28 BOTANICAL GAZETTE [SEPTEMBER 
Thuja have come in very thickly, forming a bog thicket which in 
some cases apparently may develop into a cedar forest. The 
tamaracks in this region are all small, with the exception of those 
between the shrub belt and the cedar forest already noted between 
Long and Platte Lakes. The occurrence of bog vegetation in the 
depressions among sand ridges has already been noted. The varia- 
tion in depth on opposite sides also applies to some extent to the 
larger ponds and lakes. In the latter cases the shallow portions 
are characterized by extensive Scirpus colonies, but the bog asso- 
2.—Lower reaches of Platte River, bordered by grass meadows, with some 
eel; growth birches; sand ridges in distance. 
ciations do not appear along the shores of the lakes. In open 
bays and lagoons there was no general uniformity, but swamp 
or bog types were found corresponding to the varying local condi- 
tions. From this it is concluded that the development of the later 
stages of the hydrosere into the swamp or the bog type is dependent 
chiefly on depth of water and temperature. Investigations as to 
acidity have not been made as yet, but it is assumed that here, as 
elsewhere, the swamp type will be associated with a neutral or 
alkaline condition and the bog type with high acidity. The tree 
stage in the hydrosere was found to follow only the quaking mat 
