BOTANICAL ECOLOGY OF THE DRY TORTUGAS. 



BY H. H. M. BOWMAN. 



As the name of these islands indicates, their vegetation is character- 

 istically xerophytic, although the rainfall is sufficient to assure the 

 plants the necessary amount of water. Living in such a difficult envi- 

 ronment, as may be supposed, the plants are very interesting when a 

 close study is made of their individual characteristics. The oppor- 

 tunity for such study was given the writer during the summers of 1915 

 and 1916, while pursuing another line of botanical research under the 

 auspices of the Carnegie Institution of Washington, which partially 

 controls one of them (Loggerhead Key), where a marine laboratory 

 is maintained. 



The Dry Tortugas are situated in the Gulf of Mexico between 24 

 34' and 24 42' north latitude and between 82 48' and 82 58' west 

 longitude. The relative distances in the cardinal directions of points 

 on land are as follows: approximately 920 miles east of Tampico, 

 Mexico ; 625 miles southeast of New Orleans ; 470 miles south by east 

 of Pensacola ; 70 miles west of Key West, and 90 miles north by west of 

 Havana. The Tortugas are really the westernmost of all the Florida 

 Keys, but are more detached from them and have different geological 

 and botanical aspects. 



GEOLOGIC FORMATION. 



The geological formation and the history of south Florida and the 

 adjacent regions have been investigated thoroughly by T. Wayland 

 Vaughan, of the United States Geological Survey, and in one paper he 

 discusses particularly the Tortugas Atoll. 1 From this paper the 

 writer quotes largely as to the geology of these islands. The Dry 

 Tortugas consist of eight small islands: Loggerhead Key, Bird Key, 

 Garden Key, Long Key, Bush Key, Sand Key, Middle Key, and East 

 Key; these, together with large submerged banks and several shoals 

 which were, until recent hurricanes, charted as islands, form an irreg- 

 ular ellipse with its longer axis directed from northeast to southwest. 

 The lagoon inside of this atoll has a depth of 5 to 7 fathoms. 



As stated in Vaughan's interesting account, two lines of investiga- 

 tion were taken up in determining the geology and origin of the Tor- 

 tugas atoll: first, whether submarine solubility phenomena played an 

 important part in their history; second, whether the wind and current 



'Vaughan, 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. 1S2, pp. 55-67. 



Ill 



