Human and animal influence on vegetation.—Man's influence on the plants in the Tortugas may be designated as constructive and destruc tive. The introduction of plants for use or ornament on Loggerhead and Garden Keys and their introduction unwittingly with ballast, etc. (such as Leptilon, Sonchus, Syntherisma, Portulaca), together with coconuts, papaws, Casuarinas, and Salvia serotina (used in earlier years as a febrifuge), may all be classed as constructive influences. The cutting down of the white buttonwood trees by the old fishermen, according to tradition, the burning off of the Suriana on Loggerhead, and the burning over of the parade-ground in the fort on Garden Key are destructive activities. On Bird Key the presence of the warden does not seem to have produced any change in the vegetation, while the terns which breed on the island have very little effect, as they do not feed on any plant substances and it is only the noddy terns which make nests of a few dry sticks; the other two species of terns lay their eggs on the bare sand.
Millspaugh ascribes to birds a large influence in the distribution of plants in this region, but in the writer's opinion this has been over estimated; the only birds in the region are the terns and a few frigate birds which prey on the terns. None of these birds are waders, nor do they spend much time on land, according to the author's observation. Millspaugh mentions 8 plant species as avevectant by the feet of sea birds, six of these occurring in the Tortugas, but as there are few birds in these islands and those are of such habits as scarcely to permit of carrying seeds in the webbed-toe expansion, the hypothesis is hardly tenable. A more reasonable factor for distribution in the Tortugas is the sea alone. The undoubtedly aquavectant plants (as Cakile) appear on the islands no sooner than such supposedly bird-borne plants as Cenchrus and Sccevoks.
Other animals associated with plants are the red land-crab, Gecar cinus lateralis, which lives on various islands, but was observed espe cially on Loggerhead, where it fed on decaying leaf-mold accumulating in the grove of Sebesten trees, but it probably has no effect on this tree's economy. A small black beetle also lives in this tree's deep salver-form corolla and perhaps aids in fertilization. Another insect is the tiny butterfly Thecia, which lives on the nectar of Melanthera and incident ally pollinates its stigmas.
of the Suriana on Loggerhead and Bird Keys by driving in the salt spray, which kills the bushes, or by the mechanical injury of under mining the roots by high waves and later leaving the absorptive system exposed to the drying air. This latter case is observed on the eastern shore of Loggerhead, where the bank was eroded and long ridges of the Suriana were dead, with their roots sticking up in the air.
Structure in relation to environment. This subject is well illustrated in the Tortugas. All the plants of these dry islands, in which there are no springs of fresh water of any sort and where the substratum is of a loose, porous nature, must be adapted to conserve moisture. The structures facilitating conservation of moisture in the Tortugas flora are various. In Tournefortia and Suriana the leaves are densely cov ered with tomentum and pubescence; they are spatulate in shape, which permits their being set closely together, and as a further reaction they assume an appressed hyponastic position during the middle portion of the day or in prolonged droughts. Sccevola and Ipomcea, both of which have large leaves, are heavily cutinized and frequently have sunken stomata; they also assume the hyponastic appressed position in unfa vorable conditions and seasons. Chamcesyce has small, reduced leaves, heavily cutinized, appressed, sessile, and flap-like, which transpire very slowly. ha has glabrous, heavily cutinized leaves with thick, fleshy mesophyll, containing water-storage tissue. Cenzhrus, Cyperus, and Uniola have narrow, reduced leaves with small stomata and, together with many grasses, roll inward during dry seasons or in dry situations, the reaction being due to the thin-walled water-storage cells in the sinuses of the delicate ridges losing water, the lowered turgescence causing the leaf to involute.
A peculiar reaction to environment was noted in the Poinsettias. Individuals of species occurring in the Tortugas were found in more favorable situations on the mainland of Florida with rather broad blades; with variations in environment and decreasing moisture and shade, all stages were seen, down to leaves which were little more than midribs. Boerhaavia has a rather broad blade, but is protected from excessive transpiration by tomentum and glandular hairs. Thus it is seen that on minute examination nearly every plant in this region has some special protective feature to guard against loss of water, the acme being reached, of course, in Opuntia, which has no leaves whatever, a thickly cutinized epidermis, and mucilaginous sap.