22 INTKODUCTIOIf. IV. 



tation, and strongly resemble the efflorescent forms, like cauliflowers, seen so fre- 

 quently in the sparry concretions of limestone caverns. Others, moi-e perfect, 

 become branched like corals ; and the most organised of the group, or the true 

 corallines, have symmetrical, articulated fronds. This stony vegetation affords 

 suitable food to hosts of zoophytes and moUusca, which require lime for the con- 

 struction 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 generally a marked 

 difference between the specimens. Thus, when an individual plant grows either in 

 shallower or in deeper water 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 dasyphylla, various Hypnece, and many others) that the 

 specimens from deep water have divaricated branches and ramuli, and a tendency 

 to form both hooks and discs or supplementary 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 difficult 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 plants ; but in a far less degree ; as it is 

 obviously easier for the spores of the Algas 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 barrier, 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 distribution of the Fucacece 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 Algte 

 not before known in those waters have, according to Professor L. E. 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 this 

 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 Algje along the eastern and southern shores of the United States ; 

 here recording the substance of some verbal observations which I made at the 

 Meeting of the American Association, held in Charleston, in March, 1850. 



EASTERN SHORES OF NORTH AMERICA. 



In comparing the marine vegetation of the opposite shores of the northern Atlantic, 



* Manual of British Marine Algcv, Introd., p. xxxvi. et seq. ed. 2. 



