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 oolitic 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 



