176 



THE ALG^E 



may be considered a diffused chromatophore. There is no nucleus 

 in the usual sense of the term, although the central region of the 

 cell has a different structure from the outer and probably contains 

 chromatin. The small granules arranged along the cross walls 

 are believed to be food products built up by the activities of 

 the blue-green pigment in sunlight (photosynthesis). Oscillato- 



ria takes its name from 

 the remarkable move- 

 ments of the filaments, 

 whose free ends swing 

 back and forth describ- 

 ing a circle or an ellipse, 

 while the filaments may 

 glide slowly forward. 

 The cause of this move- 

 ment is not understood. 

 Oscillatoria is found in 

 greatest abundance in 

 open drains, ditches, or 

 pools, where the water 

 is foul with decaying 

 organic matter. There 

 it may form thick felts 

 on the bottom, or rise 

 to the surface in slimy 

 masses because of the 

 bubbles of gas, largely 

 oxygen, formed during the processes of photosynthesis and held 

 within the tangle of filaments. 



Andbcena and Nostoc are closely related genera. The fila- 

 ments are chains of round or elliptical cells. Besides the blue- 

 green vegetative cells there are present at intervals curious cells 

 termed Tieterocysts (meaning other cells), which are generally 

 larger than the vegetative cells, lighter in color, and often empty 

 of protoplasm. Their function is not clearly understood. The 



FIG. 174. Filamentous blue-green algse 



A, Oscillatoria; d, dead cell, indicating a point 

 where the filament might break apart ; /, stages 

 of cell fission; J3,Anabsena; h, heterocyst; 

 s, resting cells ; C, Nostoc, habit sketch of a col- 

 ony and the details of a single filament ; h, heter- 

 ocyst; D, Gloeotrichia, portion of a colony and 

 the base of a single filament in detail; h, heter- 

 ocyst; s, resting cell 



