Botanical Ecology of the Dry Tortugas. 117 



are uprooted and blow about the beaches as tumble-weeds. Growing 

 with Cakile is the sand-spur, Cenchrus incertus M. A. Curtis, which 

 forms large flat mats on the beaches, bearing its spikelets in a heavy 

 spiny involucre, making a painful wound if stepped upon. Associated 

 with this in dense masses the sea purslane (Sesuvium portulacastrum L.) 

 is often seen; resembling it superficially is Alternanthera maritima St. 

 Hildmann. Back from the reach of spray is Sporobolus virginicus (L.) 

 Kunth; frequently out in line with it is Tournefortia gnaphalodes (Jac- 

 quin) R. Brown, forming large characteristic, flat-topped bushes 1 to 

 1.5 meters tall, clothed with soft, whitish-gray, tomentose leaves. The 

 long, trailing vines of the purple beach morning-glory, Ipomcea pes-caprce 

 (L.) Sweet, are found here, and also Sccevola plumieri Vahl, in clumps 

 with thick, glossy leaves, nauseating black berries, and white blossoms 

 with the peculiar corolla split down one side and the odd ciliated indu- 

 sium on the stigma. This last is one of the most singular of all the 

 plants of the region. On the higher portions of the beaches and a 

 meter or two back from the water-line, sea oats, Uniola paniculata L., 

 will be observed, a grass about 2 meters tall, with a gracefully drooping 

 panicle of heavy spikelets. This grows in rather close formation over 

 large areas, forming thick tufts as it grows from season to season, and 

 the dead culms remain clustered about the living ones. 



The most important association in the Suriana group covering very 

 large tracts, is formed by the bay cedar, Suriana maritima L., a shrub 

 of the Geraniales order, 1 to 2 meters tall. The deep green of this 

 shrub forms a pleasant and lively contrast to the shore plants covered 

 with gray tomentum, as, for instance, Tournefortia. With Suriana, and 

 often climbing up over it and hiding its small yellow star-shaped blos- 

 soms, is the beach bean, Canavalia lineata (Thunberg) De Candolle. 

 A common epiphyte on the lower branches of the Suriana bushes is 

 the lichen Usnea barbata L., with its small gray branches sticking out 

 like a sort of a mossy covering on the twigs. 



The third association is the Chamwsyce group, which also covers 

 large areas on some of the keys, its members growing in situations 

 not so suitable for the previous group. The chief component of the 

 association is Chamcesyce buxifolia (Lamarck) Small. This spurge forms 

 tracts thickly covered with its small gray-green boxwood-like leaves 

 and tough brown stems filled with abundant latex. The rooting sys- 

 tem of the plant is remarkable; a small individual, only 3 decimeters 

 tall, may have a root system 9 decimeters in diameter, and were the 

 plants growing in a soil less loose and coarse than the Tortugas sand 

 they could scarcely be pulled up by a strong man. An ally of the 

 Chamcesyce is a composite, the marsh elder, Iva imbricata Walter, 

 which forms a rather tall bush, about 10 to 15 decimeters, with small, 

 light-green succulent leaves, which have a rather pleasant bitter, acrid 

 odor and heads of yellowish, inconspicuous flowers. A third member 



