V*a 



102 FRESH-WATER BIOLOGY 



longer-celled forms, such as Nostoc and Tolypothrix, the small 

 granules generally occur abundantly in all parts of the peripheral 

 protoplasm. These minute granules are generally regarded as the 

 "cyanophycin granules" (Borzi), and they are probably albumi- 

 nous in their nature and serve as reserve food. The few larger 

 granules mentioned above are more hyaline and transparent than 

 the cyanophycin granules, and they appear to lie in or near the 

 center of the cell. These larger granules have been called by 

 Palla "slime globules"; by Zacharias "Centralkorner." Their 

 function is in dispute. 



The cells of favorable forms of the blue-green algae, e.g., Oscil- 

 latoria, show two more or less evident portions of the protoplasm 

 a peripheral layer, to which the pigment is confined and in which 

 the cyanophycin granules lie, and a central colorless part, the 

 so-called " central body." The majority of recent studies on the 

 subject maintain that the central body is a nucleus, although this 

 conclusion has been several times disputed. Carefully stained, 

 thin sections show, however, that it is made up of both chromatic 

 and achromatic substances. Moreover, Macallum and others find 

 in the central body complex proteid substances containing phos- 

 phorus and ''masked" iron to a marked degree, which they regard 

 as characteristic constituents of chromatin. Fischer claims, how- 

 ever, to have demonstrated by means of a tannin-safranin stain 

 that the central body is filled with certain carbohydrates, of the 

 nature of glycogen. 



The central body divides according to some, by simple amitotic 

 division; while others believe that the division is mi to tic. At any 

 rate, the division of this nucleus, or central body, precedes the 

 division of the cell, and, as in other lower plants, the two processes 

 appear to take place independently of each other. Cell division is 

 accomplished in these forms in the same manner as has been 

 described for many other filamentous Thallophytes, by constric- 

 tion: a ring-formed wall grows in from the outer wall, similarly to 

 the closing of an iris diaphragm, and finally cuts the cell in two. 



The varying shades of color shown by these organisms are caused 

 by varying mixtures of the green chlorophyll and the reddish or 

 bluish phycocyanin, the pigments being apparently confined to 



