Spirogyra : General Description of Habitat, Structure and Sexual and Asexual Reproduction

BIOLOGY | BOTANY
UNIT FOUR : KINGDOM PLANTAE
Class : 11

Spirogyra 

Habitat : They are abundantly found in fresh water resources like ponds pools, ditches, springs, lakes, streams etc. they are widely distributed throughout the world. 

Structure : The plant body is thalloid. The thallus is unbranched or unattached. The thallus is called filament. The young filament is attached to some substratum by the help of basal cells. Each filament consists of many cylindrical cells. The cells are longer than breadth. The cell is surrounded by gelatinous sheath. Cell wall is outermost layer, which is rigid and consists of two layers. The outer layer is made up of pectose and inner layer is made up of cellulose. Two cells in the filament are separated by a common wall, which is called septum. The cytoplasm contains cell inclusions and cell organelles. There is a central vacuole in the centre of well which is surrounded by a layer called tonoplast. The vacuole is filled by a liquid called cell sap. There is a nucleus at the centre of vacuole. In each cell, spirally coiled chloroplast is present in the cytoplasm. In chloroplast, round and spherical bodies are found which are called pyrinoid. 

Reproduction 

Asexual reproduction 

Fragmentation : It is common method of vegetative reproduction. In this method, the filament may breaks up into small fragments and each such fragment grows into new filament. 

Akinetes : Under unfavorable condition, some cells of the filament become thick walled which are called akinetes. The akinetes germinate to produce new filaments. 

Aplanospores : The aplanospores are thin walled spores, which arise singly in the filament. The protoplast of the cell loses water and contracts. It rounds off and secrets thin wall around it to become an aplanospore. It germinates to produce new filament. 

Sexual reproduction 

Sexual reproduction takes place by conjugation method. When cells of two filaments conjugate then it is called as conjugation. There are three methods of conjugation 

a) Scalariform conjugation 

• In this method, two filaments of spirogyra come close and lie parallel to each other in opposite direction. 
• Two filaments get surrounded by common mucilage sheath. 
• The cells of the filaments produce small outgrowths towards each other, which are called papillae. 
• The papillae of opposite filament get enlarged and fuse together. 
• The common wall of papillae gets dissolve forming tube like structures, which is called conjugation tube. At this stage, it appears ladder like structure, which is called scalariform. 
• The protoplast of conjugating cells contract by losing water forming gametes. The gamete bearing cells are called gametangia. 
• The gametes of one filament move towards the cells of another filament through conjugation tube. 
• The migratory gametes are male gametes and stationary gametes are female gametes. 
• Two gametes fuse together to from zygote in one filament to form zygote and the zygote secretes wall around it, which is called as zygospore, but the cells of another filament, remain empty. 

b) Lateral conjugation 

• The conjugation takes place between adjacent cells of the same filament is called lateral conjugation 

There are two types of lateral conjugation 

Direct lateral conjugation 

• In this method, the protoplast of adjacent cells contracts by loosing water and from gametes. 
• In the upper protoplast a conical tube like structure is formed which is called male gamete. The lower protoplast is now called female gamete. Now male gamete penetrates common septum, move downwards and fuses with female to from zygote and then zygospore. 

Indirect lateral conjugation 

• Two adjacent cells develop small outgrowth on either side i.e. near septum, which is called papillae. 
• The common wall between two papillae gets dissolved and conjugation tube is formed between two adjacent cells. 
• The protoplast of two adjacent cells becomes contract, which is called gametes. The upper cell is called male gamete and start to move to lower cell through conjugation tube. 
• Now, male and lower female gamete fuse together to form zygote and the zygote secrets wall around it to form zygospore. 

Zygospore germination 

The filament of the zygospores decay and zygospore becomes thick walled. The thick walled zygospore sinks at the bottom of water. The diploid nucleus of zygospore undergoes meiosis division and produces four nuclei. Out of four nuclei three degenerate and one remains functional. Now the outer wall of zygospore ruptures and inner wall produce small tube called germ tube. Now the germ tube divides transversely with nucleus and septa are formed with the formation f one new filament.