Spirogyra: Freshwater Green Algae

Spirogyra

• Spirogyra is a genus of filamentous green algae that belongs to the division Chlorophyta.
• It is commonly found in freshwater environments around the world.
• Spirogyras are unicellular organisms that grow in long, unbranched chains of cylindrical cells.
• They have helical chloroplasts and unbranched filaments, playing a significant role in aquatic ecosystems.

Structure

• Spirogyra exhibits a unique structure composed of unbranched filaments, each consisting of a single row of cylindrical cells.
• Each cell is enclosed by a thin cell wall composed of cellulose and pectin, providing support and protection.
• A large central vacuole occupies most of the space, maintaining cellular turgor and buoyancy.
• Embedded in the cytoplasm is a prominent nucleus, responsible for genetic control.
• Numerous spiral chloroplasts are present, containing chlorophyll, the pigment responsible for photosynthesis.

Life History

• Asexual reproduction occurs through fragmentation, where portions of the filament break off and develop into new individuals.
• Sexual reproduction occurs through conjugation, involving the fusion of two compatible filaments.
• During conjugation, cells from adjacent filaments form conjugation tubes, allowing the contents of one cell to transfer into the other, forming a diploid zygote.
• The zygote undergoes meiosis, producing haploid zoospores.
• Gemmae are produced inside gemma cups, which are asexual buds that are green and multicellular.

Bryophytes

• Bryophyta refers to embryophytes, which are land plants, especially the non-vascular ones.
• This division includes mosses (class Bryopsida), liverworts (class Marchantiopsida), and hornworts (class Anthocerotopsida).
• The only prime feature of a bryophyte is that it does not have true vascular tissue.
• Bryophytes are believed to evolve from charophytes and are considered to have been the first true plants to have ever evolved.

Bryopsida (Mosses)

• Bryopsida is the largest class of Bryophyta, with around 1400 species.
• Examples of mosses include Funaria, Polytrichum, and Sphagnum.
• Bryopsida is further divided into 5 orders: Bryales, Andreales, Sphagnales, Polytrichales, and Buxbaumiales.
• The main features of Bryopsida include:
  • Gametophyte is differentiated into protonema and foliose gametophore.
  • Foliose is made up of stem as an axis and leaves without midrib.
  • Rhizoids are multicellular with oblique septa.
  • Sex organs are borne apically on stem.
  • Elaters are absent.
  • The sporophyte is differentiated into foot, seta, and capsule.
  • Sporogenous tissues develop from endothecium.
  • Columella is present.
  • Dehiscence of the capsule takes place by separation of the lid.

Hepaticopsida (Liverworts)

• Liverworts come under this class.
• Hepaticopsida is further divided into 4 orders: Marchantiales, Sphaerocarpales, Calobryales, and Jungermanniales.
• The main characteristics of the class Hepaticopsida are:
  • Gametophyte plant is either thalloid or foliose.
  • In foliose forms, leaves are without midrib and dorsiventral.
  • Thalloid is dorsiventral, lobed, and dichotomously branched.
  • Each cell of thallus contains many chloroplasts without pyrenoids.
  • Rhizoids are unicellular, branched, and aseptate.
  • Sex organs are borne dorsally embedded in gametophytic tissues.
  • The sporophyte is made up of only capsule (in Riccia) or foot, seta, and capsule (in Marchantia).
  • The columella is absent in the capsule.
  • Sporogenous tissues develop from endothecium.
  • Asexual reproduction takes place by fragmentation or by the formation of gemmae.