Oscillatoria autumnális ([Pl. VI.] fig. 1) occurs everywhere upon damp shaded banks of ditches, especially when newly made, forming a greenish-black closely adherent stratum. Under the microscope it is seen to consist of innumerable palish-green filaments; these are jointed or transversely striated, some being straight, others curved, the ends often exhibiting a writhing or worm-like movement. The appearance of these fibres is peculiar, seeming as if they were solid throughout, and so differing from that of the Confervaceæ, in which the cell-walls are readily distinguishable from the cell-contents. The fibres easily break across at the joints; and the last few segments are often narrowed and rounded, so as to form a blunt point. When they have been left in water, they exhibit colourless tubular sheaths surrounding and extending beyond them. These sheaths consist of the consolidated outer portions of the cell-walls; for when the cells undergo transverse division, and expand by growth in the direction of the length of the filament, the original septa or inner walls are broken through, and their remains may often be seen on the inner surface of the sheath, appearing as little teeth.
Oscillatoria nigra ([Pl. VI.] fig. 2) is another very similar species, forming blackish-green masses, and is common in ditches. It has longer filaments than the last, with narrowed and slightly curved ends; and the endochrome is distinctly granular.
In two other genera of this family, Vib´rio and Spirulína, the filaments are spiral. Vib´rio spiril´lum is excessively minute, colourless, and found in decomposing vegetable mixtures. The short filaments move rapidly through the water, with a corkscrew-like motion. In Spirulína oscillarioídes ([Pl. VI.] fig. 12), which is more rarely found in clear pond-waters among Confervæ, the filaments are greenish, and form a beautiful simple spiral, resembling that of a very slender spiral vessel.
Lyng´bya murális ([Pl. V.] fig. 2) is very common on damp walls, gravel walks, &c. It forms a bright grass-green layer, consisting of somewhat rigid curled filaments. The endochrome is usually broader than long; and the cells of the filaments are often found empty, the endochrome having escaped in the form of gonidia.
[Pl. VI.] fig. 6 represents a species of Bactérium which is not uncommon in decomposing vegetable liquids; the filaments are short, curved, pointed at the ends, and have four joints.
Fig. 26 represents a Schizogónium, found upon damp paths. The filaments resemble those of Lyngbya, but are united in pairs.
Fig. 3 represents a filament of a U´lothrix, which is common in freshwater pools, showing the curious manner in which the endochrome is arranged in the cells, forming bands partially lining the cell-walls.
Nostocha´ceæ.—Two species of the typical genus Nos´toc will serve to represent this family. Nos´toc commúne is found on damp ground or in ponds, and forms to the naked eye firmish, olive-green, skin-like, plaited masses, an inch or more in diameter. Under the microscope it is seen to consist of numerous beaded fibres, imbedded in worm-like gelatinous sheaths; these are curved and interwoven to form the compound mass. In the middle of many of the filaments is an enlarged colourless cell, called the vesicular cell, which is related to the reproduction, but in a manner not yet determined.
Nostoc minutis´simum ([Pl. VI.] fig. 4) forms solid gelatinous bluish-green masses, varying in size from a pin’s head to a pea; it is found upon unhealthy water-plants kept in glass vessels. The component filaments are very slender, wavy, and the sheaths often have a brownish tinge.
Ulvaceæ.—These Algæ are mostly marine—some, however, being found in brackish or fresh water, or on damp ground, thatch, moss, &c. They are generally of considerable size, forming flat or tubular fronds, often several inches long, a few being filamentous. They consist of one or more sheets or layers of cells, containing mostly green endochrome. This at first fills the cells, but subsequently becomes converted into single spores, or subdivided into numerous ciliated zoospores.