20 BRITISH SEA-WEEDS. 



not for nourishment. But though the material of the 

 rock is unimportant, the conformation is not ; and those 

 which are rough and rugged, and abound in tiny bays 

 and creeks, are always more productive than those with 

 smooth surfaces, perpendicular sides, and regular outline. 

 Not only rocks, but mud, sand, shells, floating or sub- 

 merged wood, the copper sheathing or the iron sides of 

 ships all have their tenants ; nor do these suffice, for even 

 yet some of our Nereids are homeless, and the mighty mass 

 of Sargassa, known as the Gulf-weed, grows unattached 

 floating on the surface of the sea, and there, unaided by 

 roots or, I believe, by spores or tetraspores, has attained 

 a bulk which equals, if, indeed, it does not exceed, that 

 of any other single kind throughout the world. 



The depth to which Sea-w r eeds extend cannot be very 

 exactly defined; but there is no doubt that, as a rule, 

 they only form a fringe a mile or so wide around the 

 land. In cold and temperate climates a depth of a dozen 

 fathoms would be probably a liberal allowance for any 

 of the more perfect forms, but in tropical and sub-tropical 

 latitudes perhaps twice or thrice that depth would not 

 be an exaggeration, and in exceptional instances even 

 these limits are occasionally exceeded. 



These remarks do not apply to the lower forms of 

 Algce, for myriads of Diatomacea have been found to 

 exist at every depth of the ocean which has hitherto 

 been fathomed. There are also a vast number of calcare- 

 ous plants, belonging to the family of Corallines, whose 

 substance is principally composed of lime extracted 

 from the sea-water. These grow at considerably greater 

 depths than sea-weeds proper, and are, with the exception 

 of Diatomacea, the sole representatives of the Vegetable 

 Kingdom, over considerable regions of the bottom of the 

 sea. 



