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Phylum Chlorophyta; sub-phylum Chlorophyceae; Order Zygnematales; Family Zygnemataceae
Spirogyra species are unbranched filamentous green algae. The cell wall is characteristically straight and parallel-sided (compare with Oedogonium, Microspora or Stigeoclonium , which sometimes have bulbous cells).
They have a single chloroplast in the form of a spiral ribbon which usually almost fills the length of the cell and is the most striking characteristic of this genus.
There are several pyrenoids which are starch containing bodies and can be revealed by staining with Lugol’s iodine (the starch turns black).
Spirogyra are members of the Zygnematales or Conjugales, which as the name suggests enjoy sexual reproduction; neighbouring filaments send out processes which fuse into a tube, then the cell contents pass into the conjugation tube and form the zygospore.
The appearance of the zygospore is used to identify to species, but this characteristic is rarely found in field material.
Many species show scalariform conjugation; many adjoining cells in the two filaments form conjugation tubes at the same time, giving the appearance of a ladder. Other common members of the Conjugales are Zygnema and Mougeotia.
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