Spirogyra is a genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...

 of filamentous green algae
Green algae
The green algae are the large group of algae from which the embryophytes emerged. As such, they form a paraphyletic group, although the group including both green algae and embryophytes is monophyletic...

 of the order
Order (biology)
In scientific classification used in biology, the order is# a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family...

The Zygnematales + νήμα , νήματος ), also called the Conjugales, are an order of green algae, comprising several thousand different species in genera such as the well-known Zygnema and Spirogyra. All the members of this group develop into unbranched filaments, one cell thick, which grow longer...

, named for the helical
A helix is a type of smooth space curve, i.e. a curve in three-dimensional space. It has the property that the tangent line at any point makes a constant angle with a fixed line called the axis. Examples of helixes are coil springs and the handrails of spiral staircases. A "filled-in" helix – for...

 or spiral arrangement of the chloroplast
Chloroplasts are organelles found in plant cells and other eukaryotic organisms that conduct photosynthesis. Chloroplasts capture light energy to conserve free energy in the form of ATP and reduce NADP to NADPH through a complex set of processes called photosynthesis.Chloroplasts are green...

s that is diagnostic of the genus. It is commonly found in freshwater
Fresh water is naturally occurring water on the Earth's surface in ice sheets, ice caps, glaciers, bogs, ponds, lakes, rivers and streams, and underground as groundwater in aquifers and underground streams. Fresh water is generally characterized by having low concentrations of dissolved salts and...

 areas, and there are more than 400 species of Spirogyra in the world. Spirogyra measures approximately 10 to 100μm in width and may stretch centimeters long. This particular algal species, commonly found in polluted water, is often referred to as "pond scum".
Spirogyra is unbranched with cellular cells connected end to end in long green filaments.