Cameron: Morphology of Blue-Green Algae 417 



In the Rivulariaceae plants are aquatic or in moist habitats, spherical, cush- 

 ion-shaped, crustaceous, velvety, feltlike, or brushlike. The filaments are 

 branched or unbranched, radiate from the center of the plant outward, or are 

 parallel and tufthke. Trichomes are unbranched, thick at the base, tapering 

 above, each ending in a colorless hair. Heterocysts are basal or intercalary, 

 although absent in some species. Cell division is transverse and primarily in 

 the middle of the trichome above the heterocyst. Reproduction is by frag- 

 mentation and spores. Amphithrix is a thin crustaceous plant, which lacks 

 heterocysts and has terminal ephemeral hairs. Filaments of Calotlirix, as 

 represented by the most frec^uently collected species, C. parietina (figure 6) 

 (Fan, 1956) is usually unbranched, whereas the filaments of Dichothrix are 

 more or less dichotomously branched, the bases of the branches included for a 

 short distance within the parent sheath. Rkularia and Gloeotrichia have 

 filaments of coalesced sheaths that develop radially to form spherical or cush- 

 ion-shaped plants. No spores are formed in Rivularia but in Gloeotrichia they 

 are thick walled and next to the basal heterocysts. 



The Scytonemataceae contain irregularly cushion-shaped or matlike plants 

 with branched filaments that are single or geminate. The sheaths are firm, 

 tubular, at first colorless, but later yellow, or brown. Trichomes each consist 

 of a single row of cells, one or more included in a sheath. Heterocysts and 

 spores are variously disposed. Cell division primarily occurs behind the tip 

 of the trichome, resulting in lateral perforation of the sheath by dividing and 

 elongating cells which then give rise to single or geminate branches. Repro- 

 duction is usually by fragmentation of the trichome or filament, although one 

 genus, Aulosira, is unique in that all vegetative cells are capable of forming 

 thick walled cylindrical spores or heterocysts. Branching varies with the 

 genera, depending upon its relation to the heterocyst. In species of Scylonema, 

 e.g., S. hofmannii (figure 7), branches may be single and near a heterocyst, 

 but commonly arise at a point somewhat remote from the heterocyst and are 

 geminate. Branches in Tolypothrix are single and arise at the heterocysts. 

 Branches of Desmonema are included within a common sheath. Filaments of 

 Fremyella are short, uncommonly branched, and have basal heterocysts. 



The Oscillatoriaceae is the largest family of the group. It is comprised of 

 plants developing as layers or cushions and is differentiated from other families 

 in that the trichomes do not form spores, heterocysts, or hairs. The cylindri- 

 cal trichomes consist of 1 row of cells in branched or unbranched filaments; 

 the broken ends or hormogonia regenerate in a mode characteristic for the 

 various taxa. In many species, a terminal cell develops a thickened outer 

 membrane. Cell division occurs throughout the entire trichome and at rela- 

 tively the same time. Reproduction is by fragmentation. The current divi- 

 sion of the genera is based largely upon the structure of the sheath (Gomont, 

 1892) and is in need of further study for clarification. The sheaths of Oscilla- 

 tor ia, Arthrospira, and Spirulina are seldom discernible even by application of 

 various staining technicjues. The sheaths of Microcoleiis, e.g., M. vaginatus 

 (figure 8), and Sckizothrix contain one to many trichomes within diffluent 

 or firm sheaths. Usually only one trichome is found in firm sheaths of Plec- 

 lonema, Lyngbya, and Porphyrosiphon. Sheaths of the latter become red or 

 purple; sheaths of Lyngbya may be hyaline or become yellowish-brown. 



