CLASSIFICATION OF DIATOM
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.
STUDY OF A CELL
- 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
- In centrales, walls possess areolae or striae arranged radially and symmetrically around a central point,
- 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.
REPRODUCTIVE STRUCTURES OF DIATOM
- 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.
- Sub-division– Algae
- Presence of a simple thallus.
- Chlorophyll present
- Cell wall made of cellulose.
- Class – Bacillariophyceae
- Chromatophores 1-2 or more, golden-brown or yellow, with or without pyrenoids,
- Food reserve oil.
- The cell wall of two overlapping halves, highly silicified.
- Reproduction by flagellated swarmers.
- Sexual reproduction by conjugation.
- Order 1 – Centrales
- Valves circular, polygonal or irregular in outline.
- Ornamentation radial or concentric about a central point.
- Raphe or pseudoraphe absent.
- Order 2– Pennales
- Valves bilaterally symmetrical orasymmetrical in surface view
- Ornamentation always bilaterally symmetrical with respect to a line.
- Raphe and pseudo-raphe present.