THE BLUE-GREEN ALG& 



233 



Fig. 192. 



A, Oscillatoria princeps: a, terminal cell; b, c, portions 

 from the middle of a filament. In c, a dead cell is shown 

 between the living cells. B, Oscillatoria froelichii: b, with 

 granules along the partition walls. 



375. Oscillatoria. Oscillatoria is one of the filamentous 

 forms, and some of the species are very common. They grow in 

 small pools or in large bodies of rather shallow water. They 

 often grow on the soil at 



the bottom, forming dis- 

 tinct films or sheets 

 which are lifted by the 

 bubbles of oxygen, as a 

 result of photosynthesis, 

 which are entangled in 

 the meshes of the film, 

 and then float as a scum 

 on the surface of the 

 water. The threads are 

 made up of thin disk- 

 like cells which are 

 broader than long. 

 Near the cross walls are seen distinct but small granules which are 

 supposed to be reserve food products. The threads of Oscilla- 

 toria exhibit a peculiar motion; they sway or oscillate back and 

 forth slowly, something like a pendulum. This peculiar motion 

 of the threads suggested the name Oscillatoria. They also glide 

 slowly in the direction of the axis of the thread. The division of 

 the cells is a transverse fission. The threads multiply by break- 

 ing up into shorter threads. The threads are surrounded by a 

 thin gelatinous layer, in nature similar to that of the cell wall. 

 In some related genera the threads are enclosed in a thick gelati- 

 nous layer. 



376. Nostoc. The Nostoc plants occur on damp soil or wet 

 rocks in thick gelatinous masses or nodules, sometimes quite firm. 

 The cells are rounded and connected like a string of beads. 

 Numbers of these bead-like chains are scattered through the jelly- 

 like mass. Here and there in the chain are larger colorless cells.* 

 Division of the cells and multiplication of the threads takes place 

 much as in Oscillatoria. A related plant is Anabcena, which is 



* Called heterocysts, which means other cells, or other kind of cells. 



