General characteristics of Bacillariophyta
Division - Bacillariophyta:-
General characteristics:-
> Diatoms are cosmopolitan and ubiquitous in distribution, present in freshwater and oceans. Diatoms can also be found in terrestrial environments in the soil.
> Grow epiphytically on other algae and higher plants.
> Exist in two major forms-
i. Benthic:- Which are non planktonic, attached to some substratum such as rocks, sand or mud or be epiphytic, epizoic or endozoic.
ii. Planktonic:- Free floating.
> They are commonly unicellular and free- living but some members form colonies of various shapes like filaments, mucilaginous colonies etc.
> Microscopic cells are of different shapes. They may be oval, spherical, triangular, boat- shaped etc.
> Plant bodies are either bilateral or radial in symmetry.
> The cells are surrounded by a rigid cell wall, called frustule, consisting of upper epitheca and lower hypotheca; arranged in the form of a box with its lid.
.bmp)
> The cell wall is composed of pectic substances impregnated with high amount of siliceous substance.
> The wall may have secondary structures like spines, bristles etc.
> The frustules show beautiful sculpturing and ornamentations for this diatoms are described as “jewels of the sea" and "living opal” and "Gems of the plant kingdom".
> The cells generally have many discoid or two large plate-like chromatophores. Some cells possess stellate chromatophore.
> The photosynthetic pigments are chlorophyll a, chlorophyll c along with xanthophylls like fucoxanthin, diatoxanthin and diadinoxanthin.
> Reserve food is oil, volutin and crysolaminarin.
> Some vegetative cells show gliding movement.
> Motile structure (antherozoid) has single pantonematic flagellum.
> Vegetative multiplication takes place by cell division, which is very common. Some of the cells become very much reduced in size.
> They produce characteristic spore, the auxospore which develops to regain the normal size.
> Sexual reproduction takes place by isogamy and oogamy.
Classification:-
Class:- Bacillariophyceae
a. Order:- Centrales
Families:-
i. Discoideae:- Eg.- Melosira.
ii. Solenoideae:- Eg.- Corethron
iii. Biddulphioideae:- Eg.- Biddulphia
iv. Rutilarioideae:- Eg.- Rutilaria
b. Order:- Pennales
Families:-
i. Fragilarioideae:- Eg.- Synedra
ii. Eunotioideae:- Eg.- Eunotia
iii. Achnanthoideae:- Eg.- Cocconeis
iv. Naviculoideae:- Eg.- Navicula
v. Epithemioideae:- Eg.- Epiyhemia
vi. Nitzschioideae:- Eg.- Bacillaria
vii. Surirelloideae:- Eg.- Surirella