Order 4. =Pleurococcaceæ.= In this order the swarm-stages and sexual
reproduction are entirely absent. Vegetative reproduction by division.
The principal genera are: _Pleurococcus_ (Fig. 47), _Scenedesmus_
(Fig. 48), _Raphidium_, _Oocystis_, _Schizochlamys_, _Crucigenia_,
_Selenastrum_.--_Pleurococcus vulgaris_ (Fig. 47) is one of the
most common Algæ throughout the world, occurring as green coverings
on tree-stems, and damp walls, and it is one of the most common
lichen-gonidia.
[Illustration: FIG. 47.--_Pleurococcus vulgaris._]
[Illustration: FIG. 48.--_Scenedesmus quadricauda._]
Order 5. =Protococcaceæ.= The cells are motionless, free or affixed
on a stalk (_e.g. Characium_, Fig. 49), either separate or loosely
bound to one another; they never form multicellular individuals.
Multiplication by division is nearly always wanting. Reproduction
takes place by swarmspores, which have 1 or 2 cilia, and sexual
reproduction in some by gamete-conjugation. The principal genera are:
_Chlorococcum_, _Chlorochytrium_, _Chlorocystis_, _Scotinosphæra_,
_Endosphæra_, _Phyllobium_, _Characium_, _Ophiocytium_, _Sciadium_.
[Illustration: FIG. 49.--_Characium strictum._
_A_ The cell-contents have divided into many swarmspores.
_B_ Swarmspores escaping.]
Order 6. =Hydrodictyaceæ.= The individuals are unicellular but
several unite after the zoospore-stage into definitely formed families
(cœnobia). Ordinary vegetative division is wanting, but asexual
reproduction takes place by zoospores (or by motionless cells without
cilia), which unite and form a family similar to the mother-family,
inside the mother-cell, or in a mucilaginous envelope. Where sexual
reproduction is found it takes place by gamete-conjugation. The
principal genera are: _Pediastrum_ (Fig. 50), _Cœlastrum_,
_Hydrodictyon_ (Fig. 51).
[Illustration: FIG. 5O.--_Pediastrum asperum._]
[Illustration: FIG. 51.--_Hydrodictyou reticulatum. A_ A cell
where the zoospores are on the point of arranging themselves to
form a net. _B_ A cell with gametes swarming out.]
The cœnobium of _Hydrodictyon reticulatum_ (Water-net) is formed of
a large number of cells which are cylindrical, and attached to one
another by the ends (Fig. 51). The asexual reproduction takes place by
zoospores, which are formed in large numbers (7,000–20,000) in each
mother-cell, within which they move about for a time, and then come
to rest and arrange themselves into a new net (Fig. 51 _A_) which is
set free by the dissolution of the wall of the mother-cell, grows,
and becomes a new cœnobium. The sexual reproduction takes place by
gamete-conjugation. The gametes are formed in the same manner as the
zoospores, but in larger numbers (30,000–100,000), and swarm out of the
mother-cell (Fig. 51 _B_). The zygote forms, on germination, 2–5 large
zoospores, each with one or two cilia, these generally swarm about
for a time, and after a period of rest become irregular thorny bodies
(polyhedra); their contents again divide into zoospores, the thorny
external coating of the polyhedra is cast off, and the zoospores,
surrounded by the dilated internal coating, unite to form a small
family, which produces several others in the manner described.