154 Plant Biology 



tlon is by cell division (fission). Sometimes, soft gelatinous areas develop 

 between cells, thereby breaking the filament into pieces known as hor- 

 mogonia (hor mo -go' nia). (Gr. hormos, chain; gonos, offspring). A 

 hormogonium may form a new colony. Oscillatoria is common on damp 

 earth, stones, flower pots, and other damp places. 



Nostoc (nos' tok) (F. nostos, return). — This blue-green alga is uni- 

 cellular, with the individual, globose cells arranged as a chainlike colony, 

 resembling a necklace of beads (Fig. 29). The strands of cells are en- 

 closed in a hall of jelly. Each cell contains chlorophyll, phycocyanin, 

 and chromatin granules as in Gleocapsa and Oscillatoria. At certain 

 intervals in the chain are thick-walled, transparent cells known as het- 

 erocysts (het' er o sists) (Gr. heteros, different; kystis, sac or pouch) 

 which serve to break the filaments into hormogonia, as in Oscillatoria. 



