to revert to natural conditions would exhibit a mixture of associations 
of native and cultivated grasses impossible to decipher on an ecological 
basis unless ecological equivalents are recognized and appreciated. The 
above data are important in suggesting an explanation of the conditions 
existing on the Chicago plain, to be discussed later. 
According to the county soil reports (3) the surface soils of these 
flood-plain areas consist in the main of brown silt loam and sandy silt 
. loam derived from materials washed from the upland and deposited at 
periods of overflow. Usually there is present a considerable amount 
of sand. The subsoil as a rule is sandy silt loam, sand, or clay. In 
some of the swamps peat is abundant. Enormous areas of peat occur 
on the wide flood-plains of the Rock and Green rivers in Bureau, 
Henry, Lee, Whiteside, and Rock Island counties. The peat beds occur 
in depressions between the sand hills and in morainal depressions. 
With the exception of peat all of these soils are rich in the minerals 
required by plants, and their plant associations are similar. Peat, on 
the other hand, is exceptionally rich in organic material and poor in 
potassium, but its plant associations are dominated by the same species 
that dominate wet areas on other types of soil. Calamagrostis canadensis 
is particularly prominent on wet peat. The height of the water-table 
and not the chemistry of the soil is the limiting factor determining the 
order of associations. 
Some progress toward an explanation of the order of occurrence 
of the associations in the hydrarch succession has been made, but the 
data at present are little more than suggestive. Miss Hayden (13, 14) 
found that parenchyma and aerenchyma are more prominent in roots and 
rhizomes of swamp plants than of upland plants on the prairie of central 
lowa, and that the leaves of upland plants have thinner epidermal walls, 
a greater amount of palisade-tissue and more compact mesophyll, and 
are more frequently hairy than leaves of plants on the lowlands. -We 
are greatly in need of more ecological anatomy of this type which should 
be extended to include the dominant species of the prairie associations. 
It should also be checked by experiments similar to those of Cannon 
and Free (2) to determine the effectiveness of aerenchyma in supplying 
oxygen to roots in soils of low oxygen content. It should further be 
checked by actual measurements of transpiration rates to determine 
the comparative effectiveness of the various modifications of leaf 
structure. Transeau in some unpublished work has found that a heavy 
cuticle on leaves does not necessarily mean that they have a lower trans- 
piration rate than leaves with thin cuticle. Sayre (21) found that hairs 
on mullein leaves do not retard transpiration under ordinary intensities 
of wind and light, and that eighty per cent. of the transpiration from 
these leaves is stomatal transpiration. Some of the modifications in leaf 
structure, therefore, affect only the twenty per cent. of transpiration 
directly from the epidermal cells, and there is great danger of over- 
emphasizing their importance in the absence of experimental data. 
