Spirogyra

Spirogyra is a genus of filamentous green algae of the order Zygnematales, named for the helical or spiral arrangement of the chloroplasts that is diagnostic of the genus. It is commonly found in freshwater areas, and there are more than 400 species of Spirogyra in the world.^{[citation needed]} Spirogyra measures to around 10 to 100μm in width and may stretch centimeters long.

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General Characteristics

Spirogyra is unbranched with rectangular cells connected end to end in long filaments. The cell wall has two layers: the outer wall is composed of cellulose while the inner wall is of pectin. The cytoplasm forms a thin lining between the cell wall and the large vacuole it surrounds. Chloroplasts are embedded in the cytoplasm; their numbers are variable. The chloroplasts are ribbon shaped, serrated, and spirally arranged, resulting in the prominent and characteristic green spiral on each filament. Each chloroplast contains several pyrenoids, centers for the production of starches, appearing as small round bodies.

Spirogyra is very common in relatively clean eutrophic water, developing slimy filamentous green masses. In spring Spirogyra grows under water, but when there is enough sunlight and warmth they produce large amounts of oxygen, adhering as bubbles between the tangled filaments. The filamentous masses come to the surface and become visible as slimy green mats. Mougeotia and Zygnema are often found tangled together with Spirogyra.

Identification of particular Spirogyra species is accomplished by microscopic examination of the spores.

Reproduction

  Spirogyra can reproduce both asexually and sexually. In asexual reproduction, fragmentation takes place, and Spirogyra simply undergos mitosis to form new filaments.

Sexual Reproduction is of two types:

1. Scalariform conjugation requires association of two different filaments lined side by side either partially or throughout their length. One cell each from opposite lined filaments emits tubular protuberances known as conjugation tubes, which grow toward each other. The protuberances then elongate and unite. Walls at the point of contact dissolve to form a passage between the two cells, called the conjugation canal. The gametangia form male and female gametes. The male gamete travels through the conjugation canal by amoeboid movement and fuses with the female gamete to form a zygote/zygospore. Shortly after the zygote forms, the filaments undergo meiosis.
2. In lateral conjugation, gametes are formed in a single filament. Two adjoining cells near the common transverse wall give out protuberances known as conjugation tubes, which further form the conjugation canal upon contact. The gametangia form gametes, and the male gamete migrates through the conjugation canal, fusing with the female gamete. The rest of the process proceeds as in scalariform conjugation.

The essential difference is that scalariform conjugation occurs between two filaments and lateral conjugation occurs between two adjacent cells on the same filament.

References

John Whitton, B.A. and Brook, A.J. (editors) 2002. The Freshwater Algal Flora of the British Isles. Cambridge University Press, Cambridge. ISBN 0 521 770513.