8 _Oscillatoriaceæ_.--Plants growing either in the sea, fresh water, or on damp ground, of a gelatinous substance and filamentous structure. Filaments very slender, tubular, continuous, filled with colored, granular, transversely striated substance; seldom blanched, though often cohering together so as to appear branched; usually massed together in broad floating or sessile strata, of a very gelatinous nature; occasionally erect and tufted, and still more rarely collected into radiating series bound together by firm gelatine and then forming globose lobed or flat crustaceous fronds. Fructification: the internal mass or contents separating into roundish or lenticular gonidia. 9. _Nostochacæ_.--Gelatinous plants growing in fresh water, or in damp situations among mosses, etc.; of soft or almost leathery substance, consisting of variously curled or twisted necklace-shaped filaments, colorless or green, composed of simple, or in some stages double rows of cells, contained in a gelatinous matrix of definite form, or heaped together without order in a gelatinous mass. Some of the cells enlarged, and then forming either vesicular empty cells or densely filled sporangial cells. Reproduction: by the breaking up of the filaments, and by resting spores formed singly in the sporanges. 10. _Ulvaceæ_.--Marine or aquatic algae consisting of membranous, flat, and expanded tubular or saccate fronds composed of polygonal cells firmly joined together by their sides. Reproduced by zoospores formed from the cell contents and breaking out from the surface, or by motionless spores formed from the whole contents. 11. _Palmellaceæ_.--Plants forming gelatinous or pulverulent crusts on damp surfaces of stone, wood, earth, mud, swampy districts, or more or less regular masses of gelatinous substance or delicate pseudo-membranous expansion or fronds, of flat, globular, or tubular form, in fresh water or on damp ground; composed of one or many, sometimes innumerable, cells, with green, red, or yellowish contents, spherical or elliptical form, the simplest being isolated cells found in groups of two, four, eight, etc., in course of multiplication. Others permanently formed of some multiple of four; the highest forms made up of compact, numerous, more or less closely joined cells. Reproduction: by cell division, by the conversion of the cell contents into zoospores, and by resting spores, formed sometimes after conjugation; in other cases, probably, by fecundation by spermatozoids. All the unicellular algæ are included under this head.