Oedogoniales

(botany) An order of fresh-water algae in the division Chlorophyta; characterized as branched or unbranched microscopic filaments with a basal holdfast cell.

An order of filamentous fresh-water green algae (Chlorophyceae) with unique morphological features including (1) an elaborate method of cell division that results in the accumulation of apical caps, (2) zoospores and antherozoids with a subapical crown of flagella, and (3) a highly specialized type of oogamy. There is a single family, Oedogoniaceae, comprising three genera. See also Chlorophyceae.

Oedogonium has the largest number of species (several hundred) and is the most common of the three genera. Its unbranched filaments are initially attached by a holdfast cell to submerged vegetation, stones, or wood, usually in permanent ponds or pools, but at maturity they may form free-floating masses. The cells are cylindrical, each containing a reticulate chloroplast with numerous pyrenoids.

Vegetative multiplication by fragmentation is common. In asexual reproduction, zoospores with a subapical crown of up to 120 flagella are formed singly within a cell. In sexual reproduction, there is a highly specialized interplay between female and male elements. The egg is a metamorphosed protoplast of an enlarged spherical cell (oogonium). Antherozoids, with a subapical crown of about 30 flagella, are produced in pairs or tetrads in very small discoid antheridia. The two types of sex organs may occur on the same filament (homothallic species), or on different filaments (heterothallic species), but in either case certain species have an indirect development of antherozoids. These species, termed nannandrous in distinction to those with direct development (macrandrous), form short cylindrical cells which initially appear like antheridia, but in which the protoplast metamorphoses into a single swarmer bearing a subapical crown of flagella.