THE GREAT GROUPS OF 



19 



two heterocysts. The fragments wriggle out of the jelly 

 matrix and start new colonies of chains, each cell dividing 

 to increase the length of the chain. This cell division, 

 to form new cells, is the characteristic method of repro- 

 duction. 



At the approach of unfavorable conditions certain cells 

 of the chain become thick- walled and well-protected. These 

 cells which endure the cold or other hardships, and upon 

 the return of favorable conditions produce new chains of 

 cells, are often called spores, but they are better called 

 " resting cells." 



20. Oscillatoria. These forms are found as bluish-green 

 slippery masses on wet rocks, or on damp soil, or freely 

 floating. They are simple filaments, composed of very short 

 flattened cells (Fig. 5), and the name 

 Oscillatoria refers to the fact that they 

 exhibit a peculiar oscillating move- 

 ment. These motile filaments are iso- 

 lated, not being held together in a 

 jelly-like matrix as are the chains of 

 Nostoc, but the wall develops a cer- 

 tain amount of mucilage, which gives 

 the slippery feeling and sometimes 

 forms a thin mucilaginous sheath 

 about the row of cells. 



The cells of a filament are all alike, 

 except that the terminal cell has its 

 free surface rounded. If a filament 

 breaks and a new cell surface ex- 

 posed, it at once becomes rounded. 

 If a single cell of the filament is 

 freed from all the rest, both flattened ends become rounded, 

 and the cell becomes spherical or nearly so. These facts 

 indicate at least two important things : (1) that the cell 

 wall is elastic, so that it can be made to change its form, 

 and (2) that it is pressed upon from within, so that if free 



FIG. 5. Oscillatoria, a 

 blue-green alga, show- 

 ing a group of filaments 

 (J.), and a single fila- 

 ment more enlarged (B). 

 CALDWKLL. 



