Botanical Ecology of the Dry Tortugas. 127 



Southwest of the southern angle of the fort are several large patches 

 of Opuntia and a single Avicennia nitida tree. This black mangrove is 

 the sole representative of the species on the island. Lansing mentions 

 30 of these trees having been planted inside the fort, but these must 

 have perished, for not a vestige of them remains. This tree, which is 

 about 2.5 meters tall, is probably a seedling drifted in by the waves or 

 from seed carried in ballast from Key West. On the southern shore, 

 near the dock, are numerous small patches of Atriplex cristata. 



The interior of the fort has a large parade-ground (plate 4) and vari- 

 ous buildings and ruins. This parade-ground has been planted with 

 some introduced trees, as mentioned by Millspaugh, but a curious 

 omission in that survey was the grove of large and old white button- 

 wood trees just within the sally-port. This grove is quite large and thick, 

 and numerous young seedlings have sprung up among the older trees. 

 While the fort was a military fortress, then during its period of use as a 

 federal prison, later as a quarantine station and coaling-depot, and now 

 under the jurisdiction of the naval authorities, up to date the order 

 has been handed along that these old trees (the only remnant of the 

 traditional forest or thickets which covered these islands) shall be 

 untouched (see plate 4). 



Notwithstanding the tendency and inclination of various care-takers 

 of the fort to cut down and destroy all plants springing up inside the 

 fort and the practice of burning over the parade-ground at regular 

 intervals, and the serious fires in the fort which destroyed the large 

 barracks and the keeper's cottage, these old trees have persisted to the 

 present and are now strong and healthy. An interesting sport was 

 noted in the grove, viz, a few young plants of Conocarpus erectus var. 

 sericius. This variety probably arose quite recently as a genetical 

 variation, very likely a mutation, as these gray tomentose trees are all 

 quite young and have started as seedlings right in the midst of the old 

 trees, which all have dark-green and glabrous foliage. The center of 

 the old parade-ground is occupied every year by a thick stand of the 

 weed Glottidiwn vescarium, which the care-taker tries to destroy by 

 burning over the ground annually, but to no avail. 



Of the trees mentioned, there are a few specimens of Terminalia 

 and a tamarind (plate 5) which shows beautifully the direction of the 

 prevailing wind by the one-sided development of the tree after it had 

 grown so tall as to get the effect of the winds sweeping over the para- 

 pets. Here also about the buildings are several Phoenix palms, both 

 P. dactylifera (see plate 5) and P. canariensis, a number of coconuts, 

 some sea-grape trees, Cocolobis uvifera (which are quite old), a number 

 of Thespesia trees, gum-elemi trees, Elaphrium simaruba, as well as 

 Cordia and Oleander bushes. On the west side are several clumps of 

 Agave decipiens (see plate 6) and in a ruined powder magazine there 

 has sprung up, with a thicket of gum-elemi trees, a fair-sized guava 



