216 VEGETATION 



well as the underground ones, are very spongy, and the air taken in through 

 their large lenticels can easily pass to the subterranean parts. The leaves 

 of this plant secrete large quantities of salt, which collects in crystals on 

 the surface. 



In regard to the dispersal of seeds, there is a marked difference between 

 the plants of the Bahamas and those of the United States. In the former 

 there is a great preponderance of plants bearing fleshy fruits, i. e., fruits 

 adapted for distribution by birds, rather than those with hairy, winged, or 

 barbed seeds and fruits fitted for transportation by the wind or by clinging 

 to the hair of animals. 



Of plants with barbed seeds or fruits we found only three species 

 Petiveria alliacea L., a common weed about Nassau; Pavonia spinifex Cav., 

 a clambering shrub, and Meibomia supina (Sw.) Britton, a low herb. Those 

 with seed for wind distribution are the epiphytic Tillandsias and Orchidacece, 

 most of the Composite, Typlia, Swietenia, Casua/rind, and a few others. A 

 number of beach plants produce seeds which are able to float for a long time 

 without being damaged by the salt water, and these may thus become dis- 

 tributed over great distances through the movements of ocean currents. 



PLANT FORMATIONS. 



We will now pass to a systematic consideration of the various plant for- 

 mations of the islands which were visited by the Expedition. 



NEW PROVIDENCE. 



In describing the vegetation of this island, we shall begin at the south 

 shore, at the point where the South-side road meets the beach, and take up 

 the formations observed in crossing to the north side. 



SAND-STRAND FORMATION. The south side of New Providence differs 

 from the north side in its more regular and more sandy beaches, and it pre- 

 sents the typical sand-strand formation of tropical shores. At certain points 

 along this exposure there are large areas of shoal water stretching out for 

 hundreds of yards from the beach where the depth seldom exceeds a foot. 

 Occupying this area is an open growth of Avicennia nitida Jacq., of stunted 

 and contorted habit and with a height not often exceeding 2 or 3 feet. At 

 high tide the smaller plants are almost covered with water. The appearance 

 of these shrubs is well shown in Plate XLVII, Fig. 2. Along this southern 

 coast the sand-strand formation may be divided, beginning seaward, into the 

 five following associations : 



