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Algae: Intro. Colonial Diatoms Filamentous Desmids Motile

  The Filamentous Algae.

Filamentous Algae: Page 1. Page 2

This gallery includes only the filamentous green algae. The group is a heterogeneous one in which the members, although superficially similar, show a wide diversity in their life cycle and modes of reproduction. Spirogyra, Oedogonium and Cladophora are amongst the varieties most frequently encountered.
All blue-green algae are now classified amongst the Bacteria, and will be found in the Cyanobacteria gallery.

Spirogyra is a filamentous green alga which is common in freshwater habitats. It has the appearance of very fine bright dark-green filaments moving gently with the currents in the water, and is slimy to the touch when attempts are made to collect it. The slime serves to deter creatures which otherwise attatch themselves to underwater plants, so Spirogyra under the microscope is usually spotless.

Spirogyra. A field of Spirogyra filaments. Their appearance is not quite typical in that the nuclei are unusually prominent, and the characteristic spiral chloroplasts are so fine and tightly wound that close examination is needed to confirm the identification. In any case the possession of spiral chloroplasts is sufficient to positively identify Spirogyra to genus.
Darkfield, x120.
Spirogyra: cell showing nucleus. The central portion of a cell of Spirogyra showing the nucleus and giving an insight into the way the spiral chloroplast contacts with the wall of the cell. The filament in the background provides another view.
Brightfield. x1000.
Spirogyra: showing nucleus. Central portion of a Spirogyra cell showing nucleus and chloroplasts.
Brightfield, x1000.
Spirogyra: filament about to break. This filament of Spirogyra is about to break into two filaments. The wall of each cell (centre of picture) has developed an inward indentation at the junction between the cells. Increase in pressure in each cell will cause the indentation to pop out, forcing separation of the filaments, and leaving them with highly convex ends.
Brightfield. x1000.
Spirogyra: showing nucleus. Two filaments of Spirogyra, the lower one clearly showing the nucleus. This picture also gives a good insight into the way the chloroplasts line the wall of the cell.
Brightfield. x1000.

  Conjugation in Spirogyra.

In common with other members of its phylum (Gamophyta) Spirogyra lacks a motile variant at all stages of its life history; ie, no motile gametes (ova or sperm), no zoospores etc. Sexual reproduction is by a process called conjugation -- another of the famously remarkable sights available to the microscopist.
Although it is not possible to distinguish them visually, certain filaments in a loose parallel bundle of Spirogyra assume the female, and others the male, role in the process which follows. The cells of adjacent filaments develop bumps which grow towards one another and eventually fuse to form a continuous tube between the cells. Meanwhile the contents of each cell have detatched themselves from their respective cell walls and have formed a round ball. Over a relatively short space of time (minutes), the green spheres from the male filament squeeze their way down the connecting tubes to fuse with a similarly contracted female cell in the other filament. The result of this sexual union is the formation of a zygospore with a tough resistant outer covering within the chambers of the female filament. After a dormant period, these zygotes undergo meiosis and germinate, resulting in new filaments of Spirogyra.
Once seen never forgotten.

Click for a diagram showing conjugation in Spirogyra crassa.

Spirogyra: conjugation. This picture shows two stages in Spirogyra conjugation. The central pair of cells are joined by a conjugation tube which has yet to fuse into form a continuous passage. The cell contents are at a similarly early stage of detatching themselves from the cell wall to form a ball.
By contrast, the two cells to the right contain newly formed zygospores as a result of consummated conjugation.
Brightfield, x500.
Spirogyra: conjugation beginning. A detail of the above specimen at higher magnification.
Brightfield, x1000.
Spirogyra: zygospore formation. This picture shows the contents of three male cells about to migrate down the conjugation tubes joining them to the female filament below. The cell third from the left is already half way down the tube. The process was completed in a minute or so.
Darkfield, x300.
Spirogyra: zygospore formation. Male and female cells now occupy the same space, and are pictured before fusion to form a zygospore has taken place. The filament designated female is the one in which the zygospores have formed.
Darkfield, x300.
Spirogyra: zygospores. Two mature zygospores of Spirogyra from another part of the specimen which provided the above pictures. In this form, Spirogyra can survive winter or other adverse conditions and germinate in the spring to form new filaments. The hardened outer spore wall can be seen reflecting the light from the darkfield condenser.
Darkfield. x400.
Spirogyra: zygospore. A zygospore of Spirogyra against a background of decaying plant remains and other algal forms.
Darkfield, x400.
Spirogyra: recent zygospore. It's hard to say what is happening here. It looks like the stage in conjugation of Spirogyra in which the contraction of the cell contents to a ball is not quite complete, and the spiral nature of the chloroplast is still discernable.
Darkfield, x1000.

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