The thallus has one or more apical cells, grows principally by apical
growth, and may be differentiated into root, stem, and leaf. The
chromatophores vary in form, but have a red or brownish colour, due
to chlorophyll and phycoerythrin. Asexual reproduction by motionless
tetraspores, which generally arise by the division into four of the
contents of the tetrasporangium. The carpogonium has a trichogyne,
and the carpospores, which are formed indirectly from the fertilised
oosphere, possess a cell-wall.
[Illustration: FIG. 73.--_Callithamnion elegans_: _a_ a plant
with tetraspores (× 20); _b_ apex of a branch with tetraspores(×
250).]
[Illustration: FIG. 74.--_Polysiphonia variegata_: _a_ a portion
of a male plant with antheridia; _b_ spermatia; _c_ transverse
section of thallus.]
The thallus may assume very different forms. In the simplest species
it is filamentous and formed of single, branched rows of cells
(_Callithamnion_, etc., Fig. 73). _Ceramium_ has a filamentous thallus,
generally dichotomously forked (Fig. 75), or sometimes pinnately
branched, which, at the nodes, or throughout its entire length, is
covered by a layer of small cortical cells. _Polysiphonia_ (Fig.
74) has a filamentous, much branched thallus, made up of a central
cylindrical cell, surrounded by a layer of other cells, cortical cells,
which in length and position correspond to the central ones. In many of
the Red Algæ the vegetative organs are differentiated into stems and
leaves, the former having, as in _Chara_, unlimited growth in length,
whilst the latter soon attain their full development. _Chondrus_
has a fleshy, gelatinous thallus, without nodes; it is repeatedly
forked into flat branches of varying thickness. _Furcellaria_ has a
forked thallus with thick branches and without nodes. The thallus of
_Delesseria_ (Fig. 76) consists of branches, often bearing leaf-like
structures, with a midrib and lateral ribs springing from it. These
ribs persist through the winter, and at the commencement of the
succeeding period of vegetation the lateral ribs become the starting
points for new leaves. In _Corallina_ the thallus is pinnately
branched, and divided into nodes and internodes. The name has been
given to this genus from the fact that the thallus is incrusted with
carbonate of lime to such a degree that it becomes very hard, and the
whole plant adopts a coral-like appearance. Other genera which are
similarly incrusted, and have a leaf-like or even crustaceous thallus
(such as _Melobesia_, _Lithothamnion_), are included in this family.
In some instances the cells of the thallus may be found
_differentiated_ into more or less well defined tissues, so that it
is possible to find special assimilating, mechanical, and conducting
tissues, the last named in some cases having the double function of
conducting and of serving as a reservoir in which starch is found as
a reserve material. The cells of the Florideæ, which are formed by the
division of a mother-cell into two daughter-cells of unequal size, have
always larger or smaller pits in the cell-walls, and the thin cell-wall
separating two pits from each other is perforated by a number of small
holes. These pits are particularly developed in the conducting tissues,
but sieve-tubes are very rarely to be found.
[Illustration: FIG. 75.--_Ceramium diaphanum_ (nat. size).]
[Illustration: FIG. 76.--_Delesseria sanguinea_ (about ⅓).]
_Tetraspores_ may be wanting (_e.g. Lemanea_) or may often arise on
special, non-sexual individuals. In some (_e.g. Batrachospermum_)
only one tetraspore is formed in each tetrasporangium, but the number
is generally four, which may be formed tetrahedrally (Fig. 73) or by
divisional walls perpendicular to each other, or even in a single row.
The tetrasporangia in some species are free (Fig. 73), but in the
majority they are embedded in the thallus.
[Illustration: FIG. 77.--_A Lejolisia mediterranea_: _r_
haptera; _s_ longitudinal section through a cystocarp; _p_ the
empty space left by the liberated spore (_t_). _B-E Nemalion
multifidum_: _a_ antheridia; _b_ procarpium with trichogyne, to
which two spermatia are adhering.]
The sexual reproduction (discovered by Thuret and Bornet, 1867)
differs in the essential points from that of all other plants, and
approaches most nearly to the sexual reproduction of the _Bangioideæ_.
The sexual cells are developed from the terminal cells (never nodal
cells) of the branched cell-filaments, which constitute the thallus.
The mother-cells of the spermatia (_spermatangia_) are generally
arranged in a group, in the so-called _antheridia_ (Figs. 74, 77 _A_,
_a_). On becoming ripe the membrane of the spermatangium ruptures
and the _spermatia_ emerge as spherical or ovoid, naked (a little
later they may possess a cell-wall) masses of protoplasm which are
not endowed with the power of motion, and hence are carried passively
by the current of the water in which they may happen to be, to the
female cell. This latter is analogous with the oogonium of the Green
Algæ. The female reproductive organ is termed the _procarpium_, and
consists of two parts, a lower swollen portion--the _carpogonium_
(Fig. 77 _b_ in _A_ and _B_)--which contains the cell-nucleus, and an
upper filamentous prolongation--the _trichogyne_ (Fig. 77 _B_)--which
is homologous with the colourless receptive spot of the oosphere of
the Green Algæ, and the _Porphyraceæ_. In the sexual reproduction
of the majority of the Florideæ, a very important part is played by
certain special cells, rich in cell-contents--the _auxiliary cells_.
These are either dispersed in the interior of the thallus, or are
arranged together in pairs with the cell-filament which bears the
carpogonium, and are generally united with this to form an independent
multicellular _procarpium_. The spermatia attach themselves firmly to
the trichogyne and surround themselves with a cell-wall. The dividing
wall at the point of contact is perforated, and the nucleus of the
spermatium probably travels through the trichogyne to the swollen part
of the procarpium--the _carpogonium_--and fuses with its nucleus. After
fertilisation the trichogyne withers (Fig. 77 _C_), but the lower
portion of the procarpium, constituting the _fertilised oosphere_,
grows out and forms in various ways, first a tuft of spore-forming
filaments known as _gonimoblasts_, and finally the _carpospores_. These
latter form a new asexual generation (compare the germination of the
oospore of _Œdogonium_ and _Coleochæte_).