SCHIZOPHYTA 



457 



Oscillatoria is a filamentous type, which is common on damp walls 

 and rocks. Its pale green filaments show slow swinging movements, 

 hence the name (Fig. 390). They consist of disc-shaped cells, 

 which multiply by division. In some of the larger forms granules 

 of irregular form are found in each cell, giving the staining re- 

 actions of chromatin, and dividing before division of the cell, but 

 without a definite nuclear spindle. The filaments may break up at 

 any point into shorter lengths, and are unattached. In other cases 

 special cells (heterocysts) occur at intervals in the filaments, which 

 appear to determine their breaking up into shorter lengths, as in 



-t 



FIG. 390. 



A , Oscillatoria princeps : a terminal portion of a filament ; b, portions from the 

 middle of a filament, properly fixed and stained; t, cells k in division. ( x 1080). 

 B, Oscillatoria Froelichii. (x54o). S. 



Nostoc or Rivularia. Some of these fission-Algae take part in the 

 formation of Lichens : thus Collema has Nostoc as it's Algal constituent. 

 Others, such as Anabaena, lead an endophytic life, contributing 

 probably to an irregular nutrition, as in the roots of Cycads. In the 

 establishment of the new Flora of the sterilised Island of Krakatoa 

 (p. 295), blue-green Algae were among the first colonists, taking their 

 part in the preparation of an organic soil for larger developments 

 to follow. Certain of these Algae allied to Anabaena often appear 

 suddenly in large quantities on the surface of fresh water, causing the 

 phenomenon known as " water bloom," or the " breaking of the 

 meres." One of these, with a deep red colour (Trichodesmium ery- 

 thraeum), floats in ocean-waters, and becomes prominent when massed 

 together by wind and tidal streams. It has thus attracted attention 

 in various oceans, and has given its name to the " Red Sea." 



