258 KELP. PART ii. 



quently its habits exactly resemble those of the Macro- 

 cystis, and as that plant becomes detached and floats 

 after fructification, it is supposed that the Sargassum 

 bacciferum may grow on rocks at the bottom of the 

 Atlantic, between the parallels of forty degrees north 

 and south of the equator, and when detached after 

 fructification that it is uniformly drifted to particular 

 spots which never vary. e Multiplication is so rapid in 

 the floating beds of the Sargassum and Macrocystis, as 

 to render fruit needless ; and even the common Fucus 

 vesiculosus occurs in the Mediterranean under a peculiar 

 form consisting entirely of specimens derived from sea 

 borne weed carried in by the current which sets in 

 towards the Mediterranean from the Atlantic.' 6 



Kelp, the ashes of sea weeds, is the commercial source 

 of iodine. Algae growing in deep water contain most of 

 that substance ; consequently the kelp made at Guernsey, 

 consisting chiefly of the ashes of Laminaria digitata, is 

 richer in iodine than that made elsewhere. 



Marine vegetation varies both horizontally and ver- 

 tically with the depth, and it seems to be a general law 

 throughout the ocean, that the light of the sun and ve- 

 getation cease together. It consequently depends upon 

 the power of the sun, and the transparency of the water ; 

 so that different kinds of sea weeds affect different depths, 

 where the weight of the water, and the quantity of light 

 and heat, suit them best. One great marine zone lies be- 

 tween high and low water marks, and varies in species 

 with the nature of the coasts, but exhibits similar phse- 

 nomena throughout the northern hemisphere. In the 

 British seas this zone does not extend deeper than thirty 

 fathoms, but it is divided into two distinct provinces, 

 one to the south and another to the north. The former 

 includes the southern and eastern coasts of England, 

 the southern and western coasts of Ireland, and both 



4 Berkeley's ' Cryptogamic Botany.' 



