Oedogonium

Oedogonium (Class Chlorophyceae, Order Oedogoniales) is a freshwater, filamentous green alga found attached to rocks in ponds, lakes and still water; some species are epiphytic on other aquatic plants, and O. terrestre even grows on moist soil. Young filaments stay attached, but older filaments often break free and float.

The thallus is an unbranched filament of cylindrical cells. The basal cell is colourless and modified into a hold-fast with finger-like projections that grip the substratum; the distal end of certain cells carries ring-like markings called apical caps, so those cells are termed cap cells — the single most diagnostic feature of the genus. Each vegetative cell has a cellulose inner wall and a pectin outer wall (with a thin chitin layer above the pectin), a large vacuole, and a reticulate (net-like) chloroplast that runs the length of the cell, containing a single nucleus and numerous pyrenoids.

Oedogonium reproduces vegetatively (fragmentation, akinetes), asexually and sexually. In asexual reproduction a vegetative cell functions as a zoosporangium and normally releases one large zoospore whose colourless, beak-like anterior end bears a ring of short flagella — a flagellar arrangement unique to this genus called stephanokont. The zoospore swims off, settles, and divides so its lower cell becomes a new hold-fast while its upper (green) cell divides repeatedly to build a new filament. Sexual reproduction is oogamous, with the male organ (antheridium) and female organ (oogonium) distributed in one of two patterns: macrandrous species have full-sized male filaments, either monoecious (antheridia and oogonia on the same filament, e.g. O. fragile) or dioecious (on separate filaments, e.g. O. crassum); nannandrous species instead produce antheridia on tiny, specialised 2–4-celled dwarf males, themselves grown from androspores released by an androsporangium — if the androsporangium and oogonium sit on the same filament this is called gynandrosporous (O. concatenatum), and if on different filaments, idioandrosporous (O. conferatum). The antheridium releases multiflagellate antherozoids that are chemot …