Algae are chlorophyll bearing organism with a thalloid plant body and they are the simplest of all multicellular green plants. The branch of science which deals the study of algae is called phycology. On the basis of photosysthetic pigments, they are of following types:
Chlorophyceae (Green algae)
- Mostly algae are green in color due to the presence of photosynthetic pigments like chlorophyll-a and b, carotenes and xanthophylls.
- The inner layer of cell wall is made up of cellulose and outer layer is made up of pectin.
- They store food material as starch.
Rhodophyceae (Red algae)
- The photosynthetic pigments are chlorophyll –a and b, carotenes, xanthophylls, phycoerythrin and phycocyanin.
- The inner layer of cell wall is made up of cellulose and outer pectin layer.
- The reserve food material is floridean starch.
Phaeophyceae (Brown algae)
- They possess the brown pigments like chlorophyll –a and c, carotenoids and fucoxanthin.
- The cell wall is composed up of outer alginic acid layer and inner cellulosic layer.
- The reserve food material is laminarian starch.
Structure and reproduction of Spirogyra
Division : Thallophyta
Class : Chlorophyceae
Order : Conjugales
Family : Zygnemaceae
Genus : Spirogyra
Species : Zonata
Spirogyra is a filamentous green alga, found in fresh water, stagnant water, slow running streams and rivers etc. The plant body is haploid, multicellular, unbranched and filamentous. It is also called as pond scum or water silk, attached to the substratum by means of hapteron or holdfast. Due to the presence of spirally coiled chloroplast, the alga is given the name spirogyra. It is slimy to touch due to the presence of mucus or mucilaginous sheath. The filament consists of green cylindrical cells (except hapteron) placed end to end in a single row. The cell consists of two parts, cell wall and protoplast. The cell wall is the outer most permeable covering made up of cellulose and pectin. It provides protection, rigidity and shape to the protoplast. The mucilage protects from decaying effect of water, epiphytic growth and herbivores (Due to its bitter taste.).
The protoplast is differentiated into cytoploasm, central vacuole and nucleus. The protoplasm is differentiated into cytoplasmic matrix and endoplasmic reticulum. The matrix contains ribosome, mitochondria, chloroplast, golgi bodies etc. the choloroplast contains many spherical proteinaceous bodies called pyrenoids. Spirogyra cells are uninucleate, each cell posses large nucleus with in which nucleolus occur.
Reproduction of Spirogyra
It reproduces by all three methods :
Vegetatively it reproduces by means of fragmentation in which filament breaks into many pieces and each piece develop into new plant.
Asexually, it reproduces by means of akinets and aplanospores. Akinets are thick walled spores arises singly by means of contraction of protoplast which after the death of parental cell germinates directily into new filaments.
It reproduces sexually by conjugation (union of gametangia) where the entire content of two cells acts as gametes. It is of two types:
It takes place between the cells of two different filaments (heterothallic species). Two filaments come to lie opposite and parallel to each other and surrounds by mucilaginous sheath. Then , the cells produce lateral outgrowths opposite to each other which soon meet by their tips. Later the end dissolves and forms a conjugation tube. The protoplasmic content of the cell contracts and functions as a gamete. The formation of male gametes takes place and pass through conjugation tube into female gametangium. The gametes fuse and form a zygote which secretes wall and becomes zygospores.
The conjugation between the two adjacent cells of the same filament is termed as lateral conjugation. It is of two types:
Indirect lateral conjugation:In this type papilla is formed on either side of the septum between two adjacent cells. Then the septum stretched and dissolves on the sides and forms a conjugation tube. Later one of the active gamete (male) moves towards female gamete through conjugation tube. Then, zygote and zygospores are formed.
Direct lateral conjugation
In this type, the lower of two cells, becomes the female while the upper behave as male. It does not forms conjugation tube , but male gamete develops a rod like out growth which creates a pore on septum mechanically or by secreting enzyme. Then, male gamete moves towards female gamete and forms zygote and later the resting spore zygospore.
The zygospore (2n) undergoes meiotic division and forms four haploid nuclei, three degenerate and one functional nucleus divides transversely as a result upper and lower cells are formed. The lower cell develops hold fast while upper cell forms small threads of cells.
Note : In some cases (Spirogyra mirabilis) the gametes fail to fuse and contracts itself and act as zygospore is known as parthenospore and phenomena is known as parthenogenesis.
Economic importance of Algae
- Spirogyra is an edible alga which is dried and used in soup.
- It helps in purification of raw.
- It increases soil fertility.