VI. MARINE ALGAL VEGETATION AND SEA- 

 GRASS VEGETATION. 



THE vegetation of the sea is naturally divided into two principal 

 groups: the Plankton composed of the small plants floating 

 passively in the water, and the Benthos which comprises the 

 species attached to a substratum. In this article only the latter is 

 dealt with. 



The Benthos 1 falls naturally into two divisions, viz. the litho- 

 philous vegetation, the communities of marine algae (suhformation 

 of marine algae, Halo-nereid communities, Warming, 72, p. 170), 

 and the vegetation of loose soil (Enhalid-formation, Warming, 

 72, p. 177). 



The Benthos has been divided in different ways. J.G. Aeardh 



/ o 



in 1836 (Novitise Florae Sveciae) divided the marine vegetation into 

 three zones, a green zone above, a brown zone in the middle and 

 a red zone lowest of all. Lyngbye in the same year, also divided 

 the vegetation into three zones (Rariora Codana, printed 1879 80), 

 a zone of green algae (Ulvacece) being above, a zone of red algae 

 in the middle and a zone of Laminarice below this. 0rsted (77), 

 like Agardh, also divided the vegetation in the 0resund into three 

 zones, but 0rsted has the merit of being the first to explain that 

 the division of the zones depends upon the depth to which the 

 light penetrates, and upon the colour of the light at the various depths. 

 Kj el 1m an has divided the algal Benthos into regions. Where 

 there is a tide, the littoral region is reckoned as being between the 

 highest high-water mark and the lowest ebb-tide mark; where, on 

 the other hand, there is no tide Kj ell man reckons the littoral 

 region as extending from the uppermost limit of the algal vegetation 

 to a depth of I 1 / 2 2 fathoms (34, p. 7). The sublittoral region ex- 

 tends from the lower limit of low-tide, or else from a depth of I 1 / 2 



1 No notice is taken of the bacterial flora of the sea. 



