The thallus has a stem with nodes and internodes; and whorls of leaves,
on which may be developed the antheridia and oogonia, are borne at
the nodes. Vegetative reproduction by bulbils and accessory shoots.
Zoospores are wanting. The antheridia are spherical, and contain a
number of filaments in which the spirally coiled spermatozoids, each
with two cilia, are formed. The oogonium is situated terminally, and
is at first naked, but becomes later on surrounded by an investment,
and forms after fertilisation the so-called “fruit.” The oospore, after
a period of rest, germinates by producing a “proembryo,” from which
the young sexual plant arises as a lateral branch. The Characeæ are
distinguished by the structure of their vegetative system as well as by
the spirally-coiled spermatozoids, and stand as an isolated group among
the Thallophytes, of which, however, the Siphoneæ appear to be their
nearest relations. They were formerly, but wrongly, placed near the
Mosses. The class contains only one order, the Characeæ.
Order 1. =Characeæ.= Algæ with a peculiar odour, often incrusted with
lime, and of a brittle nature. They generally grow gregariously in
large masses at the bottom of fresh and brackish water, and are from a
few inches to more than a foot in height. The stem has long internodes
which in _Nitella_ are formed of one cylindrical cell; in _Chara_ of
a similar cell, but closely surrounded by a cortical layer of smaller
ones. The protoplasm in contact with the cell-wall exhibits in a
well-marked degree the movement of rotation (cyclosis), carrying the
chlorophyll corpuscles along with it. The internodes are separated
from each other by a layer of small cells (nodal cells) from which the
leaves are produced. The leaves are borne in whorls of from 5–12 which
regularly alternate with one another as in the higher verticillate
plants; a branch is borne in the axil of the first formed leaf of each
whorl (Fig. 61 _A_, _n_).
[Illustration: FIG. 61.--_Chara fragilis. A_ Portion of a
plant, natural size. _B_ Portion of a leaf _b_, with leaflets
β′-β′′; _a_ antheridium; _c_ oogonium. _C_ A shield.--_Nitella
flexilis. D_ Filament from antheridium with spermatozoids. _E_
Free spermatozoids.]
The leaves are constructed in the same manner as the stem; they are
divided into a series of joints, but have only a limited power of
growth; their terminal cell, too, is not enclosed by a cortex. Leaflets
are borne at their nodes. The growth of the stem is unlimited, and
proceeds by means of an apical cell (Fig. 62 _s_). The apical cell
divides into a segment-cell and a new apical cell. The segment-cell
then divides by a transverse wall into two cells, one lying above the
other; the lower one, without any further division, becomes one of the
long, cylindrical, internodal cells (Fig. 62 _in_), and the upper one
(Fig. 62 _n_) divides by vertical walls to form the nodal cells. The
cortical cells (Fig. 62 _r_) which surround the long internodal cells
of _Chara_, are derived from the divisions of the nodal cells; the
cells covering the upper portion of an internodal cell being derived
from the node immediately above it, and those in the lower part of the
internode from the node below it.
[Illustration: FIG. 62.--_Chara fragilis_: _s_ apical cell; _n_,
_n_ nodal cells; _in_ internodal cells; _bl_, _bl_ leaves; _r_,
_r_ the cortical cells.]
[Illustration: FIG. 63.--Oogonium of _Chara_: _k_ “crown”; _u_
receptive spot; _s_ spermatozoids.]
The organs of reproduction are very conspicuous by their colour
and form. They are always situated on the leaves, the plants being
very frequently monœcious. The antheridia (Fig. 61 _B_, _a_) are
modified leaflets or the terminal cell of a leaf; they are spherical
and become red when mature. Their wall consists of 8 “shields,”
_i.e._ of plate-like cells, 4 of which cover the upper half, and are
triangular; the 4 round the lower half, to which the stalk of the
antheridia is attached, being quadrilateral, with sides of unequal
length. The shields (Fig. 61 _C_) have dentated edges, with the teeth
fitting into one another, and their faces ornamented with ridges. From
the centre of the internal face of each shield (_C_) a cylindrical
cell, the _manubrium_, projects nearly as far as the centre of the
antheridium; at the inner end of each of the manubria a spherical
cell, the _capitulum_, is situated. Each capitulum bears six secondary
capitula, from each of which four long coiled filaments (_C_, _D_)
project into the cavity of the antheridium. These filaments are divided
by transverse walls into from 100–200 discoid cells, in each of which
a biciliated, coiled spermatozoid is developed (_D_, _E_) from the
nucleus. The spermatozoids escape from their mother-cell and are set
free by the shields separating from one other.
The female organ of reproduction (Fig. 61 _B_, 63) is a small modified
shoot, whose apical cell functions as an oogonium, its protoplasm
forming the oosphere, which has a colourless receptive-spot at the
summit (Fig. 63 _u_). The oogonium is situated on a nodal cell, from
which 5 cells grow out in a circle and coil round the oogonium,
covering it with a close investment. These cells divide once or twice
at the top, so that 5 or 10 small cells are cut off, which project
above the oogonium and form the so-called “crown” (Fig. 63 _k_). The
crown either drops off at fertilisation, or its cells separate to form
a central canal for the passage of the spermatozoids. The wall of the
oosphere[9] above the receptive spot becomes mucilaginous, and allows
the spermatozoid to fuse with the oosphere. The oospore, on germination
(Fig. 64 _sp_), becomes a small filamentous plant of limited growth
(Fig. 64 _i_, _d_, _q_, _pl_)--the proembryo--and from this, as a
lateral outgrowth, the sexual generation is produced.