Nostoc

 Classification

                                                   Division- Algae

                                                      Class -    Chlorophyceae

                                                      Order -      Nostocales

                                                    Family -          Nostocaceae

                                                     Genus -             Nostoc

 

Occurrence

·         It is filamentous form of both terrestrial and aquatic habitats.

·         It does not occur in single filaments but grows in large colonies of closely packed trichomes embedded in firm matrix of gelatinous materials

·         Nostoc colony occurs floating or attach to substratum.

·         The thalli are various size and shape and may be solid or hallow. They may be ball of jelly like substance or may be irregular shaped.

·         The colony may be olive green or blue green and the surface of the colony warty and smooth.

·         Terrestrial species of Nostoc grows on damp soil and forms leathery or rubbery sheath and common in artic and alpine meadows.

·         Aquatic species occur either a free floating thalli in the sunny pool, ponds and lakes at the bottom attached to the substratum.

·         Nostoc also occurs in symbiotic association with fungi to form lichens.

Structure of the colony           

            Innumerable chain of bead like cell (trichome) of varying length is intricately tangled in a copies gelatinous matrix to form a colony. Each trichome is usually enclosed by its own mucilaginous sheath and called as filament. The numerous filaments in the colony are held together by a soft mucilaginous envelope formed by a fusion of the individual trichome sheath. The mature one is large. When number of colony growing in neighborhood within a common gelatinous mass called as compound colony

Structure of trichome

           Each trichome is composed of numerous rounded or oval cells. The cells are joined loosely form end to end due to which resembling a string of beads. At frequent intervals along with trichome are found colourless, empty looking, spherical or barrel shaped cells called the heterocysts. They are slightly large and have thicker wall than the vegetative cells and are intercalary sometime terminal. Under certain condition vegetative cells become greatly enlarge with stored food and greatly thickened to become resting bodies called as akinetes. Each trichome with its individual sheath is called as filament.

Cell structure

            The cell consist of the cell wall surrounding the protoplast. The protoplasm here differentiated into two regions. Outer pigmented region called as chromoplasm and inner colour less region is called as centroplasm. The chromoplasm has usual pigments located in the lamellae which form a system of peripheral area of parallel paired membranous embedded in the granular matrix. The centroplasm is not separated from the chromoplasm by any kind of membrane.  

Reproduction

Noctoc reproduces entirely vegetatively by the following methods

·         Colony fragmentation

The Nostoc as filaments gets breaks up into the smaller filament due to storm and other disturbance. Each of these grows up into the size of the parent filament.

·         Harmogonia

Hatmogonia formation is very common in Nostoc.  The trichome rapture at some places where a heterocyst and vegetative cell join with each other.  This junction is weakest link in the chain. In this way shor t segments of living cells called as the harmogonia become isolated. The harmogonia slip out of enclosing gelatinous matrix and established new colonies by division. Sometime the harmogonia develop into fresh trichome without being liberated from the colonial sheath and form adult colony.

·         Resting spore or akinetes

Under certain condition any cell or some of the vegetative cells of the trichome  become enlarged and each secrets a thick, highly resistant wall around it. They have reserve food materials are called as akinetes or resting spore. These are well adopted to survive in unfavorable condition. With the return of favorable season each akinetes germinates to form new filament of Noctoc.

·         Endospore formation

The content of the heterocyst of Nostoc divide to produce endospores. The endospore on liberation gives rise to new filaments.  

References

1.  Vashishta, B. R., Singh, V. P. & Sinha, A. K. (2012). Botany for Degree Students - ALGAE. S. Chand, 1-624.

2.9eed7ce0820d5fb37542bcdbea84fb56.jpg (1560×1259) (z-dn.net)

3. Nostoc: Features, Occurrence, Structure, Reproduction (biologylearner.com)