THE FRESH-WATER ALGAE 1 17 



This usually is surrounded by starch and is the center of reserve 

 material. 



Davis regards the pyrenoid as the center of activity of the 

 chroma tophore. Certain it is that the division of the pyrenoid is 

 the first visible stage in the division of the chromatophore and of 

 the entire cell, and takes place in some cases at least before the 

 division of the nucleus. 



On the basis of the color of the chromatophore of the different 

 forms, together with the mode of reproduction, are formed the chief 

 divisions of the algae. The different classes are as follows and 

 each of them is treated separately in a synoptic key at the place 

 indicated: 



Chlorophyceae, color green, page 134. 



Cyanophyceae, color blue-green, page icxj. 



Phaeophyceae, color brown, page 174. 



Rhodophyceae, color red or purplish green, page 175. 



Bacillariaceae, color yellow, page 125. 



In all cases where the color is other than green there is in the 

 chromatophore a coloring matter which screens the chlorophyll 

 and gives the characteristic hue to members of the group. In 

 the Cyanophyceae the coloring matter is phycocyan; in the Phaeo- 

 phyceae, phycophaein; in the Rhodophyceae, phycoerythrin; and 

 in the Bacillariaceae, diatomin. 



Reproduction in the algae is either sexual or asexual. 



Asexual reproduction may take place either by simple cell 

 division or by the formation of some cell specially modified for 

 that purpose. Cell division may occur in one of two ways: first, 

 by fission, where a membrane is formed across a cell, dividing the 

 original membrane and contents, as in Pleurococcus; second, by 

 internal division, where the contents are simply divided into two, 

 then four, and perhaps eight or more portions, as in Dactylococcus 

 and CMorella. The membrane remains for a time unaltered, but 

 finally becomes ruptured when the daughter cells increase in size, 

 thus setting free the new individuals. They germinate iramedi- 

 ately and each produces a new plant. 



Oblique divisions of cells, so frequently attributed to the algae, 

 rarely if ever occur. They appear to take place very frequently, 



