Sentence:  A number of cryptogamic plants swim about at random in the waters, among which the most interesting, perhaps, in our present state of knowledge, is the <WSD>sargasso</WSD>, or gulf-weed of voyagers (Fucus "natans"), which is found in the Gulph of Florida, and some other parts of the ocean floating in masses or fields, many miles in length. 
Meaning of the word marked with <WSD> tags:  A brown alga, of the genus "Sargassum", that forms large, floating masses.