The Genus Oedogonium
Phylum Chlorophyta; sub-phylum Chlorophyceae; Order Oedogoniales; Family Oedogoniaceae
Oedogonium species are unbranched filamentous green algae. There is a dense chloroplast, usually filling the cell, with pyrenoids. Cells frequently are wider at one end than the other; occasionally one finds bulbous, almost globular cells. The chief diagnostic character is the presence of rings at the wider end, which arise as a consequence of cell division, one ring per division which that cell has undergone, which can usually be seen in the filament by careful focussing under favourable lighting (condenser iris shut down). Oedogonium reproduces sexually as well as vegetatively, and the larger female plant may carry a much smaller male plant attached to it.
The zoospores resulting from vegetative reproduction are motile and may often be found swimming around prior to settling on a substratum by means of a holdfast disc (usually quite elaborate) and growing into a filament.
There are a number of species. Mean cell diameter is an unreliable guide to species discrimination as filaments vary in width from one end to the other; however, median diameters of 8, 14, 30 and 45Ám. seem to differentiate taxa in field samples. Some carry a long spike on the terminal cell (calyptra).
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This filament has both a holdfast disc and a calyptra
Formation and germination of zoospores.
These bulbous cells undergo changes in appearance:
the contents darken as a zoospore is formed
The arrow marks a cell which is forming a zoospore
The empty cell on the bottom right has just released the spherical zoospore
(seen in better focus in the second image), while the cell to the left of it is almost ready also.
Within a few seconds of release, the zoospore starts to move
by means of a ring of cilia surrounding a small clear swelling
(at which point it could not be photographed)
After swimming around for a time, the zoospores settle on a surface.
The clear swelling lengthens into an attachment organ (holdfast) and secretes an attachment disc.
The filament grows vegetatively from then on.
This picture shows a zoospore which has settled on an Oedogonium filament.