Botanical Ecology of the Dry Tortugas. 115 



GENERAL SKETCH OF THE VEGETATION OF THE TORTUGAS. 



On ecologic and geologic grounds the Florida Keys have been divided 

 into four groups by Small :^ 



(1) The upper sand keys, which are really detached portions of the 

 coastal peninsula and support a sand-dune and hammock flora closely 

 related to the mainland. 



(2) The Upper Keys, extending from Soldier Key to Spanish Har- 

 bor, and which are composed of a coral rock. These have a vegetation 

 of tropical hardwood trees and shrubs and palms resembling those of 

 the Bahamas which lie slightly to the east of them. 



(3) The Lower Keys from No Name Key to Key West. These have 

 a basic formation of Miami limestone. The flora of the group, accord- 

 ing to Small, who has done much systematic work in this region, is 

 quite varied, having large areas of pineland and palms as well as exten- 

 sive hammocks. This flora is more closely related to Cuba, which lies 

 only 90 miles to the south of this group. 



(4) The gi'oup most directly considered in this paper is called the 

 Lower Sand Keys. They are all the keys lying west of Key West and 

 are composed of sand or, more strictly, according to the above analysis 

 of Vaughan,'of coarse calcareous detritus, the remains of various organ- 

 isms. Of these Lower Sand Keys SmalP says ''they are little more 

 than sand bars and they support, Uke the ocean side of all the Florida 

 Keys, only or mainly the characteristic strand flora of most of the West 

 Indies." 



The origin and relationship of the Tortugas flora may thus easily 

 be traced to the adjacent large islands of the Antilles, Cuba, etc. The 

 majority of the plants in the Tortugas are easily transported by the 

 sea, as indeed are most strand floras. 



A typical ecological formation in the Florida Key region is the man- 

 grove association, Rhizophora mangle, but this association is entirely 

 lacking in the Tortugas, owing to the physical nature of the islands. 

 In the writer's work with Rhizophora, he observed that the floating 

 hypocotyls must have a secure anchorage, either in the deep, soft 

 mud amid the entangled roots of a mangrove swamp, or in the clefts 

 and cracks of a coral rock, a mud-flat, or oolite bottom. The same 

 plununet-Uke action of a young mangrove hypocotyl in boring a rest- 

 ing-place for itself in soft mud, or in finding a cleft by its twirling action 

 in the water-currents, was observed by Crossland^ on the Zanzibar 

 reefs. The sand beaches of the Tortugas do not furnish the required 

 conditions for the young viviparous seedling. The lack of sufficient 



'Small, J. K., Flora of the Florida Keys. 1913, III-IV. 



-Vaughan, T. Wayland. The Building of the Marquesas and Tortugas Atolls and a Sketch of 

 the Geologic Historj' of the Florida Reef Tract. Carnegie Inst. Wash. Pub. No. 182, pp. 55-67. 

 "Crossland, C, Note on Dispersal of Mangrove SeedUngs, Annals of Botany, XVII, p. 267. 



