96 OSCILLATORIACE^. 



Order IX.— OSCILLATORIACE^. 



Harv. Man. Ed. 1. p. 219. Oscillatoriece, Harv. in Mack. Fl. Hib. part S,p. 164. 

 Endl. Sd. Suppl. p. 12. Oscillatoriece and Rivulariece, Harv. Br. Fl. J. Ag. Alg. 

 Medit. p. 8,10. Oscillatorece, Lindl. Veg. Kingd. p. 18. Oscillariece, Leptotrichiece, 

 Lyngbgece, Scytonemece, Mastichotrichece, Rivulariece, Kiitz. Sp. Alg. pp. 235-344. 



Diagnosis. Green, (rarely olive-brown, blue, or purple) marine or fresh water Algaj, 

 composed of simple or slightly branched filaments ; each filament having a membranous 

 unicellular sheath, enclosing an annulated medullary chord of very short cells. 



Natural Character. Boot either a simple point of attachment, or, in most cases, 

 not obvious. Filammts of small size, and often very minute, rarely solitary, variously 

 aggregated together. In some microscopic forms, as in Trichodesmium, a number of 

 minute filaments lie close together, cohering by their edges and parallel to each other, 

 forming little bundles, resembling faggots in miniature ; and these float freely in the 

 water, through which they move by a slow, proper motion, rising to the surface or 

 sinking, according to the season. In others, as in Oscillatoria, an indefinite number 

 of similar filaments lie loosely in a gelatinous matrix, within which they are developed, 

 and from the edges of which they radiate ; but they have no definite or determinate 

 arrangement in the mass. Again, in Calothrix, the filaments are fixed at the base, and 

 stand erect in minute tufts, or spread in a velvetty pile over the surface of various 

 objects. In Lyngbya the arrangement of the threads is similar, but they are 

 of much greater length, more curved and flexible, resembling tufts of hair or silky 

 Avool. Lastly, in Rivularia, there is a compact gelatinous frond of sub-definite 

 form, constructed of a multitude of symmetrically arranged filaments ; each one 

 springing from a minute, spherical, bulb-like cell, by which it is attached to the 

 neighbouring filament. These basal cells have been called " connecting cells," and 

 also " heterocysts." Their peculiar function has not been clearly ascertained. Through 

 all the genera of the Order considerable uniformity prevails Jin the structure of the fila- 

 ments. The external coating or peripheric portion, called the sheath., is a tubular 

 membrane, destitute of markings, hyaline, and apparently formed by the lengthening 

 of a single generating cell. In many cases it is delicately membranous and thin ; in 

 others it is thickened ; and in some (as in Petalonema), the sheath consists of many 

 foliations, one inside the other. In several of the Bivulariece also, the sheath is 

 similarly compound, and frequently plumoso-multifid at the extremity. Within the 

 sheath is the medullary column, or endochromatic part of the filament. This ahvays 

 consists of a series of short, lenticular, densely coloured cells, which in the full grown 



