Life cycle polysiphonia Polysiphonia is a genus of red alga with about 19 species on the coasts of the British Isles and about...
Life cycle polysiphonia
Polysiphonia is
a genus of red alga with
about 19 species on
the coasts of the British Isles and about 200 species
world-wide, including Crete in Greece, Antarctica and Greenland.
Its
members are known by a number of common names. It is in the Order
Ceramiales and Family Rhodomelaceae.
Description
Polysiphonia is
a red alga, filamentous and usually well branched some plants reaching a length
of about 30 cm. They are attached by rhizoids or
haptera [3] to
a rocky surface or other alga. Thethallus (tissue) consists of fine branched filaments each
with a central axial filament supporting pericentral cells. The number of these
pericentral cells, 4–24, is used in identification. Polysiphonia elongata shows
a central axial cell with 4 periaxial cells with cortical cells growing over
the outside on the older fronds. Its cuticle contains bromine.
Features
used in identification include the number of pericentral cells, the cortication
of main branches, constriction of young branches at their base, whether the
branching dichotomous or spiral, and the width and length of thalli.
Distribution and ecology
Species
have been recorded from Europe, Australia and New Zealand, North America and South America,
islands in the Pacific Ocean, South Africa,
southwest Asia, Japan, Greenland and
Antarctica.
The
species are entirely marine, found growing on rock, other algae, mussels or limpets and
artificial substrata etc. from mid-littoral to at least 27 m depth. Many
species are abundant in rock pools. Polysiphonia lanosa is commonly
found growing on Ascophyllum nodosum.
Life cycle
The
life-cycle of the red algae has three stages (triphasic). In Polysiphonia it
consists of a sequence of agametangial, carposporangial and tetrasporangial phases. Male
(haploid)
plants (the male gametophytes) produce spermatia and the
female plants (the female gametophytes) produce the carpogonium (the haploid carpogonium) which remains
attached to the parent female plant. After fertilization the diploid nucleus migrates
and fuses with an auxiliary cell. A complex series of fusions and developments
follow as the diploid zygote develops to become the carposporophyte, this is a
separate phase of the life-cycle and is entirely parasitic on
the female, it is surrounded by the haploid pericarp of the parent female
plant. The diploid carpospores produced in the
carposporangium when released are non-motile, they settle and grow to form
filamentous diploid plants similar to the gametophyte. This diploid plant is
the tetrasporophyte which when adult produced spores in fours after meiosis.
These spores settle and grow to become the male and female plants thus
completing the cycle.
Species
British Isles.
Polysiphonia
atlantica Kapraun &
J.Norris.
ليست هناك تعليقات