JANUARY, 1902. BRUNCKEN STUDIES IN PLANT DISTRIBUTION. 
21 
the veg-etable mould, and in such places the humus is deeper and 
richer than is usual in the oak woods, with their insufficient shade. 
As one travels eastward, the maple makes its appearance, at first 
sparingly and confined to such localities as have just been de- 
scribed. By the time one is fairly within the territory of the Oak 
and Maple Group of the map, maples and basswood and their asso- 
ciated trees mentioned above occur with growing frequency on the 
ordinary uplands a well as in ravines and coves. Going still far- 
ther east, and coming within two or three miles of Lake Michigan, 
one begins to find the beech, the tree which of all trees of the re- 
gion throws the densest shade and grows in the places with the 
deepest and richest humus. 
This geographical arrangement suggests that its cause is the 
amount of humus required for the various species. The oaks and 
their associates demand the least of it. The basswood comes 
next, while maples and beeches demand the richest soil. It might 
be supposed that the distribution of these trees is determined 
by the mineralogical character of the soil on which they grow. 
But this is negatived by a comparison of the vegetation map and 
the soil map found in the Geological Survey atlas. From this it 
appears that maples and beeches are found indiscriminately on 
''heavy marly clay," "lighter marly clay," "calcareous sand" and 
"red clay" ; while the oak forests, although mostly on "lighter 
marly clay," are by no means confined to that soil formation. 
Although sufficient sunshine reaches the forest floor under an 
oak canopy to cause comparatively rapid oxydization of the vege- 
table mould, that process is not rapid enough to prevent the slow 
accumulation of humus even in an oak forest. It is conse- 
quently a mere question of time until there will be sufficient humus 
in all the oak woods to satisfy even the hard maple. But as soon 
as the maple has gained a foothold it prevents by its dense shade 
the growth of oak seedlings, while its own seedlings, with their 
remarkable shade tolerance, can flourish well enough. At the 
same time it gives a great impetus to the further accumulation of 
humus. In the course of ages, therefore, with no new factor en- 
tering into the process, the maple and its associates would supplant 
the oak and its accompanying species everywhere in this region. 
The maple-basswood-beech sub-association is consequently the 
culmination type of forest in this locality. It is safe to assume 
that at one time Milwaukee County was covered by almost pure 
oak forests (with the quahfication to be discussed forthwith), such 
as are now struggling with the prairie formation in the Rock 
River valley and the country immediately to the east of that. 
The further question presents itself : Why are the maples, 
