FILAMENTOUS BLUE GREENS 165 
ORDER HORMOGONALES. Fiamentous Bive Greens 
221. These plants consist of simple or branched rows 
(filaments) of cells, which are usually enclosed in a 
sheath. There are half a dozen families, the lowest of 
which is Oscillatoriaceae, with cylindrical filaments of 
uniform cells. There are many genera, as Microcoleus, 
Lyngbya, Spirulina, Oscillatoria, etc., which occur in 
quiet waters. Oscillatoria and Spirulina are interesting 
because of their marked motility. 
222. The Nostocs (Family Nostocaceae) are filamen- 
tous with more or less spherical cells, some of which 
are larger (rarely smaller) than the others and have 
thickened, cellulose walls (heterocysts). Spores are 
common as larger, denser cells which serve to carry the 
species through adverse conditions. The genera Nostoc. 
Anabaena, and Cylindrospermum are common. ‘ 
Jer 
Fic. 61.—Oscilla- Fig. 62.—Scytonema and 
toria and Nostoc. Rivularia. 
223. The Scytonemas (Family Scytonemataceae) have 
cylindrical (often branched) filaments which contain 
heterocysts also. Scytonema and Tolypothrix are 
common genera. 
224. The Rivularias (Family Rivulariaceae) are taper- 
ing filaments with a heterocyst at the base. They 
usually occur in jelly-like masses. The principal genus 
is Rivularia. 
225. The Stigonemas (Family Stigonemataceae), while 
filamentous, have their larger filaments composed of more 
than one row of cells. Haplosiphon and Stigonema are 
common genera. 
