152 BOTANICAL GAZETTE [MARCH 
Possibly local prairies will be the final type, or it may be that 
the forest will come in. Fig. 2z, which shows pines encroaching 
upon the grassy areas, favors the latter view. So do some of 
the facts seen in the Calumet valley. 
All of the peat bog types have a characteristic marginal 
flora, 2. ¢., the vegetation at the margin of the original lake is 
essentially alike in all cases. These plants, as well as those of 


Fic. 22.— Encroachment of bulrushes on Calumet lake, showing how plants may 
destroy lakes. 
Cassandra bogs, are the same over wide areas. The most 
common members of the bog margin flora are the sour gum 
(Nyssa sylvatica), the aspen (Populus tremuloides), [lex verticu- 
lata, Pyrus arbutifolia (including var. melanocarpa) , Spiraea salict- 
folia and S. tomentosa, Rubus hispidus, Gaultheria procumbens, 
Osmunda cinnamomea, O. Claytoniana, O. regalis, Betula papy’ t- 
fera, and Polytrichum commune. This vegetation originates out- 
side the swamp, and may be regarded as xerophytic ; however, 
it often encroaches upon the swamp as the latter develops. At 
Thornton there isa dead swamp which is now almost entirely 
occupied by this xerophytic bog margin flora, only a few of the 
original swamp plants now remaining. Near Morgan park is a 

