THE SMITHSONIAN INSTITUTION. 109 



are scarcely distinsuishaLlc, by the naked eye, from any ordinary 

 calcareous incrustation, and strongly resemble the etHorescent forms, 

 like cauliflowers, seen so frequently in the sparry concretions of lime- 

 stone caverns. Otlicrs, more perfect, become branched like corals ; 

 and the most organized of the group, or the true corallines, have sym- 

 metrical, articulated fronds. This stony vegetation affords suitable 

 food to hosts of zoophytes and mollusca, which require lime for the 

 construction of their skeletons or shells, and it probably extends to a 

 depth as great as such animals inhabit. 



When the same species is found at different depths, there is gener- 

 ally a marked difference between the specimens. Thus, when an indi- 

 vidual plant grows either in shallower or in deeperwater than that 

 natural to the species, it becomes stunted or otherwise distorted. I 

 have noticed in many species (as in Plocamium coccineum, Dasya 

 coccinea, Laurencia dasijphylla, various Ili/pnece, and many others) 

 that the specimens from deep water have divaricated branches and 

 ramuli, and a tendency to form both hooks and discs or supplement- 

 ary roots, from various points of the stem and branches. Sometimes 

 the outward habit is so completely changed by the production of 

 hooked processes and discs, that it is difiicult to discover the affinity 

 of these distorted forms ; and such specimens have occasionally been 

 unduly elevated to the rank of species. 



When water of great depth intervenes, on a coast between two 

 shallower parts of the sea, it frequently limits the distribution of 

 species, acting as a high mountain range would in the distribution of 

 land plaats, but in a far less degree ; as it is obviously easier for the 

 spores of the Alg;e to be floated across the deep gulf, than for the 

 seeds of land plants to pass the snowy peaks of a mountain. 



The intervention of sand, already alluded to, is a far greater bar- 

 rier, because sandy tracts are usually of much greater extent than 

 submarine obstacles of any other kind. To the prevalence of a sandy 

 coast, in a great measure probably, is owing the very limited distribu- 

 tion of the Fiicacece on the eastern shores of North America, where 

 plants of this family are scarcely found from New York to Florida. 

 Since the erection of a breakwater at Sullivan's Island, S. C, many 

 Alga3 not before known in those waters have, according to Pro- 

 fessor L. R. Gibbes's authority, made their appearance, but none of 

 the Fucaceas are yet among them. In due time Sargassum vulgare 

 will probably arrive from the south. 



Some attempt has been made to divide the marine flora into separate 

 regions, the particulars of which I have detailed elsewhere.* In the 

 descriptive portion of my work I shall notice the distribution of the 

 several families, where it offers any marked peculiarity, and I shall 

 at present confine myself to some remarks on the distribution of Alga3 

 along the eastern and southern shores of the United States ; here re- 

 cording the substance of some verbal observations which I made at 

 the Meeting of the American Association, held in Charleston, in 

 March, 1850. 



« Manual of British Marine Algce, Introd., p. xxxvi et seq. ed. 2. 



