ALGJE 



365 



are also able to live independently. Such a chain of cells 

 as we find in Oscillatoria is called a filament. The filaments 

 of this plant have the peculiar power 

 of rotating and of oscillating from side 

 to side. 



C. Many-celled Fresh-water Forms. 

 - The largest many-celled forms are 

 some of the seaweeds, but just now we 

 shall consider only those which grow in 

 fresh water. In these the individual 

 cells do not exist independently; ex- 

 cept temporarily as spores. The in- 

 dividual plant is composed of a group 

 of cells, usually in the form of a fila- 

 ment. Probably you have noticed such 

 filaments. They are green, thready 

 growths which are common in brooks 

 and ponds and lakes. In brooks you may have seen them 

 fastened to rocks with their long green threads spread out 

 in the water. They make the rocks slippery. In ponds 

 you have seen them floating on the surface and under the 

 water. When they are not attached to sticks or stones, 

 the bubbles of gas they give off often lift them to the sur- 

 face. Perhaps you have noticed these bubbles in the 

 light-green pond scum which appears on stagnant water. 

 Unattractive though this scum may be to the naked eye, 

 under a microscope it is a thing of beauty. It is made up 

 of thousands of filaments whose chloroplasts have graceful 

 and symmetrical forms. (See Figure 168.} 



These many-celled algae have cells of more than one 

 kind, especially in connection with reproduction. Dif- 



FiG. 163. Oscillatoria. 

 Portions of three fila- 

 ments very highly mag- 

 nified. 



