Family 1. Schizophyceæ,[5] Blue-Green Algæ.
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.
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).
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.