The Algæ which belong to this group are uni- or multicellular with
the cells more or less firmly connected, sometimes in a definite,
sometimes in an indefinite form (Fig. 47). Colonies are formed either
by division or by small unicellular individuals becoming united in a
definite manner; the colonies formed in this latter way are termed
_Cœnobia_. Apical cells and branching are absent. Multiplication
by division; asexual reproduction by zoospores, rarely by akinetes.
Sexual reproduction may be wanting, or it takes place by isogamous,
rarely by oogamous fertilisation.
Some are attached by means of a stalk to other objects (_Characium_,
Fig. 49), others occur as “Endophytes” in the tissues of certain Mosses
or Phanerogams, _e.g. Chlorochytrium lemnæ_, in _Lemna trisulca_;
_Endosphæra_, in the leaves of _Potamogeton_, _Mentha aquatica_,
and _Peplis portula_; _Phyllobium_, in the leaves of _Lysimachia
nummularia_, _Ajuga_, _Chlora_, and species of Grasses; _Scotinosphæra_
in the leaves of _Hypnum_ and _Lemna trisulca_; the majority, however,
live free in water and in damp places. Many species which were formerly
considered to belong to this family have been proved to be higher Algæ
in stages of development.
Order 1. =Volvocaceæ.= The individuals in this order are either
uni- or multicellular, and during the essential part of their life are
free-swimming organisms. They are generally encased in a mucilaginous
envelope, through which 2–6 cilia project from every cell. The
vegetative reproduction takes place by the division of all, or a few,
of the cells of the individual; in some a palmella-stage is found in
addition. The sexual reproduction takes place by isogamous or oogamous
fertilisation.
The Volvocaceæ may be considered to include the original forms
of the Chlorophyceæ, because, among other reasons, the motile
stage is here the most prominent; they also form the connecting
link between the animal Flagellata, and forms intermediate to
the _Syngeneticæ_ may perhaps be found amongst them. Three
series of green Algæ may be supposed to have taken their origin
from the Volvocaceæ: CONJUGATEÆ (_Desmidiaceæ_) which have lost
the swarming stage, but whose conjugation is the nearest to the
fertilisation in _Chlamydomonas pulvisculus_: the PROTOCOCCACEÆ
in which the vegetative divisions have disappeared, while the
swarming stage continues to be present, though of shorter
duration; and TETRASPORACEÆ, in which the vegetative divisions
are more prominent, whilst the swarming stage is less so.
A. UNICELLULAR INDIVIDUALS. The principle genera are: _Chlamydomonas_,
_Sphærella_, _Phacotus_.--_Sphærella nivalis_ is the Alga which
produces the phenomenon of “Red Snow,” well known on high mountains and
on ice and snow fields in the polar regions. The red colouring matter
which appears in this and other green Algæ, especially in the resting
cells, is produced by the alteration of the chlorophyll.
_Phacotus lenticularis_ has an outer covering incrusted with lime,
which, at death, or after division, opens out into two halves.
Species may be found among _Chlamydomonas_, in which conjugation
takes place between gametes of similar size without cell-wall, but
in _C. pulvisculus_ conjugation takes place between male and female
aplanogametes which are surrounded by a mucilaginous envelope.
[Illustration: FIG. 44--_Gonium pectorale._]
[Illustration: FIG. 45.--_Pandorina morum._]
B. MULTICELLULAR INDIVIDUALS. The most important genera are _Gonium_,
_Stephanosphæra_, _Pandorina_, _Eudorina_, _Volvox_.--_Gonium_ has 4
or 16 cells arranged in a definite pattern in a flat plate (Fig. 44).
_Pandorina_ (Fig. 45), has 16 cells arranged in a sphere (Fig. 45 _A_).
The vegetative reproduction takes place in this way: each cell, after
having rounded off, and after the withdrawal of the cilia, divides
itself into 16 new ones (Fig. 45 _B_), each forming a new individual,
which soon grows to the size of the mother-individual. It was in this
Alga that the conjugation of self-motile gametes was first discovered
by Pringsheim, 1869. When conjugation is about to take place, each
cell divides into sixteen, as in vegetative reproduction, but the 16
× 16 cells all separate from one another (Fig. 45 _C_, female gametes,
and _D_, male gametes), and swarm solitarily in the water. The male
are, most frequently, smaller than the female, but otherwise they are
exactly alike; they are more or less pear-shaped, with a colourless
anterior end, 2 cilia, a red “eye-spot,” etc. After swarming for some
time they approach each other, two and two, generally a large and a
smaller one, and come into contact at their colourless end; in a few
moments they coalesce and become one cell (Fig. 45 _E_, _F_), this has
at first a large colourless anterior end, 4 cilia, and 2 “eye-spots”
(Fig. 45 _G_), but these soon disappear and the cell becomes uniformly
dark-green and spherical, and surrounds itself with a thick cell-wall,
losing at the same time its power of motion: the zygote (Fig. 45 _H_)
is formed, and becomes later on a deep red colour. On the germination
of the zygote, the protoplasmic cell-contents burst open the wall (Fig.
45 _J_), and emerge as a large swarmspore (Fig. 45 _K_) which divides
into 16 cells, and the first small individual is formed (Fig. 45 _L_,
_M_).
_Eudorina_ is like _Pandorina_ in structure, but stands somewhat
higher, since the contrast between the conjugating sexual cells is
greater, the female one being a motionless oosphere.
[Illustration: FIG. 46.--_Volvox globator_, sexual individual:
_a_ antheridia which have formed spermatozoids; _b_ oogonia.]
The highest stage of development is found in _Volvox_ (Fig. 46). The
cells are here arranged on the circumference of a sphere, and enclose
a cavity filled with mucilage. The number of these cells may vary from
200–22,000, of which the majority are vegetative and not reproductive,
but some become large, motionless oospheres (Fig. 46 _b_); others,
which may appear as solitary individuals, divide and form disc-shaped
masses of from 8–256 small spermatozoids (Fig. 46 _a_). After the
oosphere has been fertilised by these, the oospore surrounds itself by
a thick, sometimes thorny cell-wall, and on germination becomes a new
individual of few cells. A few cells conspicuous by their larger size
may be found (1–9, but generally 8) in certain individuals, and these
provide the vegetative reproduction, each forming by division a new
individual.
Order 2. =Tetrasporaceæ= reproduce both by vegetative divisions and
swarmspores, some have also gamete-conjugation. The principal genera
are: _Tetraspora_, _Apiocystis_, _Dactylococcus_, _Dictyosphærium_,
_Chlorangium_.