classification of algae into four sub-groups, founded on
the nature of the colouring matters present in the plant:--
I. CYANOPHYCEAE, or Blue-green Algae.
II. CHLOROPHYCEAE, or Green Algae.
III. PHAEOPHYCEAE, or Brown Algae.
IV. RHODOPHYCEAE, or Red Algae.
The merits and demerits of this system will appear during the
description of the characters of the members of the several subdivisions.
I. CYANOPHYCEAE.--This group derives its name from the
circumstance that the cells contain in addition to the
green colouring matter, chlorophyll, a blue-green colouring
matter to which the term phycocyanin has been applied.
Sub-divisions.
To the eye, however, members of this group present a greater
variety of colour than those of any other--yellow, brown,
olive, red, purple, violet and variations of all these being
known. They undoubtedly represent the lowest grade of
algal life, and their distribution rivals that of the Green
Algae. They occur in the sea, in fresh water, on moist
earth, on damp rocks and on the bark of trees. Certain
species are regularly found in the intercellular spaces of
higher plants; such are species of Nostoc in the thallus of
Anthoceros, the leaves of Azolla and the roots of Cycads.
Many of them enter into the structure of the lichen-thallus,
as the so-called gonidia. It is remarkable that species
belonging to the Oscillatoriaceae are known to flourish in hot
springs, the temperature of which rises as high as 85 deg. C.
The thallus may be unicellular or multicellular. When unicellular,
it may consist of isolated cells, but more commonly the cells
are held together in a common jelly (Chroococcaceae) derived
from the outer layers of the cell-wall. The multicellular
species consist of filaments, branched or unbranched, which
arise by the repeated divisions of the cells in parallel
planes, no formation of mucilage occurring in the dividing
walls. Such filaments may not give rise to mucilage on the
lateral surface either, in which case they are said to be free;
when mucilage does occur on the lateral wall, it appears as
the sheath surrounding either the single filament, or a sheaf
of filaments of common origin. The mucilage may also form an
embedding substance similar to that of Chroococcaceae, in which
the filaments lie parallel or radiate from a common centre
(Rivulariaceae). The cells of the filament may be all alike,
and growth may occur equally in all parts (Oscillatoriaceae);
or certain cells (heterocysts) may become marked off by
their larger size and the transparency of their contents;
in which case growth may still be distributed equally
throughout (Nostoc), or the filament may be attached
where the heterocyst arises, and grow out at the opposite
extremity into a fine hair (Rivulariaceae). An African form
(Camptothrix), devoid of heterocysts and hair-like at both
extremities, has recently been described. Branching has been
described as ``false'' and ``true.'' The former arises when
a filament in a sheath, either in consequence of growth in
length beyond the capacity of the sheath to accommodate it,
FIG, 1.--Cyanophyceae, variously magnified.
A. Gloeocapsa sp.,colony in mucilage.
B. Phormidium sp., single filament with hormogonium.
C. Microcoleus sp., several filaments in common sheath.
D. Nostoc sp., young colony-filament with heterocysts.
E. Scytonema sp., false branching.
F. Rivularia sp.
G. Stigonema sp., with hormogonium and true branching.
H. Spirulina sp.
(From Engler and Prantl, Pflanzenfamilien,
by permission of Wilhelm Engelmann.)
or because of the decay of a cell, becomes interrupted by
breaking, and the free ends slip past one another. ``True''
branching arises only by the longitudinal division of a
cell of a filament and the lateral outgrowth of one of
the cells resulting from the division (Sirosiohonaceae).
The nature of the contents of the cells of Cyanophyceae has
given rise to considerable controversy. The cells are for
the most part exceedingly minute, and are not easy to free
from their colouring matters, so that investigation has been
attended with great difficulty. Occupying as these algae
do perhaps the lowest grade of plant life, it is a matter of
interest to ascertain whether a nucleus or chromatophore is
differentiated in their cells, or whether the functions and
properties of these bodies are diffused through the whole
protoplast. It is certain that the centre of the cell, which is
usually non-vacuolated, is occupied by protoplasm of different
properties from the peripheral region; and A. Fischer has
further established the fact that the peripheral mass, which
is a hollow sphere in spherical cells, and either a hollow
cylinder or barrel-shaped body in filamentous forms, must
be regarded as the single chromatophore of the Cyanophyceous
cell. But what precisely is the nature of the central
mass is still uncertain. Some investigators, such as R.
Hegler, F. G. Kohl and E. W. Olive, claim that this body is
a true nucleus comparable with that of the higher plants.
It is said to undergo division by a mitosis essentially of
the same character, with the formation of a spindle and the
differentiation of chromosomes. It is further stated by Olive
that the chromosomes undergo longitudinal fission, and that for
the same species the same number of chromosomes appear at each
division. H. Wager speaks with greater reserve, acknowledging,
however, the central body to be a nucleus of a rudimentary
type, but devoid of nuclear membrane and nucleolus. He
thinks it may possibly originate in the vacuolization of the
central region, and the accumulation of chromatin granules
therein. He finds no spindle fibres or true chromosomes, and
considers the division direct, not indirect. With reference
to the existence of a chromatophore, he with others finds
the colouring matter localized in granules in the peripheral
region, but does not consider these individually or in the
aggregate as chromatophores. Among other contents of the cell,
fatty substances and tannin are known. A curious adaptation
seems to occur in certain floating forms, in the presence of a
gas-vacuole, which may be made to vary its volume with varying
pressure. There is evidence that the dividing wall of filamentous
forms is deeply pitted, as is found to be the case in red
algae. Reproduction is chiefly effected by the vegetative
method. Asexual reproductive cells are not infrequent, but
sexual reproduction even in its initial stages is unknown.
Nor is motility by means of cilia known in the group. In the
unicellular forms, cell-division involves multiplication of the
plant. In all the multicellular plants of this group which
have been adequately investigated, vegetative multiplication
by means of what are known as hormogonia has been found to
occur. These are short segments of filaments consisting of a
few cells which disengage themselves from the ambient jelly,
if it be present, in virtue of a peculiar creeping movement
which they possess at this stage. After a time they come
to rest and give rise to new colonies. True reproduction
of the asexual kind occurs, however, in the formation of
sporangia, particularly in the Chamaesiohonaceae. Here
the contents of certain cells break up endogenously into
a great number of spores, which are distributed as a fine
dust. Resting spores are also known. In these cases, certain
cells of a colony of unicellular plants or of the filaments
of multicellular plants enlarge greatly and thicken their
wall. When unfavourable external conditions supervene and
the ordinary cells become atrophied, these cells persist
and reproduce the plant with the return of more favourable
conditions. The Oscillatoriaceae are capable of a peculiar
oscillatory movement, which has earned for them their
name, and which enables them to move through considerable
distances. It is not clear how the movement is effected, though
it has frequently been the subject of careful investigation.
With the Cyanophyceae must be included, as their nearest allies,
the Bacteriaceae (see BACTERIOLOGY.) Notwithstanding the absence
of chlorophyll, and the consequent parasitic or saprophytic
habit, Bacteriaceae agree in so many morphological features
with Cyanophyceae that the affinity can hardly be doubted.
A census of the Cyanophyceae with their two main groups is given below:--
1. Coccogoneae--2 families, 29 genera, 253 species.
2. Hormogoneae--6 families, 59 genera, 701 species.
(Engler and Prantl's Pflanzenfamilien, 1900)
II. CHLOROPHYCEAE.--This group includes those algae in which
the green colouring matter, chlorophyll, is not accompanied
by a second colouring matter, as it is in other groups. It
consists of three subdivisions--Conjugatae, Euchlorophyceae and
Characeae. Of these the first and last are relatively small
and sharply defined families, distinguished from the second
family, which forms the bulk of the group, by characters
so diverse that their inclusion with them in one larger
group can only be justified on the ground of convenience.
Chlorophyceae include both marine and freshwater plants.
Euchlorophyceae in their turn have been until recently regarded
as made up of the three series of families--Protococcales,
Confervales and Siphonales. As the result of recent
investigations by two Swedish algologists, Bohlin and Luther, it
has been proposed to make a re-classification of a far-reaching
nature. Algae are withdrawn from each of the three series
enumerated above and consolidated into an entirely new
group. In these algae, the colouring matter is said to
be yellowish-green, not strictly green, and contained in
numerous small discoid chromatophores which are devoid of
pyrenoids. The products of assimilation are stored up in
the form of a fatty substance and not starch. A certain
inequality in the character of the two cilia of the zoospores
of some of the members of the group has earned for it the title
Heterokontae, from the Greek kontos, a punting-pole. In
consonance with this name, its authors propose to re-name
the Conjugatae; Akontae and Oedogoniaceae with a chaplet
of cilia become Stephanokontae, and the algae remaining
over in the three series from which the Heterokontae and
Stephanokontae are withdrawn become Isokontae. Conjugatae,
Protococcales and Characeae are exclusively freshwater;
Confervales and Siphonales are both freshwater and marine,
but the latter group attains its greatest development in the
sea. Some Chlorophyceae are terrestrial in habit, usually
growing on a damp substratum, however. Trentepohlia
grows on rocks and can survive considerable desiccation.
Phycopeltis grows on the surface of leaves, Phyllobium
and Phyllosiphon in their tissues. Gomontia is a
shell-boring alga, FIG. 2.--Chlorophyceae, variously magnified.
A. Chlamydomonas sp., unicellular; chr., chromatophore; p.,
pyrenoid; n., nucleus; p.v., pulsating vacuoles; e.s., eyespot.
B1. Volvox sp., with a, antheridia, and o, oogonia.
B2. Volvox sp., surface view of a single cell showing connexions.
C. Pandorina sp., a 16-celled colony.
D. Hydrodictyon, a single mesh surrounded by 6 cells.
E. Microspora sp., showing H-pieces in the wall.
F. Entoderma sp., endophytic in Ectocarpus.
G. Coleochaete sp., growing as a plate.
H. Oedogonium sp., intercalated growth by
insertion of new piece (a) leaving caps.
K. Struvea sp., showing branches forming a net-work.
L. Caulerpa sp., showing portion of axis
with leaf-like and root- like appendages.
M1. Chara sp., axis with leaf-like appendages and a branch.
