The Botany of Iceland - 01.12.1912, Blaðsíða 109

VI. MARINE ALGAL VEGETATION AND SEA-
GRASS VEGETATION.
he vegetation of the sea is naturally divided into two principal
-l groups: the Plankton composed of the small plants floating
passivelj' in the water, and the Benthos which comprises the
species attached to a substratum. In this article only the latter is
dealt with.
The Benlhos1 falls naturallv into two divisions, viz. the litho-
philous vegetation, the communities of marine algæ (subformation
of marine algæ, 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. Agardh
in 1836 (Novitiæ Floræ Sveciæ) 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 algæ (Ulvaceœ) being above, a zone of red algæ
in the middle and a zone of Laminariœ below this. 0rsted (77),
like Agardh, also divided the vegetation in the 0resund into three
zones, but 0rsted lias 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.
Kjellman 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 Kjellman reckons the littoral
region as extending from the uppermost limit of the algal vegetation
to a depth of H/a—2 fathoms (34, p. 7). The sublittoral region ex-
tends from the lower limit of low-tide, or else from a depth of F/2
1 No notice is taken of the bacterial flora of the sea.