The following points highlight the two families under which zygamatales has been classified. The families are: 1. Zygnemataceae 2. Desmidiaceae.
Family # 1. Zygnemataceae:
This family includes fresh-water multicellular filamentous forms of algae.
Genus Spirogyra of Zygnemataceae:
It is one of the common green algae occurring chiefly in ponds, pools, ditches, and similar other places. It prefers still, rather than running water and is one of the free-floating algae known as ‘pond scum’. Quite often it is present in immense quantities, the filaments forming floating masses, sometimes several acres in extent, buoyed up by the bubbles of oxygen which their assimilation has produced.
The plant is a multicellular, unbranched filament which is slippery to the touch because it is covered with a mucilaginous sheath of pectose (Fig. 71 A).
There is no distinction of base and apex and all the cells are alike throughout the filament. Within the stratified cell wall of the cylindrical cells composing the filaments there is the primordial utricle. Embedded in this are one or more spirally arranged ribbon-like chloroplasts with un- smooth edges, which form the most striking feature of the plant (Fig. 71B & G).
Each chloroplast is studded at intervals with several pyrenoids. In every cell there is one nucleus which is, either suspended in the centre by means of cytoplasmic strands, or embedded in the primordial urticle.
The nucleus contains a large nucleolus. The cross- walls of the filaments are essentially of five types, for example, plane, replicate, semireplicate, colligate and unduliseptate. In some species the middle lemelia of the cross- wall develops on either side a ring-like in growth on which the subsequent apposition- layers are deposited, such a septum is known as replicate septum (Fig. 72A to C).
Strangely enough although Spirogyra is an aquatic alga, it produces neither zoospores, nor flagellate gametes. Except for the occasional breaking up of a filament into its constituent parts, Spirogyra possesses no asexual means of propagation. Its normal reproduction is always sexual, and is accomplished by conjugation—an aplanogametic isogamy.
In this process, two vegetative cells combine their contents to form a zygote.
Usually, conjugation takes place between the cells of two different filaments, but it may also take place between adjacent cells of the same filament. In the former type, the cells in two filaments which are in contact, form protrusions which lie opposite one another and as they elongate, the filaments are pushed apart (Fig. 73A).
Finally, the walls separating the protrusions of the conjugating cells are dissolved developing an opening and in this way a conjugation tube is formed (Fig. 73B).
Meanwhile, the contents of the two participating cells contract. Fusion of the contracted cell-contents takes place within one of the two conjugating cells (Fig. 73B & C). This type of conjugation is known as scalariform conjugation. The active gamete may be designated as a male gamete and the passive as a female gamete though they are morphologically similar.
In a few species where conjugation occurs between the adjacent cells of the same filament, small protrusions arise as a result of the growth of the lateral wall on either side of the cross-wall or the participating cells (Fig. 74A & B).
With the growth of the protrusions the cross-wall also becomes stretched. Finally, as the cross-wall cannot keep pace with the growth of the protrusions, the latter lose their contact with the former, a side link is thus established between the two cells (Fig. 74C); through which the protoplast of one cell migrates into the other (Fig. 74D) fusing to form a zygote (Fig. 74E).
This is called lateral conjugation. The fusion product of conjugation is a zygospore which develops three-layered thick wall and is highly resistant to extreme cold and drought (Fig. 74F). The zygospore generally sinks to the bottom of the pool where it is formed, and germinates after a period of rest.
During germination-the two outer layers burst open, the innermost cellulose wall elongates forming a cylindrical germ tube which divides transversely to form a new filament (Fig. 75E &F).
Meiosis usually occurs during germination of the zygospore and thus the new filament is haploid (Fig. 76). Three of the four haploid nuclei formed in meiosis degenerate, so that each filament is derived from a haploid uninucleate cell (Fig. 75A to D). Sometimes the contents of certain cells of a filament round up and secrete walls to become spores parthenogenetically without any sexual fusion.
Such spores are known as parthenospores or azygospores (Fig. 75 G & H).
The amoeboid gametes involved in the sexual process do not exhibit any morphological differences. Species in which only scalariform conjugation occurs are no doubt in part dioecious with genotypic sex determination. The morphologically similar gametes exhibit difference only in their behaviour.
A situation where one of the fusing gametes is active and moves to the other awaiting gamete, indicates a case of physiological heterogamy. From the behaviour, the active gamete may be known as ‘male’ and the passive as ‘female’. This differentiation of sex between the gametes is also accompanied by sexual differentiation between the filaments.
The filaments bearing cells which produce active gametes behave as male and those having cells with passive gametes act as female. Thus such filaments are unisexual and the species having filaments of this nature as dioecious.
Some Indian species of Zygnemataceae:
Spirogyra maxima (Hass.) Wittr.; S. neglecta (Hass.) Kutz.; S. nitida (Dillw.) lank.
Special features of Zygnemataceae:
1. Filamentous un-branched plant body without being differentiated into base and apex.
2. Presence of replicate septum.
3. Spiral chloroplast.
4. Absence of flagellate gametes.
5. Sexual reproduction by conjugation—morphological isogamy, but physiological heterogamy.
6. Conjugation both lateral and scalariform.
Sex differentiation:
The conjugating cells of the filaments of Spirogyra behave as gametangia since the entire content of each cell metamorphoses into a gamete. The gametes produced are morphologically similar and are thus isogametes. But they exhibit difference in their behaviour.
One of them is active and moves from the cell of the filament where it is developed to another, where its fusing partner waits remaining passive for fusing with it. This situation – may be defined as primitive anisogamy or physiological heterogamy.
The active gamete thus behaves as male and the passive as female. This type of sex differentiation is also associated with sex differentiation between the filaments of whose cell contents are taking part in conjugation. The filament whose cells produce active gametes are sexually distinct from that whose cells produce passive gametes.
Both the filaments are thus unisexual. In spite of such sex differentiation the sexual reproduction in Spirogyra is isogamous since there is no other visible difference between the fusing gametes.
Genus Zymes:
This alga is very commonly found to occur as yellowish free-floating masses in considerable quantities on the surface of slow flowing as well as quiet fresh-water particularly, in ponds and wayside ditches.
The genus Zygnema is closely related to Spirogyra in many respects, differs only in having two stellate chloroplasts embedded in the cytoplasm, each with a central pyrenoid, with the nucleus embedded in a mass of cytoplasm both the isogametes by amoeboid movement move towards each other and unite in the conjugation tube to form a zygospore (Fig. 77F).
Again one of the two fusing gametes may be active (male) and by amoeboid movement may pass through the conjugation tube, meet and fuse with the other passive (female) gamete lying in the cell where it is formed (Fig. 77H). The cell in which conjugation takes place is the conjugating cell. Here gametes though morphologically similar exhibit physiological anisogamy.
Zygospore formation is more common in the conjugating cells indicating a tendency of differentiation of sex. Again sexual dimorphism has been reported in certain species of Zygnema where the cells producing passive gametes are longer and broader with larger chloroplasts and pyrenoids than the cells producing the active gametic partner.
But since both isogamy and physiological anisogamy, as well as lateral and scalariform conjugation have been reported from the same species, the sex differentiation may not be very pronounced. The process of zygospore formation, and the behaviour of the zygospore and its diploid nucleus with ultimate development of a new haploid filament, all are very similar to those of Spirogyra.
When conjugation fails, azygospore may be developed (Fig. 77I).
Some Indian species of Genus Zymes:
Zygnema giganteum Randhawa; Z- gorakhporense Singh; Z- indicum Misra; Z- pectinatum (Vauch. ) Ag.; Z. terrestre Randhawa.
Special features of Genus Zymes:
1. Filamentous un-branched plant body not differentiated into base and apex.
2. Stellate chloroplast.
3. Absence of flagellate gametes.
4. Conjugation both scalariform and lateral.
Family # 2. Desmidiaceae:
This family includes fresh-water unicellular forms of algae.
Genus Cosmarium of Desmidiaceae:
The genus Cosmarium is commonly found free-floating in fresh-water small ponds and ditches, either solitary, or in amorphous colonies. Single-celled, non-motile plants of rare beauty with length rarely more than one and a half times the breadth, are characterized by the fact that the cells are usually divided into two symmetrical halves by a median constriction.
The two halves are referred to as semi cells; the constriction is called the sinus 5 and the narrow connecting zone of the two halves is known as the isthmus (Fig. 78A).
The cells may have smooth or ornamented wall (Fig. 78C) which when mature is composed of two well-differentiated layers. The inner one of cellulose and the outer is firmer and thicker and is composed of cellulose impregnated with various substances including iron compounds (Fig. 78D).
Numerous small pores occur in the cell wall except in the region of isthmus (Fig. 78B).
Through these pores mucilage is secreted which sometimes resembles like a third layer of the cell wall.
Most of the species have two relatively large chloroplasts with radiating plates having centrally placed pyrenoids, one located in each semicell. The single nucleus, embedded in a small mass of cytoplasm, is in the isthmus (Fig. 78A). Reproduction is by cell division and by the process of conjugation which resembles that in Spirogyra and Zygnema.
As soon as the cell division starts the nucleus divides mitotically into two daughter nuclei (Fig. 78E & F) and is followed by the elongation of the isthmus region.
Then there appears a constriction in the isthmus in between the two daughter nuclei dividing the parent cell into two daughter cells by the formation of a wall (Fig. 78F & G). The portion of the isthmus of each daughter cell later grows into a new semicell (Fig. 78H).
A new chloroplast is formed in each of the new semicells by the division of the existing chloroplast and pyrenoid of the old semicell.
Hence, the two semicells that constitute the Cosmarium plant, one is always younger than the other (Fig. 78H). The younger semi cell is at first very small, but it gradually attains the same size as the other semicell (Fig. 78 I).
Sexual reproduction is by conjugation, a process of aplanogametic isogamy (Fig. 80), in which the contents of the two conjugating cells behave as gametes. To start with the process of conjugation, the two conjugating cells come together, lie side by side, and are embedded in a common mucilaginous sheath (Fig. 79A).
The cells split open at the isthmus and the semicells separate allowing the protoplasts to emerge (Fig. 79B). Each protoplast behaves as a gamete.
The gametes move towards each other by slow amoeboid movement and finally fuse together forming a zygote—zygospore which surrounds itself with a three-layered heavy wall (Fig. 79G), the outermost layer of which may be smooth or spiny (Fig. 79D).
The zygospore sinks to the bottom of the pool and after a period of dormancy, the diploid zygote nucleus divides meiotically forming four haploid daughter nuclei, of which two are functional and the other two gradually disintegrate (Fig. 79E & F). The zygospore chloroplast then divides into two daughter chloroplasts (Fig. 79G).
Each daughter chloroplast with one functional nucleus forms a new plant (Fig. 79H). The formation of parthenospore has also been reported in some species of Cosmarium.
Some Indian species of Genus Cosmarium:
Cosmarium connatum Breb.; C. distichum Nordst.; C. granatum Dreb.; C. moniliforme (Turp.) Ralfs.; C. quadrilaterum Bruhl et Biswas; C. tenne Arch.
Special features of Genus Cosmarium:
1. Unicellular plant body differentiated into semicells and isthmus.
2. Star-shaped chloroplast.
3. Cell wall with characteristic pores.
4. Characteristic vegetative cell division.
5. Sexual reproduction by conjugation.
6. Gametes are non-flagellate.
Genus Closterium of Desmidiaceae:
This is essentially a free-floating alga occurring abundantly in small ponds and ditches forming a component of the planktonir flora, may also be present at the bottom of pools. Vegetative cells are solitary, elongate with no median constriction. In most species the cells are curved with attenuated apices being markedly lunate or arcuate (Fig. 81 A).
The highly perforated cell wall is composed in two layers—an inner structure-less cellulose layer surrounded by an outer firmer and thicker layer made of cellulose impregnated with pectic and iron compounds.
In some species delicate ridges at intervals running from pole to pole appear as striations. In addition to cell wall, the cells are surrounded externally by a coat of mucilage which is secreted by the protoplast through the performations. An individual cell exhibits movement in constant, with a substratum. This movement is facilitated by the secretion of mucilage by the cells.
Each cell has a single nucleus which is embedded in the cytoplasmic mass and is median in position in the region of the isthmus (Fig. 81 A). Axile chloroplast, one in each semicell bearing one to several pyrenoids arranged in linear series in the axis, is present in each cell. At the extremities there are vacuoles having one in each pole.
Cell division may continue indefinitely without conjugation. During cell division a slight internal thickening and a slight external constriction appear in the wall at the isthmus region (Fig. 81B). With the elongation of the cell this local thickening becomes drawn out into a cylindrical strip. The nucleus divides mitotically to form two daughter nuclei (Fig. 81C to E).
Then there appears from inside the strip a septum between the two nuclei resulting in the division of the cell into two daughter cells (Fig. 81F & G).
These daughter cells are cone-shaped in outline (Fig. 81G) and each one carries half of the isthmus. Each half of the isthmus then bulges out to form the new semicell. The old chloroplast divides into two daughter chloroplasts which are distributed into two semicells—the old semicell and the new semicell (Fig. 81H & I).
This is how a Closterium cell is composed of one old semicell and a newly developed semicell (Fig. 81I).
During conjugation the two individuals become enveloped by mucilage sheath (Fig. 82A). They arrange themselves parallel to each other in pairs. Protrusions arising at the isthmus from the two paired cells meet and the two cells are pushed apart (Fig. 82B).
As a result of which the cells break at the point of isthmus and the contents of the two cells are liberated out, behave as gametes fusing together to form a zygospore (Fig. 82G & D). In some species conjugation may also take place by the development of conjugation tubes. The zygospore goes to rest by developing a thick wall.
On germination the zygospore nucleus divides twice of which one is reductional, forming four haploid nuclei. Of the four haploid nuclei, two degenerate. The two surviving nuclei along with the two daughter chloroplasts take part in the formation of two new Closterium cells.
Special features of Genus Closterium:
1. Free-floating solitary, elongate single-celled plant body with no median constriction.
2. Cells are usually curved with attenuated apices possessing lunate appearance.
3. Perforated cell wall.
4. Asexual reproduction by cell division.
5. Axile chloroplast.
6. Sexual reproduction by conjugation.