Information of zygnema

Zygnema:

Taxonomic Position:

Division: Chlorophyta

Class: Chlorophyceae

Order: Zygnemales

Family: Zygnemaceae

Genus: Zygnema

 Habitat:

Zygnema is common green algae having about a hundred species. All are aquatic except Z.terrestre, Randh. They are found in considerable quantities in quiet fresh water of ponds, lakes and also on slow flowing streams as yellowish green free floating masses on the surface of the water.

 Habit:

 It is a simple, fine, unbranched thread or a filament consisting of several cylindrical, rectangular cells arranged one above the other in a row. 

 Cell Structure: 

1. Each cell contains a small mass of protoplast surrounded by a cell wall.

2. The cell wall is two layered – inner cellulosic and outer pectic, covered my mucilage sheath

3. The protoplast has a cell membrane, cytoplasm, nucleus and two stellate or star shaped chloroplasts. 

4. The chloroplasts are embedded in the cytoplasm.

5. Each chloroplast consists of a central body with a single pyrenoid at the center.

6. A number of delicate strands radiate out from the center of the body of the chloroplast and extend towards the plasma membrane.

7. A single centrally situated nucleus lies embedded in the middle of the broad strand of cytoplasm connecting the two chloroplasts.

 

 Reproduction in Zygnema:

 Zygnema shows vegetative, asexual and sexual modes of reproduction

Vegetative Reproduction – It is shown by fragmentation. The filament breaks into short segments of living cells or individual cells. These may show cell division or simply grow into new Zygnema filaments.

Asexual Reproduction – It is accomplished by Akinets or thick walled Aplanospores.

Akinetes – An Akinets is a thick walled, resting, vegetative cell. It may be formed singly, in rows of two or three or in later stages as long chains.

Aplanospores – They are produced singly in the vegetative cells. They may be round or ovoid in form and have thick, variously colored walls. However, according to Randhawa (1938) the mature Aplanospore has dark greenish blue colored wall. The shape of Aplanospore varies from oval to pyramidal or barrel shaped.

Sexual Reproduction – Zygnema shows scalariform and lateral form of conjugation. 

(a) Scalariform Conjugation:

It is similar to that of Spirogyra. At the onset, the filaments come to lie side-by-side in pairs. Small dome shaped protuberances grow towards each other from the opposite pairs of cells. The protuberances elongate and finally meet. At the point of contact, the intervening walls dissolve. With this the two outgrowths form a conjugation tube. This open passage is known as the conjugation canal. Most species of Zygnema are usually isogamous (no visible difference between the gametes and the conjugating filaments). Now the protoplasts of the conjugating cells form gametes, singly. The protoplast or the gamete, which functions as the male gamete shrinks from the parent cell wall, migrates through its respective conjugation canal and fuses with its partner in the opposite cell, which can be termed as female gamete. The movement of the male gamete is often amoeboid. This fusion leads to the formation of the zygospore.

(b) Lateral Conjugation :

 It is commonly found in Z.gangeticum and Z.heydrichii. In this case the adjoining cells of the same filament give out tube like protuberances on either side of the septum. When the protuberances finally meet, the separating septum breaks at the point of contact and a communication link is established between the two adjoining cells. The contents of one cell migrate into the other where the fusion of two protoplasts takes place to form a zygospore.

Zygospore – The fusion cell formed by the union of protoplasmic masses of the two conjugating strands is called a zygospore. It contains four chloroplasts and a single diploid nucleus. The zygospore secretes a wall around it which gradually thickens. This wall is differentiated into three layers:

1. Exospore – Thin, hyaline outer layer which is cellulosic or pectic in nature.

2. Mesospore – Thick middle layer made up of cellulose.

3. Endospore – Thin delicate innermost layer made of cellulose.

The Zygospores sink to the bottom of the pond after disintegration of the walls of the female cells or conjugation tubes. Upon release the zygospores enter a resting period. Two out of the four chloroplasts, disintegrate. The Zygospore is now capable of resisting unfavorable conditions. It is filled with starch and oil. It germinates during favorable conditions.

Germination of Zygospore – The zygospore is released after the disintegration of the walls and comes to rest at the bottom of the pond. At the beginning of the next growing season the zygospore germinates to form a new plant. Prior to germination, two of the four chloroplasts disintegrate. The diploid nucleus undergoes meiosis to from four haploid nucleii. Three of these disintegrate and only one remains. This is the functional haploid nucleus. It divides and redivides to form daughter cells. In this way a new filament is formed.