Division : Algae
Algae are mostly aquatic, some being terrestrial also. A few algae grow on other plants. Aquatic algae grow in marine water or fresh water. Most of them are free living while some are symbiotic.
The vegetative structure (thallus) of algae display variety in organisation and size. They may be small, unicellular, microscopic like Chlorella (non-motile), Chlamydomonas (motile) or multicellular unbranched filaments like Spirogyra or branched filamentous like Chara or huge macroscopic such as sea weeds like Sargassum measure more than 60 meters in length.
The algal cell wall consists of two layers i.e. inner cellulosic and outer composed of pectin. The algae consist of various types of photosynthetic pigments. Chlorophyll-a (essential pigment) is present in all groups of algae. The accessory pigment present are chlorophyll-b, chlorophyll-c and chlorophyll-d, carotenes, xanthophylls and phycobilins. Phycobilins are of two types i.e. phycocybilins and phycoerythrins. The reserve food material is in the form of starch, laminarin - starch, mannutol or floridean-starch, etc.
Algae reproduce by three different methods. Vegetative reproduction occurs by fragmentation (in filamentous forms) and cell division (in unicellular forms). Asexual reproduction occurs by the formation of various types of non-motile or motile spores. Sexual reproduction takes place by formation and fusion of gametes. The life cycle of algae exhibits the phenomenon of alternation of generations. (There are two phases; haploid and diploid).
Depending upon the predominant photosynthetic pigments, algae are divided into following major groups.
Chlorophyceae ( Green Algae) :
Their predominant photosynthetic pigments are chlorophyll-a and chlorophyll-b. Reserve food is stored in the form of starch. The cell wall is chiefly composed of cellulose. The motile cells possess 2-8, equal-sized, apical flagella. They are found mostly in fresh water and sometimes in marine and brackish water.
Examples :- Chlamydomonas, Spirogyra, Chara, etc.
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| Chlamydomonas, Spirogyra, Chara |
Phaeophyceae (Brown Algae) :
The principal photosynthetic pigments include chlorophyll-a, chlorophyll-c and fucoxanthin. Mannitol and laminarin are reserved food. Cellulose is associated with algin in cell wall. The motile spores are biflagellated with two unequal, lateral flagella. These algae commonly occur in marine water and brackish water and some are seen in fresh water.
Examples - Ectocarpus, Sargassum, Fucus, etc. (Many species of marine algae are used as food)
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| Sargassum |
Rhodophyceae (Red Algae) :
These algae contain chlorophyll-a, chlorophyll-d and phycoerythrin as chief pigments of photosynthesis. Reserve food is as special kind of starch called floridean starch. The cell wall has pectin in addition to cellulose and other carbohydrates. Motile cells are totally absent. They are usual inhabitants of marine water and brackish water, but occur rarely in fresh water.
Examples : Chondrus, Gelidium, Polysiphonia, etc. Commercially important important 'agar', which is used as solidifying agent in tissue culture is obtained from red algae.
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| Gelidium,Chondrus |