XU INTRODUCTION. 



branaceous expansion of the stem^ whicli continues 

 its course through the frond^ and assumes the cha- 

 racter of a midrib or vein— presenting in this in- 

 stance a manner of growth nearly analogous to that 

 of the fronds of ferns. At a later period, what 

 constituted originally the midrib of the frond be- 

 comes a branch, as in Delesseria. Many algse are 

 parasitical on the larger species ; others have knob- 

 like, flat, leather-like or fibrous roots, by which they 

 adhere to rocks, stones, shells, and other substances. 

 It is doubted whether they derive any nourishment 

 from these roots, which seem to partake more of 

 the character of clasping fibres or tendrils, than of 

 a true root. Some few species are always found un- 

 attached; of these the Sargassum bacciferum, or 

 Gulf-weedy is by far the most extraordinary and 

 well-known instance, occurring in extensive masses 

 or lengthened ridges on the surface of the Atlantic 

 in certain latitudes. Columbus and his companions, 

 at the sight of this to them unwonted phenomenon, 

 were struck with surprise and fear. ^^In this part," 

 he says, '' the sea was covered with such a quantity 



