CHAPTER 20
Bacillariophyceae
The forms of Bacillariophyceae are popularly called diatoms which may be a unicellular or
colonial. Their highly sculptured and symmetrically ornamented ccll wallmakes this group
separate from rest of the algae. The group contains about 190 genera classified into 5,500
species, majority of which are living and a few are fossils.
The class Bacillariophyceae are characterised by the following features.
(1) The forms may be unicellular or colonial.
(2) The vegetative cells of the thallus are diploid.
(3) The plant body consists of two overlapping halves called valves,
(4) The cell wall are silicified.
(5) Chromatophores are characteristic with the presence of chlorophyll-c togcther
with fucoxanthin, diatoxanthin and diadinoxanthin and therefore give brown colouration.
There may be one to many chromatophores in each cell.
(6) The reserve food material is oil and chrysolaminarin.
(7) Quite interestingly therc occur reduction in size of cells during vcgetative multi
plication. However, auxospores are larger in size than, the cells producing them.
Occurrence. Forms are cosmopolitan and grow in all kinds of aquatic habitats, both
in fresh and salt waters and on damp surface to planktonic conditions. They can be frequently
seen as a yellow scum on the surface of mud in ponds and ditches. They are quite important
as primary producers in the food chain of aquatic ecosystem.
The most commonfresh water forms are Asterionella, Navicula, Nitzschia, Melosira,
Synendra ctc. The cpiphytic forns growing under similar habitatare Cocconeis, Gomphonema
and Euntia. They grow on fresh water algae such as Cladophora, Oedogonium, Pithophora,
etc. Triceratium and Hyalodiscus are marine forms which grow as epiphytes on sea weeds.
Fossil diatoms. Abundant fossils of diatoms from Silurian are known and about 70
genera have been so far reported. Majority of such fossils are centric. A diatom, Pyxidicula
is supposed to be oldest one from Jurassic. Scvoral fossil forms are reported from India,
especially from Laradura, Kashmir (Shukla, 1962). Their deposists are popularly called
Kicsclguhr.
THE PLANT BODY
Structure
The thallus, Commonly thalli are unicellular þut may bc colonial also
and may be of
different sizesand shapes (boat-like, disc-like or triangular, (Fig. 20.1). The unicellular forms
have been classificd intotwo orders based on symmctry:
(i) the pennate diatoms (Pennales) with isobilateral symmery, e.g.,
(iü) the centric diatoms (Centrales) with radial symmetry, Pinnularia.
o.g., Cyclotella.
Second point of consideration in classification is based on presence or absenco of
bristles or horns on the cell surlacc of centrales while in pennales, the number and
morphology of raphes on the valves, In certain cases, colonial diatoms may be organiscd into
Bacillariophyceae
The forms of Bacillariophyceae are popularly called diatoms which may be a unicellular or
colonial. Their highly sculptured and symmetrically ornamented ccll wallmakes this group
separate from rest of the algae. The group contains about 190 genera classified into 5,500
species, majority of which are living and a few are fossils.
The class Bacillariophyceae are characterised by the following features.
(1) The forms may be unicellular or colonial.
(2) The vegetative cells of the thallus are diploid.
(3) The plant body consists of two overlapping halves called valves,
(4) The cell wall are silicified.
(5) Chromatophores are characteristic with the presence of chlorophyll-c togcther
with fucoxanthin, diatoxanthin and diadinoxanthin and therefore give brown colouration.
There may be one to many chromatophores in each cell.
(6) The reserve food material is oil and chrysolaminarin.
(7) Quite interestingly therc occur reduction in size of cells during vcgetative multi
plication. However, auxospores are larger in size than, the cells producing them.
Occurrence. Forms are cosmopolitan and grow in all kinds of aquatic habitats, both
in fresh and salt waters and on damp surface to planktonic conditions. They can be frequently
seen as a yellow scum on the surface of mud in ponds and ditches. They are quite important
as primary producers in the food chain of aquatic ecosystem.
The most commonfresh water forms are Asterionella, Navicula, Nitzschia, Melosira,
Synendra ctc. The cpiphytic forns growing under similar habitatare Cocconeis, Gomphonema
and Euntia. They grow on fresh water algae such as Cladophora, Oedogonium, Pithophora,
etc. Triceratium and Hyalodiscus are marine forms which grow as epiphytes on sea weeds.
Fossil diatoms. Abundant fossils of diatoms from Silurian are known and about 70
genera have been so far reported. Majority of such fossils are centric. A diatom, Pyxidicula
is supposed to be oldest one from Jurassic. Scvoral fossil forms are reported from India,
especially from Laradura, Kashmir (Shukla, 1962). Their deposists are popularly called
Kicsclguhr.
THE PLANT BODY
Structure
The thallus, Commonly thalli are unicellular þut may bc colonial also
and may be of
different sizesand shapes (boat-like, disc-like or triangular, (Fig. 20.1). The unicellular forms
have been classificd intotwo orders based on symmctry:
(i) the pennate diatoms (Pennales) with isobilateral symmery, e.g.,
(iü) the centric diatoms (Centrales) with radial symmetry, Pinnularia.
o.g., Cyclotella.
Second point of consideration in classification is based on presence or absenco of
bristles or horns on the cell surlacc of centrales while in pennales, the number and
morphology of raphes on the valves, In certain cases, colonial diatoms may be organiscd into
