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Cyanophyta

The   prokaryotic   division containing the blue-green *algae. It contains    unicellular,    colonial,    and    filamentous forms, many of which are surrounded    by    a    mucilaginous    sheath, hence   the   older   name   Myxophyceae. Sexual  stages  and  flagellate  forms  are unknown. The predominant pigment is phycocyanin,    which    confers    a   wide range of different colours depending on species and environment. Blue-green algae are abundant in fresh water and the soil; they also occur in the marine littoral zone, in hot springs, as symbionts in lichens and root nodules, and as parasites in certain plants. The   nuclear   and   plastid   material   of-blue-greens is not bound by membranes, chromosomes are lacking, and the pigments are dispersed in a primitive chro-matophore. Some can also fix nitrogen. These characteristics have led many to classify   them  with   the  bacteria  under various   names,   such   as   Cyanobacteria and Schizophyceae. However they differ from bacteria in their lack of motility and lack of fermentative activity. The presence or absence of *heterocysts and  hormogonia and the tendency of cells to aggregate into distinct filaments are the main features by which orders are recognized. The *Chroococcales and the *Hormogonales are the most important orders.

 
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