic 
habitats. 
The areas around the swamp, whether clay, sand, or rock, 
have been undergoing their normal changes, so that the swamp 
is eventually surrounded by a forest. The swamp is finally built 


IG. 11.—A lagoon cut off from Lake Superior by a beach line, resulting in an 
undrained swamp. In the foreground is seen a portion of the original lagoon; back 
of this is the sedge vegetation, and near the conifers is a Cassandra zone. The coni- 
fers are mostly tamaracks (Zarix Americana). 
up high enough above the water level to permit a higher type 
of tree to occupy the area. Thus the tamarack is gradually 
crowded out, and a climax forest is the final stage. 
The life-history is not always as indicated above. , The 
Swamp may become partially drained, but still be too wet to 
Support the highest tree society. In that case the abor vitae 
gradually replaces the tamarack. Associated with the arbor 
vitae are the ash ( Fraxinus sp.), the balsam, the white pine, and 
the yellow birch. Gradually, however, the arbor vitae swamps 
