TRANSACTIONS OF WAGNER 
106 
VEGETATION OF SOUTH FLORIDA 
other parts of Florida, is hardly in accord with the geographic conditions, 
as they are found in extreme southern Florida. Here the otherwise con- 
tinuous stretch of pine timber is intersected by transverse prairies some 
of them a kilometer across, which are submerged during a part of the year, 
thus serving as a partial barrier to forest fires and Big Pine Key covered with 
pines and hammocks is surrounded by the ocean. According to Harper's 
theory, these protected areas of elevated land should be covered with hammock 
vegetation, but they are not. The pine trees (Pinus caribaea Morelet) are 
supreme with their associated undergrowth. Right in the center of this pine- 
land, which owes its open condition to forest fires according to Harper, are 
the banana holes with elements of the true hammock vegetation. If the pine 
woods owed their floristic character to forest fires, as Harper emphasizes, then 
there would be no banana hole vegetation for the fire would have destroyed 
all such non-coniferous species. 
My observations on the banana holes of South Florida seem to indicate 
that they have arisen by the occupancy of a limestone hollow, or depression 
by vegetation, which gradually filled the basin with forest litter until the 
presence of this rich humus resulted in the self-perpetuation of the hammock 
formation. They are not extensive enough to make any sweeping 
generalization, but from my study of the big hammock between Miami 
and Cocoanut Grove, I wish to propose an hypothesis as to its formation. 
Given an original area of exposed oölitic limestone, it is probable that such 
an area was not perfectly level, but was marked with larger, or smaller, 
concavities. Such concavities might cover two hundred and fifty-nine 
hectares (=one square mile) of country. The center of the basin might 
be only three to six decimeters below the edge of the depression with the 
sides sloping almost imperceptibly toward the center. Such an area would 
become a shallow reservoir in rainy weather and perhaps on the disappear- 
ance of the water in drier weather its soil would retain a lot of ground water. 
Chance seeds carried by wind and by animals falling into these saucer-like 
hollows would develop into broad-leaved shrubs and trees, which would 
soon completely fill them. The soil, although over an underground drain- 
age depression, holds its soil water longer. This would exclude by close 
occupancy of the ground the pine trees, which flourish in a drier soil, 
because less retentive of soil-water. Soon the basin would fill with humus 
and the vegetal material mixing with the sand and the marl washed into the 
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