- Occurrence: Diatoms are aquatic. They are both marine and fresh water. They form brownish scum on the surface of mud and ponds. Diatoms are abundant during spring and autumn months. They are mostly planktons. Some are attached to the substratum.
- Vegetative structure: Diatoms are unicellular organisms. Some forms colonies. Mucilaginous coverings join the cells in a colony. Mortly they are non-motile. But many diatoms show characteristic creeping movement. Diatoms lack motile organ. Creeping mole ements occur due to streaming movement of cytoplasm.
- Cell wall and valves: The cell wall of diatoms is composed of pect:c substance. Silica is impregnated in it. Therefore, their wall becomes very hard. Their wall is composed of two halves called valves. These halves overlap like Petridish. The margins of the two valves are covered by a connecting band called cingulum. The two valves with their inner protoplasts are called frustule. The outerlarrr valve is called epitheca. The smaller inner valve is called hypotheca. The cell is covered by mucilaginous layer. The surface of valve has minute pores or pits. It produces characteristic markings on the surface of valves. The marking free area is called axial field. The markings are arranged in linear rows. The axial field may be homogenous. Or it may contain a longitudinal slit called raphe. An axia: field without raphe is called pseudoraphe. The raphe is not continuous throughout the length of the valve. It has circular thic.cening called central nodules in the centre. Similar thickening also present at the outer end of raphe. These are called polar nodules. These markings are of two types:(a) Radially symmetrical (Centrales): In this case, markings are arranged in radially symmetrical pattern. The diatoms are circular in shape. The group of such diatoms is called Centrales.
(b) Bilaterally symmetrical (Pennales): The have bilaterally symmetrical thickenings. These diatoms are elongated or boat shaped. The group of such diatoms is called pennales.
- Cell structure: Vacuole is present in the cytoplasm. It zontains one or more chromatophores. The chromatophores may be discoid or lamMate. Cell has single nucleus. The nucleus divides by mitosis. The spindle fibers are arranged in the form of cylinder. Chrmnosomes are arranged in metaphase in an equatorial ring. Pyrenoid may or may not be present. Starch is absent.
- Pigments and reserve food material: They have golden brown pigment called diatomine. They also have chlorophyll a, chlorophyll b, carotene and Xanthophyll. Chief reserve food material is fat droplets. Chyrsolaminarine is also a reserve food material.
- Diatomaceous earth: The frustules of dead cells deposits on the surface of ponds, lakes and sea. These deposited are called diatomaceous earth. They have many commercial uses. They are used for manufacturing dynamite, polishes and tooth pastes. They are also used as insulators in boilers and furnaces.
- Vegetative reproduction: Vegetative reproduction in diatoms occurs by simple cell division. Nuclear division occurs by mitosis and cell divides into two parts. Each half receives one half of the parent cell. It synthesizes new valve. New valve is fit into the parent valv Thus new valves are always smaller than the parent halves. Thus one generation gradually become smaller in size. It reached to minimum size. Then its size is restored by auxospore fonnation. But second generation remains of same size.
8. Auxospore formation: Sexual reproduction occurs by auxospore formation. Auxospore grow to form new cells. First division is meiotic. Four nuclei are produced. Three degenerates Auxospore is formed by following methods:
(a) Two smaller individuals act as gametes. They conjugate with each other and produce a single auxospore.
(b) Two vegetative cells produce four gametes. They fuse to form auxospore.
(c) A single cell produces two gametes They fuse with each other to Produce auxospore.
(d) A single cell produces single gametes. It undergoes parthenogenesis and develops auxospore.
(e) Single cell produces single auxospore in Centrales. The nucleus of auxospore divides by meiosis. The haploid nuclei fcse with each other.
9. Statospores formation: It is also called endospore or cyst. It is thick walled. It is produced in some diatoms. In this case, a new thick wall is formed around the protoplast. This wall also has overlapping valves. But their markings are different from the vegetat:ve cells.