Spirogyra:
Kingdom : plant
Subkingdom- : Cryptogams
Division: Thallophyta
Sub-division Algae
Class: Chlorophyta
Sub-class: Chlorophyceae
Cohort: Zygnematales (Conjugales)
Family: Zygnemataceae
Genus: Spirogyra
Occurrence :-
Spirogyra is found in the form of bright green free floating masses in freshwater reservoirs, ponds, rivers, lakes and slow flowing streams. Spirogyra is found floating on the surface of water. The thallus of spirogyra covered by mucilage therefore the filaments appear slimy & smooth. Hence spirogyra is also known as “Pond silk" or “water silk”. Genus includes about 289 species and out of these 94 have been reported from India.
Morphology: -
The plant body is called as thallus. The thallus is in the form of silky, hair-like filament. The spirogyra is multicellular, unbranched, cylindrical & filamentous. These filaments are long, cylindrical, free-floating and slimy. As all its cells are similar in structure, the thallus is termed as homothallus. The cells are arranged end to end, one above the other in a single row. All the cells in the filaments are similar and usually much longer than broad. The filament cannot be differentiated into apex or base, but some Species have basal cell with which the filament is attached to the rock. This basal cell is known as hold fast. During day time, O2 evolved due to photosynthesis fills the interstices of the filaments and keep them floating on the surface of water.
Cell - structure:
The cells are cylindrical. Their length is more than the width. It consist of two parts the cell wall and the protoplast.
The structure of cell wall is similar in all the cells of the filament. It is thin & made up of 2 layers. The inner layer is made up of Cellulose while the outer layer is formed of pectin. The outer part of pectose layer is converted into pectin, which dissolves in water to form a mucilaginous sheath. This mucilage sheath is considered as a third layer of cell wall. Due to this reason the spirogyra filaments are slippery. Normally the size of the cell is about 0.25 mm.
Protoplast: It is differentiated into plasma membrane, cytoplasm and nucleus. The plasma membrane is thin and living. The central portion of the cell is occupied by a large central vacuole because of that cytoplasm is divided into two parts. The cytoplasm which is found attached with cell wall is known as ectoplasm and the one which surrounds a centrally located nucleus is known as endoplasm. Fine cytoplasmic strands keep ectoplasm & endoplasm interconnected. A big central vacuole surrounds the endoplasm & the nucleus. The ectoplasm surrounding the vacuole is called primordial utricle. The nucleus is kept suspended within endoplasm by means of cytoplasmic strands.
The nuclear possesses 1 to 2 nucleoli. The cell possesses all the organelles.
The chloroplast are vary in numbers from 1 to 16. Each chloroplast is an elongated, green, flat, ribbon like band which is spirally arranged from one end of cell to the other end. The twisting of the spiral shaped chloroplast is on the left side. The margin of the chloroplast may be uniform, wavy or serrated. . A series of small, rounded, spherical bodies situated at equidistance in the green band of chloroplast. These structures are called as pyrenoids. These pyrenoids store protein in the centre and starch in the surrounding region.
Reproduction:
Two types of reproduction are found in spirogyra viz.
l) Vegetative (2) Sexual.
Vegetative reproduction
i) Fragmentation
ii) Aplanospores
iii) Akinete
i) Fragmentatio:
It takes place normally in favourable conditions. In favourable conditions, some of the intercalary (in between) cells are destroyed. The result is the fragmentation of the filament into small pieces. Each piece or fragment survives and when conditions are favourable, each fragment will grow in length by cell division, attaining the full length of the filament. Thus as many fragments as many filaments are formed.
a)
Mechanical injury:
the rapid flow of water, biting of an aquatic animals or the animals which come to drink water may cause fragmentation.
b)
By sudden changes in the habitat factors such as temperature or acidity of water the middle lamellae of the end walls become gelatinous. Consequently a turgor difference arises between adjacent cells, causing one cell to bulge in to the other. The strain on the junctions between the cells finally reaches the breaking point.
c)
In some species with replicate end walls the rings of one cell get evaginated forcing apart the cells.
d)
In some species formation of H- shaped piece (H0colligata) in the end walls of adjacent cells also causes fragmentation. The H-piece slips off. Consequently the two cells come apart.
Aplanospores :-
S. aplanospora (Randhava-1938) reproduces by aplanospores. Aplanospores formed singly in the vegetative cells.
iii) Akinete :-
S. farlowii (Transeau-1915) reproduces by formation of thick lamellated walled akinets.
Sexual reproduction:
spirogyra reproduces sexually by conjugation. In Spirogyra isogamous type of sexual reproduction is seen. The gametes are similar in size and shape The gametes of spirogyra are aplanogametes (non-.motile). When conditions are unfavourable, spirogyra reproduces sexually.
There are two types of Conjugation. (1) Scalariform (2) Lateral.
Scalariform conjugation:
This is seen in the beginning of summer season. During this time two filaments lie closer & remain parallel in such a way that their cells face each other. Both the filaments remain covered by the same mucilage covering. The cells face each other & by the touch stimulus, they are stimulated to form small protuberances. Both the opposite protuberances grow & meet each other. By the action of enzymes, catalase & cellulase, their contact walls dissolve forming a continuous tubular passage, called conjugation tube. Such tubes are formed successively between each pair of the cells of the 2 filament. They appear like a ladder or staircase. Hence the name Scalariform conjugation.
At this time, the protoplasm of each cell of both the filaments shrinks & forms a gamete. The cells are referred as gametangia. The gametes of one filament become active and they pass through the conjugation tube by amoeboid movement and meet the inactive gametes of the other filaments. The active gametes are known as male gametes. While the inactive gametes are known as female gametes Thus the fusion of male & female gametes takes place This Is termed as conjugation. As a result of conjugation, diploid cell, called zygote is formed. The zygote is covered by a protective layer and it is now known as zygospore. The cell if both the filaments as well as conjugation tubes disintegrate, liberating the zygospores. The zygospores sink at the bottom & remain dormant for some period
Spirogyra—Transfer of one aplanogamete to other aplanogametangia A. Two filaments lying parallel to each other, B. Formation of papillate outgrowths to form conjugation tube, C. Ladder-like (Scalariform) conjugation. One aplanogamete moving into the aplanogametangia of the other filament, D. Zygospores formed in one filament and empty aplanogametangia of the other filament
Lateral -conjugation:
In some species of spirogyra, lateral conjugation is found. A conjugation tube is formed on the lateral sides of a adjacent cells of the same filament: It is termed as lateral conjugation. Lateral conjugation is of the two types.
(I) Indirect: In this type of lateral conjugation, small protuberance develops from each of the adjacent cells on the same lateral side. The two protuberances then meet each other and as the growth of the transverse wall does not keep space with the growth of protuberances, a sort of lateral conjugation tube is formed between 2 adjacent cells.
Now, the protoplasm of these cells shrinks and forms the gametes. The active gamete i.e. male gamete now passes through the lateral conjugation tube by amoeboid movement & meets the inactive i.e. female gamete. Both these gametes fuse with each other, forming a diploid zygote. The zygote then gets covered by a protective layer and is termed as zygospore. Male gamete producing cells & female gamete producing tells may be found alternatively or in alternative pair in the filament.
(ii)
Direct: Prof. O.P. Ayenger noticed that in spirogyra jogensis this type of lateral conjugation occurs. No conjugation tube is formed in this type of lateral conjugation. First the protoplasm of cells shrinks & forms the gametes. Now the active i e male gamete directly pushes the partition wall and it gets perforated. The male gamete enters through the perforation and fuses with the inactive or male gamete forming a diploid zygote. The zygote later on changes into zygospore by enveloping itself into a protective sheath.
Zygospore and its germination:
Zygopspore is elliptical or spherical body covered by 3 layered protective walls. It is diploid and hence corresponds to sporophytic generation. The cells of the filament are destroyed and the zygospores are liberated and sink at the bottom. Here they remain in dormant condition for a period of 4 to 6 months. The chloroplast gets destroyed. Starch is converted into oil. 3It therefore appears grayish or yellowish. When the favourable season approaches, the zygospore germinates. This time the outer two layers break off and the inner cellulose layer grows as germ tube. Its diploid nucleus undergoes the reduction division forming 4 haploid nuclei. 3-of these haploid nuclei, degenerate and the one remaining shift into germ tube. Now by transverse division the germ tube divide into two cells. A proximal cell helps in attachment to the substratum while the apical cell divides and redivdes forming the filament of spirogyra.
Parthenogenesis:
Sometimes when conditions are, not very favourable, the filaments are arranged adjacent to each other and forms gametes but conjugation tubes-are not formed. So, the gametes, cannot fuse at such time, the gamete itself gets covered by a protective layer and forms a structure similar to zygospore. But it is haploid and it is known as azygospore. When conditions become favourable, the azygospore germinates without reduction division into new filament. This is known as parthenogenesis as parthenogenesis.
Life cycle of Spirogyra:
Gametophytic stage is the principal stage in the life cycle of spirogyra. It is haploid and produces haploid gametes. By conjugation a diploid zygote is formed. Being diploid, it represents a sporophytic stage. This is a very recessive and dormant stage. In favourable conditions, it germinates by meiosis forming again a gametophytic stage. Thus, in the life cycle gametophytic stage alternates with a sporophytic stage and so alternation of generation is found.
