44 



Myxophyceae 



Reproduction takes place by hormogones and by resting-spores. The latter are 

 variable in shape, and may be solitary or seriate. In Cylindrospermum the spore arises 

 from the cell next the terminal heterocyst (fig. 32 E G-'). About half the species of 

 Nostoc and Cylindrospermum occur in subaerial habitats, but all the other members of 

 the family are aquatic. Species of Nodularia are mostly brackish-water forms. 



Fain. Scytonemaceae. The Algte of this family are distinguished by their peculiar 

 type of branching. As a rule there is only one trichome within a strong tubular sheath 

 of regular thickness. The false branches arise either singly or in pairs, and are due to the 

 perforation of the sheath of the primary filament by its trichome, which issues either as a 

 single 'branch' (Tolypotliri.r; 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-brown 

 colour. The heterocysts are intercalary and rather infrequent. The normal reproduction 

 is by hormogones, but resting-spores also occur (fig. 17 C). 



Fam. Stigonemacese. The filaments are coarse and tough owing to a strong uneven 

 sheath, usually of a brown colour. As a rule the filaments are much branched, and they 

 may contain one regular, or several more or less irregular, series of cells (fig. 34). The 

 heterocysts are lateral or intercalary. Stigonema is the principal genus, most of the 

 species of which occur on damp or wet rocks, and are propagated by hormogones. 

 Hapalosiphon is an aquatic genus, with more slender filaments and longer branches. The 

 sheaths are not so strong as those of Stigonema, and the plants are largely reproduced by 

 resting-spores (fig. 17 A and B}. 



B. Trichophoreie. Trichomes markedly attenuated towards one or both 

 extremities, which are often drawn out into hair-like points. 



Fam. Rivulariaceae. In this family the trichomes are attenuated from the base 

 upwards and are often piliferous. In all except a few cases there is a single heterocyst at 

 the base. There is a well-marked tubular sheath, gelatinous or membranous, and often 

 strongly lamellose. There is a prominent false branching in the genus Dichothnx, 

 accompanied by a conspicuous fusion of sheaths. The filaments of Culotliriv generally 



