Spirogyra algae is a widely distributed large genus consisting of about 300 species found throughout the world. It grows in freshwater as a free-floating mass and is thus commonly called pond scum.
It is found growing in freshwater stagnant reservoirs and also in slow-running streams and rivers and looks like a mass of long shining silky filaments. That is why it is also known as pond silk.
Though it prefers to grow during the cold season, it thrives well throughout the year but a little rise in temperature favors the reproductive stage instead of vegetative growth. The common and frequently occurring species are S. elongata, S. communis, S. microspora, S. indica, S. crassata.
Classification Of Spirogyra
- Class: Chlorophyceae
- Order: Conjugales
- Sub-order: Zygnemoideae
- Family: Zygnemaceae
- Genus: Spirogyra
The peculiar characteristics of spirogyra (colonial algae) involve the plant body and the cell structure.
Spirogyra Plant Body
The plant body consists of filaments that are cylindrical and unbranched. The young filament is found attached to some object at the bottom by a modified basal cell.
The modified basal cell that “helps in the attachment is called hapteron or holdfast. The rhizoidal outgrowth in this region has been reported in S. rhizoides and S. dubia. The adult plants are always free-floating.
Spirogyra Cell Structure
A single row of cylindrical cells is present in each filament. Each cell consists of a firm cell wall enclosing a mass of protoplast. A dual-layered cell wall is present, the inner layer is composed of cellulose while the outer layer is made up of pectic substances.
The pectic substance becomes gelatinous in the presence of water and thus gives the plant a slimy touch. The protoplast consists of a single nucleus, a mass of cytoplasm, a variable number of flat ribbon-shaped (1 to 12 sometimes 24) chloroplast, and a large central vacuole.
The nucleus is found centrally suspended by strands of cytoplasm or it may be parietal in position. The cytoplasm is peripheral due to the presence of a large central vacuole which is traversed by several cytoplasmic strands.
The number of the chloroplast is the characteristic feature of this alga. Their arrangement is also specific and spiral, i.e., twisted to the right in the ascending order. The name of the alga is given after the spiral arrangement of a chloroplast.
Each chloroplast remains studded at intervals with linearly arranged pyrenoids. The transverse septa between the cells may be plane, replicate or semi-replicate colligate, etc.
Darves’s (1965) study with the help of an electron microscope reveals the presence of photosynthetic bands in chloroplast each with 4 to 12 thylakoids.
Numerous pyrenoids were en-sheathed in starch. Nucleus with the complex nucleolus, Golgi bodies, mitochondria, and endoplasmic reticulum.
Spirogyra Under Microscope
The microscopic view of spirogyra is as follows:
Vegetative Reproduction In Spirogyra
Spirogyra generally reproduces by vegetative and sexual methods. However, some workers have reported asexual reproduction in Spirogyra through akinetes and aplanospore formation.
In Spirogyra fragmentation is the standard method of vegetative reproduction. Accidental breaking or injury breaks the filament into 2-3 cell portions each germinating to produce a new plant.
However, in certain cases, cross-walls also play a role in separating the two cells apart by the process of invagination.
Sexual Reproduction In Spirogyra
Sexual reproduction is isogamous, which occurs by the conjugation of non-flagellated gametes. The conjugations are of two types:
Scalariform Conjugation In Spirogyra
It is of common occurrence in nature and is brought about by the participation of two parallel lying filaments. All cells of a filament are capable to produce gametangia. A single gamete is formed from each gametangium.
The cells destined to produce gametes produce lateral outgrowth in both filaments opposite to each other which soon meet by their tips. The end wall dissolves and as a result, a continuous conjugation tube, conducting the two cells of different filaments is formed.
The protoplasmic content of the cell metamorphoses into a single elliptical gamete. During fusion, one gamete after passing through the conjugation tube reaches the opposite cell and fuses with the gametes of the other filament.
As a result, a zygospore is formed in one cell and the other cell stands empty. Based on behavior, the motile cell is called the male gamete while the other is called the female gamete.
In general, a zygospore is formed in a cell of the filament and the two filaments are thus dioecious. In certain cases, the zygospore is formed in the conjugation tube in which case the gametes of both the filaments move.
In such cases, sex distinction is not possible and neither one can be called male or female. The filaments in such cases are monoecious.
In nature, at a time most of the cells of the filaments participate in the sexual reproduction process and as a result, both the filaments appear ladder-like and hence called scalariform or ladder-like conjugation.
Lateral Conjugation In Spirogyra
It occurs in between the two adjacent) cells of the same filament and, therefore, such plants are bisexual and monoecious. There, are two types of lateral conjugations:
- Direct lateral conjugation
- Indirect lateral conjugation
Direct Lateral Conjugation
This conjugation occurs between the two adjacent cells (S. jogensis).
The upper cell functions as male gametangium (structure producing gametes) and the lower cell as female gametangium.
The lower cell increases in size but the upper cell remains smaller.
The protoplast of the male cell forms an outgrowth that elongates and comes in contact with the transverse wall between male and female cells.
Later it passes through by forming an opening in it.
Thus, without forming a conjugation tube, the contents of the male cell pass into the female cell and result in zygospore formation.
Indirect Lateral Conjugation
It is of rare occurrence and reported in S. affinis. During the process, papilla-like outgrowth is formed in the transverse septum which in the further course of development breaks and forms a side passage between the two cells.
Of these, the upper cell behaves as male gametangium while the lower one is female. Through this side passage, the male gamete passes through and fuses with the female gamete, and as a result, the zygospore is formed. This typed lateral conjugation is also called chain conjugation.
A perfect conjugation tube may be formed between the two adjacent cells to function as a conjugation tube (S. collegate). There are reports of the simultaneous presence of scalariform and lateral conjugation in S. gratiana. Such a case is quite rare.
The zygote is the fusion product in either case which soon develops a thick wall and becomes a zygospore composed of a three-layered wall. The zygospores are liberated by the decay of the gametangial wall and settle down at the bottom of the water body.
It undergoes a resting period. Before germination, the zygospore nucleus divides meiotically to produce four nuclei. Three of four degenerate and only one survives. It germinates to produce a new haploid plant.
The natural form of asexual reproduction, in which the growth and development of the embryo take place without fertilization.
In plants, rare cases involve, that the gametes fail to fuse round off, and secrete a cell wall around and behave as spores (S. mirabilis and S. groenlandica). Such spores are called parthenospores. These spores germinate directly into new haploid plants.