Nostoc belongs to the group Cyanophyta of kingdom Monera. It is blue-green algae. Nostoc is common as terrestrial and subaerial cyanobacteria. It is widely distributed in alkaline soils, moist rocks, water ponds, streams, and lakes etc. Nostoc colony forms a jelly-like mass. Many filaments are embedded in this mass. The individual filament is called trichome. Trichome is unbranched and it appears bead-like structure.
Thallus Structure of Nostoc
The body of Nostoc is called thallus. It is a simple body and consists of many threads like structures, called filament. Each filament is composed of many rounded cells which are arranged in a chain like manner or in beaded form. All the cells are similar. The whole filament is covered by a gelatinous sheath.
In the filament, some cells are larger in size and thick-walled, called heterocysts. These cells perform two functions, reproduction and Nitrogen Fixation. In Nitrogen fixation, Nitrogen is converted into simpler Nitrogen compounds, which are used by the plant.
Trichomes are unbranched and appear beaded. Individual cells are mostly spherical but sometimes barrel-shaped or cylindrical. All cells in trichome are mostly similar in structure but at intervals are found slightly large, round, light yellowish thick-walled cells called as heterocysts. Trichomes mostly break near heterocyst and forms hormogonia and thus help in fragmentation.
Structure of a cell of Nostoc
Each cell of Nostoc is rounded in shape. It is covered by an outer cell wall. The protoplasm of the cell consists of two parts:
b) CENTROPLASM OR CENTRAL BODY
It is the inner colorless part of the cell, which contains stored food granules. It also acts as a nucleus. The cell does not contain a true nucleus. The central body performs the function of the nucleus. It has no nuclear membrane or nucleolus, so it is called incipient nucleus. i.e.(Incomplete nucleus).
Reproduction in Nostoc
Sexual reproduction is absent in Nostoc. In Nostoc, the reproduction takes place by the following methods:
1. By Hormogonia OR by Fragmentation
Hormogonia are formed by Fragmentation. The filament breaks at different points and each broken filament is called hormogonia. The filament breaks due to decay and death of ordinary cells. Heterocyst may form the breaking point. Each hormogonium grows into a new filament by repeated cell division.
2. By Arthrospores OR by Akinetes Formation
During unfavorable conditions, some cells of filament become enlarge and they are covered by a thick wall, they are called Akinetes or Arthrospores or Resting Spores. They also store food material. They germinate during favorable conditions into new filaments.
3. By Heterocyst
At the time of reproduction, the heterocysts are separated from Nostoc filament. These heterocysts are changed into a normal reproductive cell. By the process of cell division, it is converted into a new filament.
Nutrition in Nostoc
Nostoc contains chlorophyll, so it can manufacture its own food i.e. it is Autotroph in nature.
Importance of Cyanobacteria
- Cyanobacteria (Blue, Green Algae) use carbon dioxide and release oxygen during photosynthesis, so environment becomes fresh and is used by aquatic animals.
- They take part in Nitrogen Fixation.