4° 



THE PLANT SOCIETIES OF 



tation originates outside the swamp, and may be regarded as xerophytic; 

 however, it often encroaches upon the swamp as the latter develops. 

 At Thornton there is a dead swamp which is now almost entirely occu- 

 pied by this xerophytic bog margin flora, only a few of the original 



swamp plants now remaining. Near Morgan Park is a bog margin flora 

 without a bog; a shallow trench has been dug and in this trench there 

 have appeared various peat bog plants, e. g.. Sphagnum. These con- 



FiG. 22. — Encroachment of bulrushes on Cakiniet lake, showing how plants may destroy 

 lakes. 



siderations show that bog margin floras, though associated with most 

 bogs, are not necessarilv genetically connected with them. 



A word may be said about undrained swamps among the active 

 dunes. The conditions here, of course, are far more severe than in ordi- 

 nary peat bogs and only a few spedes are able to endure in such a hab- 

 itat. The most typical herb vs, Jmicus Balticus littoralis. Seedlings of 

 the Cottonwood, as well as the long-leaved and glaucous willows, ger- 

 minate in these wet depressions. Reference will be made to these plants 

 in connection with the dunes. 



In the morainic portions of our territory there are few if any peat 

 bogs as described above, although they are usually more typical of 

 moraines than of other topographic areas. On account of the clay soil 

 which characterizes the morainic uplands there are many patches of 

 swampy woods throughout the district. Shallow depressions of this 



