THALLOPHYTA: ALGAE 



13 



usually not in connection with a heterocyst. In Stigonema a true branch 

 arises as a lateral outgrowth from a single cell of the filament. This type 

 of branching is rare in the Cyanophyceae, occurring in only a few genera. 

 Summary. The Cyanophyceae are an ancient group of plants showing 

 an extremely primitive condition of structural organization. In addition 

 to chlorophyll and carotinoids, a blue pigment (phycocyanin) is present 

 and often a red pigment (phycoerythrin) as well. The plant body is 

 unicellular, the cells nearly always forming colonies. The cell wall is 



A B C 



Fig. 4. False branching in Tolypothrix (A) and Scytonema (B), and true branching in 

 Stigonema (C) ; A, X750; B, X200; C, X300. 



more or less unstable, usually producing abundant mucilage. The 

 protoplast shows little organization. The pigments forming the charac- 

 teristic blue-green color are diffused throughout the peripheral part of the 

 protoplast, no plastids being present. Reserve carbohydrate food is 

 stored as glycogen. A nucleus is represented only by scattered chromatin 

 granules, there being no nuclear membrane or nucleolus. Reproduction 

 occurs by fission and by nonmotile spores. It is entirely asexual. 

 Ciliated cells are never produced. The resting ceUs (akinetes) are 

 merely enlarged protoplasts with a thick wall. There is a tendency 

 toward cellular differentiation, in some forms resulting in the establish- 

 ment of a distinct apex and base. The relationships of the Cyanophyceae 

 to the other algae are obscure. They appear to be closely related to the 

 bacteria. In fact, the blue-green algae and bacteria are sometimes placed 

 in the same group, the Schizophyta, and made an independent class of 

 thallophytes. 



