22. _Chara vulgaris_, branch with nucule and globule. 23. _Tabellaria flocculosa._ 24. _Tabellaria flocculosa_, prepared frustules. 25. _Palmella cruenta._ 26. _Vaucheria Ungeri_ (_sessilis_). 27. _Vaucheria Ungeri_, capsule. [Illustration: Plate VI. _W Bagg sculp_ _London: John Van Voorst._] _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.