[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_.