The cells are rectangular; cell wall is made up of inner cellulose and outer pectin layers. Outer pectin layer swells in water into a mucilaginous sheath.
This explains the smooth silky nature of the filament. Plasma membrane encloses the protoplast and occurs beneath the cells wall. Cytoplasm occurs in the peripheral zone called primordial utricle which surrounds a large central vacuole.
The cells are eukaryotic and uni nucleate. The nucleus occurs at the centre of the cell which is held by cytoplasm strands extending from the periphery of the protoplast. Each cell is provided with one or more ribbon shaped spirals chloroplast, the margin of which maybe smooth or serrated. The number of spirals may vary from l -16 in a chloroplast. Many pyrenoids occur in the chloroplast. The pyrendoids are the site of starch formation and accumulation.
Each cell of the filament is capable of ordinary division in one plane leading to increase in length of the filament.
Spirogyra is autotrophic and prepares carbohydrate by the process of photosynthesis.
Reproduction in Spirogyra takes place by vegetative, asexual and sexual methods.
(i) Vegetative reproduction: It occurs by the usual process of fragmentation caused by mechanical means (injury) or gelatinization of some intercalary cell. Each fragment having one or more cells by repeated cell division produces a new filament.
(ii) Asexual reproduction: It is observed only in few species by forming akinetes, aplanospores and parthenospores. Cells of the filament get covered by cellulose layers resulting in thick walled akinetes, as seen in S. farlowii. In S. karnalae, the protoplasm of the vegetative cells of the filament contracts to form aplanospores.
When gametic union fails, the gametes round up and secrete a thick wall to become aplanospores. Sometimes sexual reproductive units or gametes fail to fuse owing to sudden change in the environment. They remain inside the parental cell and each gamete forms a parthenospore. Akniete, aplanospsore and parthonespore etc. germinate under favorable conditions to form new filament.
III. Sexual Reproduction:
Sexual reproduction is isogamous and accomplished by conjugation – fusion of amoeboid aplanogametes which are formed singly within a cell. The species of Spirogyra may be homothallic or heterothallic. Conjugation is of two types’ lateral conjugation and scalariform conjugation.
(a) Lateral conjugation: It takes place between two adjacent cells of the same filament (Homothallic). It is of two types such as indirect lateral conjugation and direct lateral conjugation, (a) indirect lateral conjugation: The longitudinal wall on either side of a septum protrudes out.
The septum in the region of protrusion dissolves forming a conjugation tube that connects the two gametangia (cells). The protoplasts of the cells contract and form identical gametes. The gamete of one cell which is active migrates into other (female) where it fuses with its gamete (plasmogamy and karyogamy) to from zygote.
(b) Direct Lateral Conjugation: It takes place in lower part of the filament. A pore is formed in the middle of the septum existing between two adjacent cells of the filament through which male gamete of one cell migrates into the other (female) and fuses with its gamete to form diploid zygote (2n).
In lateral conjugation gametes of alternate cells move to the neighboring cells. Thus finally the zygospore bearing cells are seen to alternate with empty cells in a filament. Zygote secretes thick three layered wall, turns black and is called zygospore.
(b) Scalariform Conjugation: This type of conjugation is more common and the species which show only scalariform conjugation are heterothallic. Here fusion occurs between two similar gametes (isogametes) coming from cell of two spearate filaments.
In the beginning, the two filaments get adhered by mucilage and lie parallel to each other. Papilla (tubular outgrowths) develops in opposite direction from the walls of cells of the adjacent filaments. Due to growth and elongation of the papillae, the two filaments are pushed apart. A tube like passage known as conjugation tube is formed by dissolution of the spearating wall of the two papillae.
These processes give the whole structure the appearance of a ladder and hence it is scalariform or ladder like conjugation. After formation of conjugation tube protoplasm of both cells start contracting and transform to non flagellate gametes.
The gamete which is more active is called male gamete. It passes through the conjugation tube and fuses with female gamete in the cell of the adjacent plant. Fusion product are known as the zygote (2n). The Zygote develops a thick wall. Zygotes, may be called zygospores since isogametangial copulation occurs here:
Zygospores are formed in a series in one filament (female) while the other filament (male) becomes practically empty except for a few vegetative cells here and there.
Sometimes conjugation takes place in three filaments, where the middle one bears the zygospores.