THALLOPHYTA i 313 
of an ordinary nucleus. The complexity of this incipient nu- 
cleus varies greatly in different forms. In the most advanced 
types it is clearly differentiated from the remainder of the 
protoplasm, while in the simplest forms such 
differentiation has not been demonstrated. 
The group as a whole is characterized by 
having the photosynthetic pigment distrib- 
uted in the peripheral zone, and not collected 
in chloroplastids as in the higher plants. 
Another very characteristic feature of the 
Cyanophyceae is the gelatinous coverings that 
surround them. The 
gelatinous coverings 
and the blue-green 
color give the Cyano- 
phyceae their charac- 
teristic appearance, as 
they can usually be 
recognized as slimy, 
blue-green masses. 
In those species in 
which the plant con- 
sists of a single cell or 
pag a colony of cells held 
cell of Spirulina, Pile. 
Me en alee. together by mucilagi- 
(x 925) nous materials all the 
cells are alike, and in 
many of the species in which the cells are 
arranged in the formofchainsthecellsare 3,4 314. ‘Two eee 
alike. In other species there are special Wostoc, each embedded in 
cells called heterocysts (Figs. 315, 316). a gelatinous matrix 
These are formed from ordinary cells, 
usually by increase in size and an almost complete loss of pigment. 
Reproduction. The Cyanophyceae have no method of sexual 
reproduction. The single-celled and colonial forms are repro- 
duced by simple division. In the filamentous species vegetative 

Fig. 813. A single 

