98 
IOWA ACADEMY OP SCIENCES. 
Prof. Lesquereux carries the idea of suitability of soil a little 
farther. He traces all prairies to old time lakes; declares 
that prairie soil is “neither peat nor humus, but a soft, black 
mould, impregnated with a large proporton of ulmJc acid, pro- 
duced by the slow decomposition, mostly under water, of 
aquatic plants, and thus partaking as much of the nature of peat 
as of that of true humus.” * * * “ It is easy to understand,” 
he says, “why trees cannot grow on such kind of ground. The 
germination of seeds needs free oxygen for its development, 
and the trees, especially in their youth, absorb, by their roots, 
a great amount of air, and demand a solid point of attachment 
to fix themselves, etc. ” That is, the reason why our prairies are 
treeless is that they are too wet, and they contain, in virtue of 
their origin, certain elements to trees inimical. Professor 
Whitney also finds explanation of our prairies in the nature of 
the soil, “as the prime cause of the absence of forests and the 
predominance of grasses over this widely extended region. 
And although chemical composition may not be without 
influence in bringing about this result, * * yet we con- 
ceive that the extreme fineness of the particles of which the 
prairie soil is composed is probably the principal reason why 
it is better adapted to the growth of its peculiar vegetation than 
to the development of forests.” 
Whitney makes also another very suggestive statement, the 
importance of which he did not himself realize. He says: 
“Wherever there has been a variation from the usual condi- 
tions of soil on the prairie or in the river bottom there is a cor- 
responding change in the character of the vegetation. Thus 
on the prairie we sometimes meet with ridges of coarse 
material, apparently deposits of drift, on which from some local 
cause there has never been an accumulation of fine sediment; 
in such localities we invariably find a growth of timber. This 
is the origin of the groves scattered over the prairies for whose 
isolated circumstances and peculiarities of growth, we are 
unable to account in any other way.” 
It is interesting to notice the emphasis which Whitney here 
places on the character of this soil. No doubt there is some- 
thing about prairie soils which makes them different from all 
other soils with which we are acquainted, and no doubt differ- 
ence in soils is responsible for the difference in the forms of 
vegetation which they carry, but while both these excellent 
students, Lesquereux and Whitney, came in their surmises 
