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Algae: Intro. Colonial Diatoms Filamentous Desmids Motile

  The Filamentous Algae (continued).

Filamentous Algae: Page 1. Page 2

  Oedogonium:  Zoospore Formation.

The green filamentous alga Oedogonium is something of an oddity amongst the members of the green algae (Chlorophycaea) for the unique and complex nature of its sexual and asexual modes of reproduction. It reproduces asexually in two ways.

The vegetative process involves the cell division of special "cap cells" resulting in the elongation of the filament. The other asexual process involves the production of motile zoospores and is shown in the pictures below.

Click for a
diagram of Oedogonium.

Oedogonium 1.

Oedogonium 2.

Oedogonium 3.
Picture captions anticlockwise from upper left.
Magnification for all (Brightfield) pictures is x800.
  1. Two adjacent cells of a filament of Oedogonium are seen in which the cell contents, including chloroplasts, have contracted from the walls of the cell and are forming a globular mass in the centre of the cell which will become a zoospore.
  2. When the filament breaks, the zoospore makes its way towards the open end and begins to emerge.
  3. A more advanced stage of emergence.
    The process takes a minute or so.
  4. A completely emerged zoospore.
  5. In this picture the zoospore has elongated a little, and the clear "eyespot" surrounded by large flagellae (undulipodia) is clearly seen.
  6. The undulipodia are less clearly seen in this shot as the zoospore swims freely in search of a suitable place to settle.
  7. The transparent "eyespot" has extended in an amoeboid fashion to find a suitable attatchment point.
  8. A firmly attatched zoospore, elongated and about to become a new filament by the vegetative asexual process of cell division.
Oedogonium 4. Oedogonium 5. Oedogonium 6. Oedogonium 7. Oedogonium 8.

Oedogonium: Settled zoospores. A couple of settled Oedogonium zoospores in a mini-jungle of other filamentous algae.
Darkfield, x400.
Oedogonium: Settled zoospore. An Oedogonium zoospore attatched to a rootlet of duckweed (Lemna).
Darkfield, x400.
Oedogonium: Settled zoospore. A newly settled Oedogonium zoospore seen in profile amongst filaments of algae. It has attatched to an older Oedogonium filament, and the point of attatchment will rapidly become brown due to absorption of iron from the water.
Darkfield, x400.
Oedogonium: Newly formed cell. This filament of Oedogonium has undergone a recent cell division. The newly formed cell has not had time to acquire the incrustation of iron-coloured organic matter seen covering the cells on either side, and shines like a lantern in the darkfield illumination.
Darkfield, x400.

  Cladophora and Microspora.

The filamentous alga Cladophora is a common inhabitatant of freshwater locations. It is called blanket weed in some places -- not an inappropriate name when in late summer dense floating rafts of Cladophora can be found both at the pond's edge and in the open water, buoyed up with the oxygen generated by its own photosynthesis.

Unlike Spirogyra, Cladophora is capable of branching, and seems to produce little or no mucilagineous secretion. This, and the fact that salts tend to crystallize on the filaments of older specimens, gives it a rougher, grittier feel than other filamentous algae. It is also more readily colonized by epiphytic diatoms and other algae, and provides a protected foraging environment for the smaller pond creatures such as protozoa, worms, small crustaceans and insect larvae.

Its springiness also makes it more difficult to prepare the thin, flat specimens required by the microscope.

Branching Cladophora. Branching in this filament of Cladophora has begun with an outgrowth of the cell at the upper end near the cell wall junction. As the branch grows, differential growth of the main cell wall causes the branch to grow forwards rather than at right angles to the original cell.

An interesting feature of the picture is the distribution of plastids in the two cells shown. Since the plastids are the energy converters of the cell, large numbers have migrated into the growing branch, where the energy requirement is greatest.
The cell on the right shows a distribution of plastids normal to a resting cell.
Darkfield, x300.
Cladophora encrusted. Picture shows Cladophora at a branching point. The filaments are encrusted with diatoms (Gomphonema) and crystals of calcium carbonate which give the plant its rough gritty feel.
Darkfield, x400.
Microspora filament. Microspora is common in ponds, especially in the winter months. It can be recognized by its reticulated chloroplast which covers the inside wall of the cell including the cell walls between one cell and the next.
Darkfield, x600.

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