24 BOTANICAL GAZETTE [SEPTEMBER 
the original advance, so that in either event the stages of the prisere 
would be approximately as outlined. 
AQUATIC SUCCESSION (HYDROSERE) 
The substratum in this succession is standing water, either in 
closed depressions such as ponds and small lakes, or in open depres- 
sions as bays or lagoons along the banks of lakes or of Platte River. 
These various bodies of water show practically the same vegetation 
for the first three stages, commonly designated as the Pota- 
mogeton, Nymphaea, and Scirpus stages. After that different lines 
of succession are found, depending on the condition of the habitat. 
SAND RIDGE DEPRESSIONS CONTAINING STANDING WATER.—In 
these depressions the water is fairly shallow, but the depression is 
surrounded by sand ridges whose slopes rise directly from the 
water’s edge. Here are to be found either swamp or bog stages 
according to the condition of the substratum. In the swamp type 
the fourth stage is a narrow sedge zone, the shrub growth is scanty, 
mostly willows, and the sand ridge vegetation descends the slopes 
almost to the water’s edge. In the bog type the quaking mat is 
seldom found, but there is a dense growth of sedges and grasses on a 
fairly solid muck foundation. This contains such bog plants as 
Menyanthes trifoliata, Potentilla palustris, and occasionally Sar- 
racenia purpurea, and Sphagnum sp. The shrub zone is a dense 
thicket of Alnus incana, Pyrus arbutifolia, with some Hamamelis 
virginiana, and sometimes with scattered specimens of Thuja 
occidentalis and even Tsuga canadensis mingling with poplars and 
the first trees of the pine-oak association. The water is frequently 
shallow on the north side of the pond, possibly from sand blown in 
by the winds from Lake Michigan, and of course is exposed to the 
heat of the sun, but protected from the colder winds. The south 
side has deeper water, and is more sheltered from the heat of the 
sun on account of the shade of the pines and oaks, but exposed 
to the cold winds. In depressions of this kind the swamp vegetation 
is found on the shallow, warmer, northern side, while the bog type 
occurs on the deeper, cooler, southern side. In depressions which 
are sheltered on all sides and in which the water is deep all over, the 
bog type generally prevails over the whole pond. 
