34 THE PLANT SOCIETIES OF 



No two floras can be more unlike in species or in adaptations than 

 are the typical brookside and swamp floras. Though each type may 

 be called hydrophytic, so far as the water is concerned, the vegetation 

 is really hydrophilous in the first case, but pronouncedly xerophilous 

 in the second. Peat bogs, which may be taken as the type of undrained 

 swamps, have a remarkable assemblage of xerophytic adaptations, such 

 as leathery or hairy leaves, and special structures for water absorption. 

 Schimper believes that these structures are due to the difficult absorp- 

 tion in peaty soil, the humus acids and the lack of oxygen being detri- 

 mental to normal root activities. For similar reasons the normal soil 

 activities of bacteria and fungi are lessened, and as a result of this 

 relative lack of decay great quantities of peat accumulate. All of 

 these peculiarities of peat bogs may be referred to the lack of drainage, 

 since the stagnant conditions prevent oxidation and the removal of 

 the humus acids. The lack of drainage is of course due to topographic 

 conditions. Peat bogs and undrained lakes, therefore, are features of 

 a young topography, since several agencies combine to cause their 

 rapid destruction. Rivers may work back and tap the undrained lakes 

 or inlets may fill them up. Probably the most important agent in the 

 death of undrained lakes, however, is the vegetation, as will be seen 

 later. The great abundance of lakes and ponds in the young glaciated 

 regions as compared with older regions to the south is a striking 

 proof of their short life. 



In the immediate neighborhood of Chicago typical peat bogs are 

 scarce. They find their best development in the depressions of the 

 dune region, where they may be called abundant. Wherever a sag 

 between two dunes is low enough to retain moisture for the greater 

 part of the season, the conditions favor the development of an un- 

 drained swamp flora. If the depression is so low that the water level 

 outcrops throughout the year, fhen there is an undrained pond or 

 lake. The first flora in this latter case consists of plants that are able 

 to exist with little or no change in the water of the pond except 

 through rain and evaporation. Among these plants the alga Chara 

 takes a prominent place. The water lilies (Nymphsea and Nuphar) 

 are an exceedingly important constituent of this first vegetation, as is 

 also Utricularia, which is represented by several species. The above 

 species, together with others, play a great part in filling up lakes, since 

 their remains accumulate with almost no decay. Chara in particular 

 is a soil former of great importance. The rapidity with which these 



