118 Papers from the Department of Marine Biology. 



of this group is a sedge, Cyperus hrunneus S. Watson, forming con- 

 spicuous tufts of grayish foUage overtopped by the deep-brown 

 inflorescences. 



The fourth association is the Opuntia community. This is made up 

 mostly of Opuntia dillenii (Ker) Haworth and Paspalum mspitosum 

 Flugge, a rather coarse grass about 4 decimeters tall, the branched 

 inflorescence proUfic with dark-brown, disk-shaped seeds. Frequently 

 occurring in this association, and also to some extent in the ChamcBsyce 

 community, are three species common in the Tortugas: (1) the Ipomcea 

 pes-caprce; (2) its more beautiful relative, the white moon-flower, 

 Calonyction tuha (Schlechtendal) CoUa, which makes a most wonderful 

 effect in the brilliant tropical moonhght with its large salver-shaped 

 corollas starring-over the clumps of Opuntia or other supporting vege- 

 tation, half hidden in its enveloping heart-shaped foUage; (3) Melan- 

 thera hrevifolia 0. E. Schultz, a scrubby composite about 6 decimeters 

 tall, bearing inconspicuous whitish blossoms, but during the hot mid- 

 dle portion of the day they are constantly surrounded by hosts of a tiny 

 butterfly, Thecla, whose source of food is the nectar of these httle 

 flowers. Into the above four gi'oups the predominating species of the 

 Tortugas may be said to fall naturally on account of the influence of 

 various factors in the environment or those inherent in the plants 

 themselves. 



DISTRIBUTION OF SPECIES AMONG THE KEYS OF THE GROUP. 



Only one other paper previous to the pubhcation of the present one 

 has dealt with the ecology of this region, viz, a series of maps with field- 

 notes collected by E. O. Lansing in the spring of 1904, and pubUshed 

 by Millspaugh in 1907.^ This work, while fairly accurate and detailed, 

 was compiled from notes evidently made in a very rapid survey of the 

 islands, and in the four days (March 19 to 22 inclusive) allotted to the 

 Tortugas group, the coUector naturally overlooked many important 

 minor features in the flora. For instance, evidence of haste is seen 

 in overlooking the groups of sisal hemp. Agave sisala^ia (Engelmann) 

 Perrine, among the bay cedars on Loggerhead, very old plants which 

 were there even in the time of the third predecessor of the present 

 Hght-house keeper, about 25 years ago; also on the same key numerous 

 clumps and patches of Opuntia dillenii among the Suriana were not 

 observed. Only one station for this plant is given for Loggerhead, viz, 

 on the west coast near the light-house boat-shed, but the writer found 

 it disseminated fairly well over the island in patches of old plants, often 

 a meter high. These patches have certainly been there since the foun- 

 dation of the Laboratory of the Carnegie Institution of Washington on 



^Millspaugh, C. F., Flora of the Sand Keys of Florida. Field Columbian Museum Publica- 

 tion 118, Bot. Series ii, No. 5. 



