The Algæ belonging to this class have chlorophyll, and pyrenoids
round which starch is formed. The cells divide only in one direction,
they live solitarily, or united to form filaments which generally
float freely (seldom attached). Swarm-cells are wanting. _The
fertilisation is isogamous (conjugation) and takes place by means
of aplanogametes._ The zygote, after a period of rest, produces,
immediately on germination, one or more new vegetative individuals;
sometimes akinetes or aplanospores are formed in addition. They only
occur in fresh or slightly brackish water.
Order 1. =Desmidiaceæ.= The cells generally present markings on the
outer wall, and are mostly divided into two symmetrical halves by a
constriction in the middle, or there is at least a symmetrical division
of the protoplasmic cell-contents. The cell-wall consists nearly
always of two layers, the one overlapping the other (Fig. 35 _C_). The
cells either live solitarily or are united into unbranched filaments.
The mass of protoplasm formed by the fusion of the two conjugating
cells becomes the zygote, which on germination produces one (or after
division 2, 4 or 8) new vegetative individual. The chromatophores are
either star-, plate-, or band-shaped, and regularly arranged round the
long axis of the cell.
[Illustration: FIG. 35.--A Cell of _Gymnozyga brebissonii_,
external view showing the distribution of the pores. _B A_
portion of the membrane of _Staurastrum bicorne_ with pores
containing protoplasmic projections. _C_ Cell-wall of _Hyalotheca
mucosa_ during cell-division: the central part, being already
formed, shows the connection with the divisional wall.]
The Desmidiaceæ are not able to swim independently, many species,
however, show movements of different kinds by rising and sliding
forward on the substratum. These movements, which are partly dependent
upon, and partly independent of light and the force of gravitation, are
connected with the protrusion of a mucilaginous stalk. The mucilage,
which sometimes surrounds the whole individual, may acquire a prismatic
structure, it is secreted by the protoplasmic threads which project
through certain pores definitely situated in the walls (Fig. 35
_A_, _B_).
VEGETATIVE MULTIPLICATION takes places by division. A good example of
this is found in _Cosmarium botrytis_ (Fig. 36 _A-D_). The nucleus
and chromatophores divide, and simultaneously the central indentation
becomes deeper, the outer wall is then ruptured making a circular
aperture through which the inner wall protrudes forming a short,
cylindrical canal between the two halves to which it is attached (Fig.
36 _C_). After elongation the canal is divided by a central transverse
wall, which commences as a ring round its inner surface and gradually
forms a complete septum. The dividing wall gradually splits, and the
two individuals separate from each other, each one having an old and
a new half. The two daughter-cells bulge out, receive a supply of
contents from the parent-cells, and gradually attain their mature size
and development (Fig. 36 _B-D_). Exceptions to this occur in some forms.
[Illustration: FIG. 36.--_Cosmarium botrytis. A-D_ Different
stages of cell-division.]
[Illustration: FIG. 37.--_Cosmarium meneghinii_: _a-c_ same
individual seen from the side, from the end, and from the edge;
_d-f_ stages of conjugation; _g-i_ germination of the zygote.]
CONJUGATION takes place in the simplest way in _Mesotænium_, where
the two conjugating cells unite by a short tube (conjugation-canal),
which is not developed at any particular point. The aplanogametes
merge together after the dissolution of the dividing wall, like two
drops of water, almost without any trace of preceding contraction, so
that the cell-wall of the zygote generally lies in close contact with
the conjugating cells. The conjugating cells in the others lie either
transversely (_e.g. Cosmarium_, Fig. 37 _d_; _Staurastrum_, etc.),
or parallel to one another (_e.g. Penium_, _Closterium_, etc.), and
emit a short conjugation-canal (Fig. 37 _d_) from the centre of that
side of each cell which is turned towards the other one. These canals
touch, become spherical, and on the absorption of the dividing wall
the aplanogametes coalesce in the swollen conjugation-canal (Fig.
37 _e_), which is often surrounded by a mucilaginous envelope. The
zygote, which is often spherical, is surrounded by a thick cell-wall,
consisting of three layers; the outermost of these sometimes bears
thorn-like projections, which in some species are simple (Fig. 37
_f_), in others branched or variously marked; in some, however, it
remains always smooth (_e.g._ _Tetmemorus_, _Desmidium_). Deviation
from this mode of conjugation may occur within certain genera (_e.g._
_Closterium_, _Penium_). Upon germination the contents of the zygote
emerge, surrounded by the innermost layers of the wall (Fig. 37 _g_,
_h_) and generally divide into two parts which develop into two new
individuals, placed transversely to each other (Fig. 37 _i_); these may
have a somewhat more simple marking than is generally possessed by the
species.
[Illustration: FIG. 38.--Desmidiaceæ. _A Closterium
moniliferum_; _B Penium crassiusculum_; _C Micrasterias
truncata_ (front and end view); _D Euastrum elegans_; _E_
_Staurastrum muticum_ (end view).]