Division : Chlorophyta
Class : Chlorophyceae
Order : Conjugates
Sub-order : Zygnemoideae
Family : Zygnemaceae
Genus : Spirogyra
Spirogyra (Gr. Speira, a coil, Gr. Gyros, twisted) is a large genus. It is found throughout the world in fresh water. Its filaments look like a shining silky mass in running water. That is why it is also called “water-silk”, “pond scum” or “mermaid’s tresses”.
The plant body is filamentous, the filaments are green, simple, unbranched. A filament is a single row of cylindrical cells. The filaments are free floating. A few species remain attached to the substrata with the help of hapteron or holdfast.
The cells are generally more in length than in breath.
- Cell wall: Cell wall is composed of two layers. The outer is pectic and inner layer is composed of cellulose. The pectin of outermost layer dissolves in water and forms a slimy mucilaginous sheath.
- Protoplast: Their protoplast is differentiated into structures such as plasma membrane, chloroplast, pyrenoid, a large central vacuole other cytoplasmic organelles and nucleus.
- a) Nucleus: The nucleus is present in the centre. It is suspended by strands of cytoplasm. b) Cytoplasm: The cytoplasm is peripheral due to large central vacuole. Cytoplasm is traversed by several cytoplasmic strands.
- c) Chloroplasts: These are ribbon-shaped and arranged spirally hence alga named as Spirogyra. The chloroplast contains many Each pyrenoid consists of a central proteinaceous core surrounded by many small starch plates.
Reproduction in Spirogyra is vegetative and sexual. However, some species reproduce asexually through akinetes and aplanospore formation.
- Vegetative Reproduction:
Fragmentation during favourable conditions, the filaments break into two or more fragments, each or which develop into a new filament.
- Sexual Reproduction:
Sexual reproduction is isogamous type and takes palace under unfavourable conditions. The conjugation may be of following types.
It takes place between the cells of two different filaments. The filaments lie side by side or parallel to one another, through their length.
- Any cell of filament can acts as gametangium and produces a single gamete.
- The gamete producing cells are opposite to each other in both filaments.
- These cells develop lateral outgrowths, which soon meet by their tips.
- The walls dissolve and a conjugation tube is formed between gametangia.
- The cell contents are changed into a single gamete. Motile gamete is male non-motile is female
- One gamete passes through the conjugation tube and reaches the opposite cell.
- It fuses with the gametes of the other filament.
- Zygote (2n) is formed in one cell. The other stands remain empty.
(B) Lateral conjugation
- It takes place between the two adjacent cells of the same filament.
- The upper cell functions as male gametangium and remains smaller. Its contents are transformed to single male gamete.
- The lower cell acts as female gametangium.It increases in size and its contents are transformed to female gamete or egg.
- The protoplast of male cell forms an outgrowth.
- An opening is formed in the wall and the contents of male cell pass into the female cell where fertilization takes place and zygote (2n) is formed.
- Zygote is converted into zygospore by developing a 3 layered thick wall.
- The outer two layers of gametangial wall burst and the zygospore comes out.
- After a period of rest zygospore divides meiotically.
- One out of four nuclei germinate to produce a haploid plant, the rest are disintegrate.