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BOTANY: PRINCIPLES AND PROBLEMS 



sometimes those of the third and fourth, are held together, 

 embedded in their swollen cell-walls. In Nostoc (Figs. 141, C 

 and D) the individuals are joined loosely into filaments which 

 somewhat resemble strings of beads, and these may also be 

 embedded in a jelly-like substance, the whole colony often reaching 

 a diameter of one centimeter and containing hundreds of filaments. 



Fig. 141. — Cyanophyceae of various sorts. ^4, Oscillatoria, X 340. B, 

 Gloeocapsa. X 450. C, iVosioc colony, natural size. Z), filament of iVostoc, with 

 a heterocyst in the middle of it. X 460. E, colony of Rivularia. X M- -P. 

 filament of Rivularia, with heterocyst at base. X 225. {E and F after Engler 

 and Prantl) . 



Here and there along the filament are frequently found large, 

 empty cells called heterocysts. Their function is not definitely 

 known but they may be concerned in breaking up the filament 

 into short segments or hormogonia. In Oscillatoria (Fig. 141, A) 

 the cells are pressed flatly against one another and the gelatinous 

 wall is so poorly developed that the filaments are free in the water, 

 where they sway or revolve slowly. It is hard to know whether 

 to regard the filament of some of these blue-green algae as a 

 colony of distinct individuals or as a single, many-celled plant. 

 Chlorophyceae or Green Algae. — This class is by far the largest 

 of the four groups of algae and its members are very diverse. 

 They contain chlorophyll but no other pigment, and the bright 



