All the Blue-green Algæ are able to assimilate carbon by means of
a colouring material containing chlorophyll (cyanophyll); but the
chlorophyll in this substance is masked by a blue (phycocyan), or red
(phycoerythrin, _e.g._ in _Trichodesmium erythræum_ in the Red Sea)
colouring matter which may be extracted from them in cold water after
death. The colouring matter, in most of them, permeates the whole of
the protoplasm (excepting the cell-nucleus), but in a few (_e.g._
_Glaucocystis_, _Phragmonema_), slightly developed chromatophores are
to be found. Where the cells are united into filaments (cell-chains) a
differentiation into apex and base (_Rivulariaceæ_) may take place, and
also between ordinary vegetative cells and heterocysts; these latter
cannot divide, and are distinguished from the ordinary vegetative cells
(Fig. 22 _h_) by their larger size, yellow colour, and poverty of
contents. Branching sometimes occurs and is either true or spurious.
[Illustration: FIG. 18.--_Microcoleus lyngbyanus_: _a_ portion of
a filament, the thick sheath encloses only one cell-chain; in one
place a cell is drawn out by the movement of the cell-chain; _b_
the cell-chain has divided into two parts (“hormongonia”) which
commence to separate from each other.]
The cell-chain in the spurious branching divides into two parts,
of which either one or both grow beyond the place of division
(Fig. 18) and often out to both sides (_e.g. Scytonema_), the
divisions however, always take place transversely to the longitudinal
direction of the cell-chain. In the true branching a cell elongates
in the direction transverse to the cell-chain, and the division
then takes place nearly at right angles to the former direction
(_Sirosiphoniaceæ_).
[Illustration: FIG. 19.--_Cylindrospermum majus_: _a_ resting
akinete with heterocyst; _b-d_ germinating stages of a resting
akinete; _e_ filament with two heterocysts and the formation
of new akinetes; _f_ part of a filament with a heterocyst, and
mature resting akinete.]
Cilia are wanting, but the filaments are sometimes self-motile (_e.g._
hormogonia in _Nostoc_) and many partly turn round their axes, partly
slide forward or backward (_Oscillaria_).
Reproduction takes place by spores and hormogonia in addition to simple
cell-division. Hormogonia are peculiar fragments of a cell-chain
capable of motion, and often exhibit a vigorous motion in the sheath,
until at last they escape and grow into a new individual (Fig. 18).
The spores are reproductive akinetes (_Chamæsiphon_, etc.) or resting
akinetes; these latter arise by the vegetative cells enlarging and
constructing a thick cell-wall (Fig. 19 _e f_). On germination,
this cell-wall bursts and the new cell-chain elongates in the same
longitudinal direction as before (Fig. 19 _b c_). Many (_e.g._
_Oscillaria_) may however winter in their ordinary vegetative stage.
Aplanospores are wanting.
The Fission-Algæ are very prevalent in fresh water and on damp soil,
less so in salt water; they also often occur in water which abounds in
decaying matter. Some are found in warm springs with a temperature as
high as 50° C.
The Family may be divided into 2 sub-families:
1. HOMOCYSTEÆ (heterocysts are wanting): _Chroococcaceæ_, _Lyngbyaceæ_
and _Chamœsiphonaceæ_.
2. HETEROCYSTEÆ (heterocysts present): _Nostocaceæ_, _Rivulariaceæ_,
_Scytonemaceæ_ and _Sirosiphoniaceæ_.
Order 1. =Chroococcaceæ.= The individuals are 1--many-celled, but all
the cells are uniform, united to form plates or irregular masses, often
surrounded by a mucilaginous cell-wall, but never forming cell-chains.
Multiplication by division and sometimes by resting akinetes, but
reproductive akinetes are wanting. _Chroococcus_, _Aphanocapsa_,
_Glœocapsa_ (Fig. 20), _Cœlosphærium_, _Merismopedium_, _Glaucocystis_,
_Oncobyrsa_, _Polycystis_, _Gomphosphæria_.
[Illustration: FIG. 20.--_Glœocapsa atrata_: _A_, _B_, _C_, _D_,
_E_ various stages of development.]
[Illustration: FIG. 21.--_Oscillaria_; =a= terminal, =b= central
portion of a filament.]
Order 2. =Lyngbyaceæ (Oscillariaceæ).= The cells are discoid (Fig.
21), united to straight or spirally twisted, free filaments, which are
unbranched, or with spurious branching. The ends of the cell-chains
are similar. Heterocysts absent. Reproduction by synakinetes, resting
akinetes are wanting. _Oscillaria_ (Fig. 21), _Spirulina_, _Lyngbya_,
_Microcoleus_, _Symploca_, _Plectonema_.
Order 3. =Chamæsiphonaceæ.= The individuals are 1--many-celled,
attached, unbranched filaments with differentiation into apex and base,
without heterocysts. Multiplication by reproductive akinetes; resting
akinetes are wanting. _Dermocarpa_, _Clastidium_, _Chamæsiphon_,
_Godlewskia_, _Phragmonema_.
Order 4. =Nostocaceæ.= The individuals are formed of multicellular,
unbranched filaments, without differentiation into apex and base;
heterocysts present. Reproduction by synakinetes and resting akinetes.
[Illustration: FIG. 22.--_Nostoc verrucosum. A_ The plant in
its natural size; an irregularly folded jelly-like mass. _B_ One
of the cell-chains enlarged, with its heterocysts (_h_), embedded
in its mucilaginous sheath.]
Some genera are not mucilaginous, _e.g. Cylindrospermum_ (Fig. 19).
The cell-chains in others, _e.g. Nostoc_, wind in between one another
and are embedded in large structureless jelly-like masses, which may
attain the size of a plum or even larger (Fig. 22); sometimes they are
found floating in the water, sometimes attached to other bodies. Other
genera as follows: _Aphanizomenon_ and _Anabæna_ (in lakes and smaller
pieces of water); _Nodularia_ is partly pelagic. Some occur in the
intercellular spaces of higher plants, thus _Nostoc_-forms are found
in _Anthoceros_, _Blasia_, _Sphagnum_, _Lemna_, and in the roots of
_Cycas_ and _Gunnera_; _Anabæna_ in _Azolla_.
Order 5. =Rivulariaceæ.= The individuals are multicellular filaments,
with differentiation into apex and base; spurious branching, and a
heterocyst at the base of each filament, reproduction by synakinetes
and resting akinetes, rarely by simple reproductive akinetes.
_Rivularia_, _Glœotrichia_, _Isactis_, _Calothrix_.
Order 6. =Scytonemaceæ.= The individuals are formed of multicellular
filaments with no longitudinal division; differentiation into apex and
base very slight or altogether absent; branching spurious; heterocysts
present. Reproduction by synakinetes, rarely by resting akinetes and
ordinary reproductive akinetes. _Tolypothrix_, _Scytonema_, _Hassalia_,
_Microchæte_.
Order 7. =Sirosiphoniaceæ.= The individuals are formed of multicellular
threads with longitudinal divisions; true branching and heterocysts,
and often distinct differentiation into apex and base. Reproduction
by synakinetes, rarely by resting akinetes and ordinary reproductive
akinetes. _Hapalosiphon_, _Stigonema_, _Capsosira_, _Nostocopsis_,
_Mastigocoleus_.