FILAMENTOUS BLUE GREENS 165 



ORDER HORMOGONALES. FILAMENTOUS BLUE 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 Oscillator iaceae, 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. 



FIG. 61. Oscilla- Fia. 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. 



