Sort REacTIONS OF FERNS 15 
optimum reaction. It is, however, noteworthy, that 
the peculiar relation found to exist among rock ferns,— 
the favoring of acid soils by southern species and of 
circumneutral soils by northern ones,—is likewise well 
marked in the present series of plants. As the same 
sort of relation appears to hold also with other plants 
than the ferns,—in particular with the native orchids,— 
it is sufficiently definite to justify inquiry into its prob- 
able origin. 
Circumneutral reactions are shown by soils which 
either: contain considerable amounts of undecomposed 
carbonate minerals; are bathed by alkaline spring 
waters; or are so situated as to favor the accumulation 
of leaf mold. An acid reaction, on the other hand, 
tends to develop in soils which either: lack carbonate 
minerals; are exposed to the action of rain water so that 
basic constituents become leached out; or are so located 
that peat can accumulate. 
In northern latitudes, or at high elevations, rocks 
disintegrate more rapidly than they decompose, and so, 
if the rocks at any locality thus situated contain suitable 
minerals in the first place, circumneutral soils may 
develop. Glacial deposits are especially likely to con- 
tain undecomposed carbonate minerals, which the ice 
has ground from rock ledges; and actual tests of the 
soils derived from such deposits, in Pennsylvania, New 
Jersey, and the New England states, have shown that 
even after exposure to the weather for many thousands 
of years, since the last ice-sheet retreated, sufficient 
quantities of undecomposed minerals are still present 
in many places to keep the reaction circumneutral. 
The territory left bare by the retreat of the great ice- 
sheet must at first have presented an almost unbroken 
expanse of circumneutral soils, and the vegetation which 
first occupied it accordingly comprised only plants 
which thrive best in such soils. Although acid soils 
