98 JOURNAL OF THE MITCHELL SOCIETY [ June 
grow for the most part on trees, in humus, or on exposed limestone, 
are largely confined to three well-defined areas. Two of these con- 
stitute ‘‘tropical Florida’’—that is to say, the Florida Keys and the 
Everglade Keys. The other area is that lime-sink region in the north- 
western part of the peninsula, mentioned above and to be referred to 
more fully further on. 
The Florida Keys consist of a chain of low islands built upon the 
Florida Reef mainly south of the peninsula. All of them are remnants 
of what were evidently larger islands in past ages.1° They are really 
situated in the waters of the Gulf Stream, and extend from the At- 
lantie Ocean on the northeast into the Gulf of Mexico on the south- 
west. They are naturally divided into two groups: those of the upper 
or more northern group, which are of coral-rock, and those of the 
lower or more southern group, which are of limestone. All the 
islands are clothed with tropical hammock, except portions of a few 
and here they are either partly heath-like or partly covered with pine. 
These hammocks and pinelands harbor but ten different kinds of na- 
tive ferns. But of these, one only (Paltonium lanceolatum) has not 
yet been discovered on the Florida mainland. 
The Upper Keys are for the most part long and narrow ridges of 
coral-rock and are clothed with evergreen hardwood forests which 
harbor the one kind of fern not yet known to occur on the Everglade 
Keys. The Lower Keys are more spread out, more even, and rather 
less elevated above the sea. They are clothed both with hardwood for- 
ests and, in the case of a half-dozen islands, with pine woods, at least 
in part. The Lower Keys have as yet yielded no ferns not already 
known on the Everglade Keys. In fact, the Florida Keys have a 
much smaller fern flora than the Everglade Keys. Only about one- 
fifth of the species of the Everglade Keys have been found there. No 
doubt in their past the fern flora was larger than it is now. It may 
have rivaled or excelled that of the Everglade Keys, for the Florida 
Keys consist of two areas of different ages, coral and limestone; but 
this region has been for a long time decidedly on the wane as regards 
area, and doubtless also vegetation. 
5 In addition to the leaching process of erosion that has reduced the 
surface of the Everglade Keys, the Florida Keys have had the me- 
chanical and chemical action of the sea to contend with and the evi- 
covered with rik maapreis er evident mage ‘ale, 3 Tcieaeine t in aa " pot fally And natered 
‘ ize, ws less 
places, but they scarcely figure in ne matter of ferns. er are Seitioate. of ferns, un 
n occasional Acrostichum aureum got a foothold there 
