Sub-division :- Algae

Class :- Bacillariopbyceae

Order 1 :- Centrales

Order 2 :- Pennales

The diatoms are cosmopolitan in distribution being present in almost all the habitats. These constitute major part of the freshwater and marine planktons. In sea-water diatoms form bottom flora. Besides these habitats, diatoms may occur on soil, rock cliffs, bark of the trees, etc.

Diatom, A-B Pinnularia, A. Girdle view, B. Valve view of a frustule.



  • Organisms are unicellular.
  • The thallus mostly occurs singly or cells may be united in colonies.
  • A diatom cell is known as a frustule. 
  • The wall is composed of two overlapping halves. The older part (valve) is known as epitheca which fits closely over the younger part (valve)-hypotheca. 
  • The silicified parts of each valve are more or less flattened. Valves are joined with connecting bands (cingulum). 
  • The two connecting bands together form a girdle. 
  • The silicification of wall varies 
    1. In centrales, walls possess areolae or striae arranged radially and symmetrically around a central point,
    2. In pennales, walls are bilaterally symmetrical or asymmetrical with respect to an axial strip. 
  • In some pennate diatoms, two systems of striae are separated from one another by a narrow, linear and smooth area occupying apical axis of the valve and is known as pseudoraphe. 
  • In some diatoms (e.g. Pinnularia, Gomphonema, etc.) the valves possess one central and two polar nodules which are internal thickenings of the wall. A longitudinal slit runs from one polar nodule to another. It is called raphe. 
  • Just inside and close to the cell wall is a cytoplasmic lining. Internal to this is situated a central vacuole. This vacuole is generally interrupted in some forms (e.g. pennate forms) by centrally located prominent band of cytoplasm, in which lies a nucleus. (In centrales this band is absent and nucleus remains near the valve). 
  • Nucleus is very prominent and is round or oval in shape. 
  • Chromatophores vary in shape. These are parietal in position. In pennales, chromatophores are richly lobed and perforated and contain many naked pyrenoids.
  • Reserve products are in the form of oils accumulated in large quantities.

Suriella. Internal structure of a cell.



  • The common method of multiplication is cell division. 
  • During division, two daughter cells of slightly unequal size are formed (comparing the size of the parent cell, one of the daughter cells is always smaller). 
  • Continuous cell division results in progressive diminution of size. It is compensated by the formation of auxospores. 
  • Auxospores are larger than the vegetative cells from which these are produced. 
  • It is primarily the rejuvenation of the protoplast. 
  • Sexual reproduction by conjugation is generally observed in pennales. 
  • The zygote formed as a result of sexual reproduction gives rise to a diploid auxospore, that produces a new set of vegetative cells. 
  • In centrales, the sexual reproduction is oogamous where spermatozoid is uniflagellate and an egg cell is non-motile.

Triceratium. Part of the wall magnified.


  • Sub-division– Algae
    1. Presence of a simple thallus.
    2. Chlorophyll present
    3. Cell wall made of cellulose.
  • ClassBacillariophyceae
    1.  Chromatophores 1-2 or more, golden-brown or yellow, with or without pyrenoids,
    2. Food reserve oil.
    3. The cell wall of two overlapping halves, highly silicified.
    4. Reproduction by flagellated swarmers.
    5. Sexual reproduction by conjugation.
  • Order 1Centrales
    1. Valves circular, polygonal or irregular in outline.
    2. Ornamentation radial or concentric about a central point.
    3. Raphe or pseudoraphe absent.
  • Order 2Pennales
    1. Valves bilaterally symmetrical orasymmetrical in surface view
    2. Ornamentation always bilaterally symmetrical with respect to a line.
    3. Raphe and pseudo-raphe present.




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