. Algæ. Vol. I. Myxophyceæ, Peridinieæ, Bacillarieæ, Chlorophyceæ, together with a brief summary of the occurrence and distribution of freshwat4er Algæ . i:ir<H<irpu Born & Flah. (All xIsO.) /;, heterocyst; gp, resting-spore. 44 Myxophyceae Reproduction takes place by hormogones and by resting-spores. The latter arevariable in shape, and may be solitary or seriate. In Cylindrospermum the spore arisesfrom the cell next the terminal heterocyst (fig. 32 E—G-). About half the species ofNostoc and Cylindrospermum occur in subaerial habitats, but all the other members ofthe family are aquatic.
. Algæ. Vol. I. Myxophyceæ, Peridinieæ, Bacillarieæ, Chlorophyceæ, together with a brief summary of the occurrence and distribution of freshwat4er Algæ . i:ir<H<irpu Born & Flah. (All xIsO.) /;, heterocyst; gp, resting-spore. 44 Myxophyceae Reproduction takes place by hormogones and by resting-spores. The latter arevariable in shape, and may be solitary or seriate. In Cylindrospermum the spore arisesfrom the cell next the terminal heterocyst (fig. 32 E—G-). About half the species ofNostoc and Cylindrospermum occur in subaerial habitats, but all the other members ofthe family are aquatic. Species of Nodularia are mostly brackish-water forms. Fain. Scytonemaceae. The Algte of this family are distinguished by their peculiartype of branching. As a rule there is only one trichome within a strong tubular sheathof regular thickness. The false branches arise either singly or in pairs, and are due to theperforation of the sheath of the primary filament by its trichome, which issues either as asingle branch (; fig. 33 E) or as a pair of branches (Scytonema; fig. 33. Fig. 33. A—D, Scytonema mirabile (Dillw.) Thtiret. A, showing pair of false branches ;B, part of filament with a heterocyst (h); C, apex of branch; D, organ of attach-ment at base of filament. E, Tolypnthnx lanata (Desv.) Wartm. All x 440. A—D), each of which develops a new sheath. The sheaths are variable in character,sometimes homogeneous and colourless, but in other cases lamellose and of a golden-browncolour. The heterocysts are intercalary and rather infrequent. The normal reproductionis by hormogones, but resting-spores also occur (fig. 17 C). Fam. Stigonemacese. The filaments are coarse and tough owing to a strong unevensheath, usually of a brown colour. As a rule the filaments are much branched, and theymay contain one regular, or several more or less irregular, series of cells (fig. 34). Theheterocysts are lateral or intercalary. Stigonema is the principal genus, most of the
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