Botanical Ecology of the Dry Tortugas. 135 



Storms. Hurricanes and great waves driven by continuous winds 

 have wrought more havoc on the vegetation of the Tortugas than per- 

 haps any other agents. Whole keys have been washed away by some 

 of the great storms, and islands have been completely denuded of 

 vegetation. Not only on these sand islands, but in the mangrove 

 swamps of larger islands (as at Boca Grande), large areas of swamp 

 were dead and piled up densely with the decaying branches of shattered 

 Rhizophora trees. To storms also may be laid some of the destruction 

 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. Scwvola and Ipomoea, 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. Iva has glabrous, heavily cutinized leaves with thick, fleshy 

 mesophyll, containing water-storage tissue. Cenchrus, 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. 



