Spirogyra (Pond Scum): Definitive Case Study To Pond Scum (Updated)
Spirogyra (Pond Scum) – Characteristics, Vegetative and Sexual Reproduction (Scalariform Conjugation, Lateral Conjugation), Parthenogenesis
Spirogyra algae is a widely distributed large genus consisting of about 300 species found throughout the world. It grows in fresh water as free-floating mass and thus commonly called pond scum. It is found growing in fresh water 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.
Life of Fresh Water Algae (Pond Scum)
Though it prefers to grow during cold season, it thrives well throughout the year but a little rise in temperature favors 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) involves the plant body and the cell structure.
The plant body consists of filaments which are cylindrical and unbranched. The young filament is found attached to some object at the bottom by modified basal cell. The modified basal cell which “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.
Single row of cylindrical cells is present in each filament. Each cell consists of a firm cell wall enclosing a mass of protoplast. 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 become 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, 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 presence of 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 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 (1965) study with the help of electron microscope reveals the presence of photosynthetic bands in chloroplast each with 4 to 12 thylakoids, numerous pyrenoids en-sheathed in starch, nucleus with complex nucleolus, Golgi bodies, mitochondria and endoplasmic reticulum.
Spirogyra under Microscope
The microscopic view of spirogyra is as follow:
Reproduction in Spirogyra
Spirogyra generally reproduces by vegetative and sexual methods. However, some workers have also reported asexual reproduction in Spirogyra through akinetes and aplanospore formation.
In Spirogyra the fragmentation is the typical/common method of vegetative reproduction. Accidental breaking or injury breaks the filament into 2-3 cell portions each germinates 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 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 reach the opposite cell and fuses with the gametes of the other filament. As a result, zygospore is formed in one cell and the other cell stands empty. On the basis of behavior, the motile cell is called male gamete and other female gamete.
In general, 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 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 sexual reproduction process and as a result both the filaments appear as 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 i) the direct lateral conjugation and ii) indirect lateral conjugation.
Direct Lateral Conjugation: This type of conjugation takes place between the two adjacent cells (S. jogensis) of which upper cell functions as male gametangium (structure producing gametes) and the lower cell as female gametangium.
The lower cell increases in size but upper cell remains smaller. The protoplast of male cell forms an outgrowth which elongates and comes in contact with transverse wall in between male and female cells. Later it passes through by forming an opening in it. Thus, without formation of conjugation tube the contents of male cell passes into the female cell and results 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 on further course of development breaks and forms a side passage between the two cells. Of these, the upper cell behaves as male gametangium while lower one as female. Through this side passage, the male gamete passes through and fuses with female gamete and as a result 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 conjugation tube (S. collegate). There are reports of simultaneous presence of scalariform and lateral conjugation in S. gratiana. Such a case is quite rare.
Zygote is the fusion product in either case which soon develops a thick wall and becomes zygospore composed of three-layered wall. The zygospores are liberated by the decay of gametangial wall and settle down to the bottom of water body. It undergoes a resting period. Prior to 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.
Natural form of asexual reproduction, in which the growth and development of embryo takes place without fertilization. In plants, rare cases involves, 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.