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 group 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 of coarse calcareous detritus, the remains of various organ isms. Of these Lower Sand Keys Small' says " they are little more than sand bars and they support, like 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 plummet-like 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 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. X., Flora of the Florida Keys. 1913, T. Wayland. The Building of the Marquesas and Tortugas Atolls and a Sketch of the Geologic History of the Florida Reef Tract. Carnegie Inst. Wash. Pub. No. 182, pp. 55-67. 'Crossland, C.. Note on Dispersal of Mangrove Seedlings, Annals of Botany, atm, p. 287 moisture in the coarse, porous sand, together with the dry winds, soon kills the drifted young mangrove. This has been demonstrated many times in the physiological experiments of the writer on Rhizophora and explains the absence of the mangrove association in the Tortugas.
The plants of the Tortugas, living as they do in such uniformly xero phytic conditions and on such small areas of land, are naturally confined to a small group called a strand flora, on account of characteristic species which occur on all maritime shores and particularly tropical coasts—such genera as Samla, and even the species Ipomcea pea-came as found on all tropical beaches, as far as the remote islands of the Pacific, according to Guppy' and the latter, in his Plant Geography,* in his classification of tropical littoral formations calls the open sandy formation a " pes-caprss formation"; also includes a great many species found scattered all over the Tortugas in his treatment of sea-strand formations in South Florida, from which, however, the Florida Keys are excluded.
Broadly, it may be said that the entire flora of the Tortugas is a strand flora, but to the close observer these plants are easily seen to fall into certain associations or groups of several species. The reason for this grouping of species in a fairly uniform flora the writer believes to be due to two factors: first, the prevailing winds, which frequently carry dense salt spray or mist inshore, and certain of these strand plants are better able to withstand this drenching with a rather strong solution of sodium chloride and magnesium and calcium carbonates, of which latter salts the sea-water contains, in this region, fairly high per centages second, the aggressiveness of certain species and the ability to hold a position occupied by them against invading species—for instance, few plants will advantageously invade an area occupied by Opuntia, even though the surface is not entirely covered by its sprawl ing joints.