Division Chlorophyta Class Chlorophyceae Order Conjugates
Family Zygnemaceae Genus Spirogyra
Spirogyra is a large genus. It is widely distributed and found throaghout the world. It has of about 300 species. It is found in fresh water. It grows as free floating mass. Therefore, it is commonly called pond scum. It commonly grows in fresh water stagnant reservoirs. It also grows in slow running streams and rivers. It looks like a mass of long shining silky filaments in running water. That is why it is also called pond silk. Suitable season for its growth is winter. But it grows well throughout the year. It develops reproductive stage if temperature rises. It develops vegetative stage if temperature lowers.
S. crassata, S. elongata, S. microspora, S indica and S. cqmmunis. Vegetative structure
The plant body consists of filaments. These filaments are cylindrical and unbranched. The young filaments have modified basal cells called hapteron or holdfast. Hapteron is used for attachment to some object at the bottom. Rhizoids also grow on hapteron in S. rhizoides and S. dubia. But rhizoids are absent in other plants. The adult plants are always free floating.
Fig: Cell of filament of Spirogyra
Each filament consists of a single row of cylindrical cells. These cells have following structures.
I. Cell wall: Each cell is enclosed by a firm cell wall. Cell wall is
composed of two layered. The inner layer is composed of cellulose. The outer layer is composed of pectic substances. Water makes the pectic substance gelatinous. It gives the plant a slimy touch.
2. Protoplast: Their protoplast consists of a single nucleus, a mass of cytoplasm, chloroplast and a large central vacuole.
a) Nucleus: The nucleus is present in the center. It is suspended by strands of cytoplasm. In some cases, it may parietal in position. it has a complex nucleolus.
b) Cytoplasm: A large central vacuole is present. Therefore, its cytoplasm is peripheral in position. Cytoplasm is traversed by several cytoplasmic strands.
c) Chloroplasts: Chloroplasts are present in cytoplasm. Their number is variable. The number of the chloroplast is the characteristic feature of this alga. Their number may be 1 to 12 30111e times 24. Chloroplasts are flat ribbon-shaped. Their arrangement is spiral. In this case, they twisted to the right in the ascending order. Spirogyra is named due spiral arrangement of chloroplast. Pyrenoids are attached with the chloroplast. They are arranged linearly On the chloroplasts at intervals. Darves (1965) studied chloroplast of Spirogyra with the help of electron microscope. It reveals the presence of photosynthetic bands (grana) in chloroplast. Each gran= has 4 to 12 thylakoids. He found numerous pyrenoids en-sheathed in starch
d) Cytoplasmic organelles: Golgi bodieg, mitochondria and endoplasmic reticulum are present in the cytoplasm.
Spirogyra reproduces by vegetative and sexual methods. However, some workers have also reported asexual reproduction in Spirogyra. Asexual reproduction occurs through akinetes and aplanospore formation.
Fragmentation is the common method of vegetative reproduction in Spirogyra. Accidental breaking or injury may occur. It breaks the filament into 2-3 cell portions. Each portion germinates to produce a new plant. In some cases, cross walls are produced by invagination. This cross wall separates two cells apart and two fragments are formed.
Sexual reproduction is isogamous. It takes place by conjugation of non-flagellated gametes. Conjugations may be of two types: the scalariform conjugation and the lateral conjugation. Simultaneous scalarifortn and lateral conjugation may occur in S. gratiana. But it occurs rarely.
(A) Scalariform conjugation
It is of most common type of conjugation. In this case, two parallel lying filaments take part. Most of the cells of the filaments participate in conjugation at the same time. Therefore, both the filaments appear as adder-like. Hence it is called scalarifonn or ladder-like conjugation.
Fig: Different stage of scalariform conjugation
a) Gamete formation: All cells of a filament can produce gametangia. A single gamete is formed from each gametangium. The gamete producing cells are opposite to each other in both filaments. These cells develop lateral outgrowths. These outgrowths soon meet by their tips. The end wall dissolves. As a result, a continuous conjugation tube is formed between ale two cells of different filaments. The protoplasmic contents of the cells change into a single elliptical gamete.
b) Fertilization: There are two methods of fertilization.
- Dioecious filaments: In this case, during fusion, one gamete passes through the conjugation tube. It reaches the apposite cell. It fuses with the gametes of the other filament. As a result, zygospore is formed in one cell. The other stands remain empty. The motile cell is called male gamete and other is female gamete. Zygospore is formed in a cell of the filament. Therefore, the two filaments are dioecious.
- Monoecious filaments: In this case, gametes of both the
filaments move. The zygospore is formed in the conjugation tube. Therefore, sex distinction is not possible. Therefore, neither one can be called male or female. Such filaments are Mr oecious.
(B) Lateral conjugation
Lateral conjugation occurs between the two adjacent cells of the same filament. Therefore, such plants are bisexual and monoecious. There are two types of lateral conjugations: direct late, II conjugation indirect lateral conjugation.
a. Direct lateral conjugation
It reurs between the two adjacent cells. In this case, ppper cell functions as male gametangium. The lower cell acts as female gametangium. The lower cell increases in size. The upper cell remains smaller. The protoplast of male cell forms an outgrowth. This outgrowth elongates. This outgrowth comes in contact with transverse wall in between male and female cells. Later, it passes through it by forming an opening in it. Thus conjugation tube is not formed and the contents of male cell pass into the female cell. As a result, zygospore is formed.
Fig: Direct Lateral Conjugation
b. Indirect lateral conjugation
It occurs rarely. It is reported in S. affinis. In this case, a papilla like outgrowth is formed in the transverse septum. This outgrowth grows and breaks the septum. Thus a side passage is formed between the two cells. The upper cell behaves as male gametangium. The lower cell acts as female gametangium. The male gamete passes through this side passage. It fuses with female gamete and zygospore is formed. This type of lateral conjugation is also called chain conjugation
Fig: Germination of Zygospore
Fig: Germination of Zygospore
Formation and germination of zygospore
Zygote is formed by the fusion of male and female gametes. It soon develops a thick wall and becomes zygospore. Zygospore is composed of three layered wall. The gametangial wall decay and the zygospore come out. It settles down to the bottom of water body. It undergoes a resting period. Before germination the zygospore nucleus divides meiotically. It produces four nuclei. Three nuclei degenerate. Only one nucleus germinates to develop a new haploid plant.
In some rare cases, the gametes fail to fuse. These gametes become rounded and secrete a cell wall around them. Now they behave as spores. Such spores are called parthenospores. These spores germinate directly into new haploid plants.
Fig: Germination of Zygospore