Biology Today — December 2017

ALGAE

Algae are chlorophyllous thallophytes characterised by absence of embryo stage and presence of non-jacketed gametangia with

fertile cells.

• As per Linnaeus’ two kingdom classification, all members of algae were grouped into plant kingdom along with bacteria and fungi.


• As per Whittaker’s classification, the members of algae were grouped in three kingdoms i.e., Monera (blue-green algae),
  Protista (dinoflagellates, diatoms, etc.) and Plantae (Rhodophytes, Phaeophytes and Chlorophytes).

General Characteristics

• Habitat : Algae are of universal occurrence and they are found in a variety of habitats, such as freshwater, sea water, on snow,
  on rocks and on/or within the plant and animal bodies. Of these, aquatic forms are most common.


• Forms : Algae may be unicellular, colonial, filamentous, siphonaceous or parenchymatous.


• The algal plant body is covered by mucilage which protects it from epiphytic growth and decaying effect of water.


• Vascular and mechanical tissues are absent, therefore the body is flexible and easily gets swayed without being torn.


• Nutrition is photosynthetic. Grana are absent in chloroplast. The chloroplast varies in appearance among different algal forms,
  e.g., cup shaped, girdle shaped, reticulate or ribbon shaped, discoid, spiral and stellate.


• Algae contain chlorophyll a, b, c, d, e, carotenes and xanthophylls. Additional pigments like phycobilins, fucoxanthin occur in
  specific groups.


• Pyrenoids are associated with chloroplast for storage of starch. The reserve food may be starch, laminarin, mannitol, oil, fats, etc.,
  which differ among different algal members.


• Vegetative and asexual modes of reproduction are present. Vegetative reproduction may take place by fragmentation
  (e.g., Ulothrix, Oedogonium), fission (e.g.,Chlamydomonas), hormogonia (e.g., Oscillatoria), tubers (e.g., Chara), budding
  (e.g., Protosiphon), etc.


• Asexual reproduction takes place by flagellated zoospores (e.g., Ulothrix, Oedogonium); non-motile, thin walled aplanospore
  (e.g., Microspora) and non-motile, thick walled hypnospores (e.g., Pediastrum, Chlamydomonas nivalis), thick walled akinetes
  (e.g., Cladophora), palmella stage (e.g., Chlamydomonas).


• Under favourable conditions, algae show sexual reproduction which may be isogamous or heterogamous (i.e., anisogamous and
  oogamous). Sex organs are non-jacketed and one celled called gametangia.


• Life cycle may be haplontic, diplontic, haplobiontic and diplobiontic.

Glomerule

Main axis

Fig.: Batrachospermum

Lateral branch

(of unlimited

growth)

Erect system

Plurilocular

sporangium

Unilocular sporangium

Rhizoid

Fig.: Ectocarpus Fig.: Chlamydomonas

Flagellum

Flagellar canal

Mucilage

Vacuole

Contractile

vacuole

Volutin

granules

Plasma

membrane

Starch sheath

Pyrenoid

Cell wall

Nucleus

Chloroplast

Centrosome

Eye spot

Basal granule

Apical papilla

Prostate system