Book : Botany
Posted by: CHELSEA
The plant body of spirogyra is a gametophytic thallus which is elongated cylindrical, unbranced silky thread like structure known as filament. Each filament consists of numerous elongated rectangular cells joined end to end in a row. The cells are separated by a partition wall known as Septum. Each cell can be differentiated In to two reasons; cell wall and protoplast. Cell wall is the outermost protective double layere structure. The inner layer of which is made of cellulose and the outer layer contains pectose. The outermost lining of Pectose turns into Pectin and get dissolved in water to form Mucilage which surrounds the filament and forms Mucilagenous sheath. The protoplast contains Plasma membrane, cytoplasm, single large centrally located vacuole and Chloroplast containing Pyrenoids. The plasma membrane lies inner to the cell wall and semi permeable in nature (allows the passage of selective substance).
During favorable conditions spirogyra reproduces vegetatively by the method of Fragmentation. Fragmentation is brought about due to the mechanical injuries caused by the biting of the dished and other aquatic animals or due to the change in temperature , PH , and other aquatic conditions or some intercalary sells of the filament (intermediate cells). Fragmentation involves the breakage of filament into small pieces known as fragments each of which can develop into new filament by the process of repeated cell division and growth.
During unfavorable conditions some species of spirogyra reproduce asexually though rarely by the formation of –Akinetes - Aplanospores, Azygospores/Parthenospores Akinetes:Few species of spirogyra such as spirogyra farlowii reproduce asexually by the formation of Skinetes. In this method the protoplast of few cells of the filaments lose water get contracted and round off and develop thick walls around themselves due to the extra deposition of cellulose and Pectin and form the akinetes which are thick walled resting spores that can develop into new filament with the coming of favourable conditions. Aplanospores: Eight species of spirogyra such as spirogyra aplospora,spirogyra kanalae form Aplanospores during asexual reproduction. Here, the protoplast of the cell after forming rounded structure develop thin cellulosic walls around themselves and form the aplonospore which are the thin walled,non motile spores that can germinate into new filament with the coming of favorable condition and after the decaying of the mother filament.
Sexual reproduction in spirogyra occurs by the method of conjugation. Conjugation is the process of fusion of two gametes out of which one acting as the male gamete passes through the conjugation tube to the female gamete. In spirogyra all the cells of the filament can form the gametes which are Morphologically similar and are referred to as Isogametes. This phenomena of sexual reproduction is known as Isogamous type of sexual reproduction. But at the time of conjugation one of the two isogametes shows motility and is considered as the male gamete. This phenomenon is referred to as Physiological anisogamy. In spirogyra,two types of conjugation occurs Scalariform and Lateral conjugation.
Occurs between the two filament and seen in heterothallic/dioeious species. Two filament of opposite strands come closer and lie parallel to each other throughout their length and get surrounded by commom Mucilagenous sheath(Due to attraction). The opposite cells of two filaments produce small papillae outgrowth /Processes from their opposir\te walls. These papillate outgrowth increase in their size and join through their anterior end. By this time filament secretes enzyme cytase. Two filament secrete the enzyme cytase due to the activity of which the partition wall is sissolved between the two process forming a continuing passage conjugation tube. The ladder like structure is formed due to conjugation of many cells. Same time the protoplast get contracted forms gametes known as Gametangia. Thus, forms gametes are Isogametes in gametangia(bag to hold gamete) out of the isogametes one gamete shows motility and can be determined as male gamete. The male gametes enters through conjugation and enters the female gametangia and fude with femate gamete and forms Zygote. This zygote after the secretion of thick wall forms Zygospores.
Occur rare type of conjugation and seen in Homothallic/Monoecious species. It is of two type direct and indirect.Indirect: On the lateral side of filament above the septum the two adjacent cells gives papillate out growth. Cytase is secrete by the cells the partition wall get dissolved forming conjugation pasage due to the activity of enzyme cytase. Simultaneously the protoplast of the adjacent develops into the gamete which is Isogametes. The cells are referred as Gametangia. After sometime one gamete out of two gamete shows motility and determined as male gamete. The male gamete passes through conjugation tube and enters the female gametangia and fuses with female gamete to form zygote then forms thick walled Zygospore. Direct: (Primitive type of conjugation) During the time of conjugation few attached species shows.Two adjacent cells which are going to conjugate. The lower cell get enlarged and form barrel shaped structure termed as Female gamete.The upper cell (male gametangia) forms male gamete which are some what elongated cylindrical and pointed lower end and here conjugation tube are not forming. The male gamete with pointed end directly pierces the septum and forms a hole in septum and passes through the hole into female GAmetangia where it fuse with female gamete to form gametes. The zygote after developing thick wall develops into zygospore.
Zygospore represents the only diploid stage in the lifecycle of spirogyra and it is yellowish brown ellipsoidal structure having diploid nucleus and covered by three layered wall : - the outer exospore/exine-The middle mesospore/ mesinem - The innermost Endospore/ intine The zygospore due to the thick wall can withstand long period of dry and dessication. They are liberated after the destruction of mother cell or mother filament and they remain at the bottom of water body. With the coming of rainy season,the zygospores start to germinate. Before germination,the diploid nucleus divides meiotically and form four haploid nuclei. Out of which three degenerate and one remains at functional nucleus. At the time of germination the exospore and mesospore rupture and the endospore along with functional nucleus comes out as single cell germinating tube which immediately divides into two celled structure. The basal cell forms the rhizoidal cell and the upper cell by repeated division and growth develops into new spirogyra filament.