96 
0SCILLAT0RIACEA2. 
Order IX.— OSCXLLATORIACE.E. 
Harv. Man. Ed. 1. p. 219. Oscillatoriece, Harv. in Mach. FI. Hib. part 3, p. 164. 
Endl. 3d. Suppl. p. 12. Oscillatoriece and Rivulariece , Harv. Br. FI. J. Ag. Alg. 
Medit. p. 8,10. Oscillatorece, Lindl. Veg. Kingd. p. 18. Oscillariece, Leptotrichiece, 
Lyngbyece , Scytonemece, Mastichotrichece , Rivulariece, Kiitz. Sp. Alg. pp. 235-344. 
Diagnosis. Green, (rarely olive-brown, blue, or purple) marine or fresh water Algm, 
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. Root either a simple point of attachment, or, in most cases, 
not obvious. Filaments 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 
wool. 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 in 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 Rivulariece 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 always 
consists of a series of short, lenticular, densely coloured cells, which in the full grown 
