CHICAGO AND VICINITT. 55 



lake bluff, higher than that at Beverly Hills and only a mile back of 

 the present lake shore, is tenanted by a high grade type of mesophytic 

 forest. 



It will be instructive to make a few comparisons between lake bluffs 

 and other plant societies. Closest to the lake bluff in a physiographic 

 sense is the river bluff. When a stream has banks of clay, the con- 

 ditions seem decidedly similar, and yet the flora is not the same. A 

 comparison of the lake bluffs at Glencoe with the bluffs along Thorn 

 creek shows that some species are common, notoriously Ostrya, Rhus, 

 Quercus, Populus. Yet the differences are still more striking, for the 

 bluffs along Thorn creek do not show Salix glaucophylla nor Shep- 

 herdia; most striking of all, however, is the entire absence of conifers. 

 When we compare the lake bluffs with the rock bluffs of the Illinois 

 river, we find that the resemblances are greater than the differences, 

 since the river bluffs have conifers, though even here some of the lake 

 bluff forms are absent. When, however, we compare the Glencoe 

 bluffs with the dunes, we find that all of the dominant shrubs and trees 

 of the bluff are found also on the dimes; not only this — the dominant 

 bluff forms are dominant on the dunes also. 



The facts of the preceding patagraph are pregnant with signifi- 

 cance. One obvious corollary is that, given similar soils but dissimilar 

 conditions of atmospheric exposure, as at Glencoe and Thorn creek, 

 the vegetation is unlike. Another and more striking corollary is that, 

 given the most dissimilar soils possible, viz., the Glencoe clay and the 

 dune sand, we still have similar vegetation, because the atmospheric 

 conditions are the same in the two cases. The evidence of the Illinois 

 river bluffs is less clear; they are more xerophytic than the bluffs along 

 Thorn creek; but whether this is chiefly due to rock as against clay, or 

 to greater exposure, is not certain. At all events, these facts show that 

 it is not enough to know about chemical or physical conditions in the 

 soil. We cannot divide plants into those of clay, rock, and sand, but 

 must take into account that most plants have a wide range of life, so 

 far as soil is concerned, provided the atmospheric conditions are con- 

 genial. The chief exception to this statement seems to be found, not 

 in the original soils, but in the superimposed humus. There are many 

 plants that require humus for their occurrence in nature, but it makes 

 no difference whether the subsoil is rock, sand or clay, provided alone 

 that the humus is present in sufficient quantity. It is by reason of this 

 last fact that the mesophytic forest can appear in all conditions in this 



