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The following points highlight the three families under which chlorellaceae has been classified. The families are:- 1. Chlorellaceae 2. Coelastraceae 3. Hydrodictyaceae.
Family # 1. Chlorellaceae:
This family includes the unicellular non-motile genus Chlorella which is a tool of many physiological experiments. It reproduces by the development of non-motile spores.
Genus Chlorella:
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This unicellular alga has a very varied habitat. Cells are solitary or sometimes aggregated into groups being enveloped by mucilage. They may be free-living in stagnant pools. Species of Chlorella may also be terrestrial or may occur abundantly in habitat rich in organic matter (sewage waters and similar other places), within the cells of some invertebrates forming a symbiotic association, and in the exudation of trees.
Being one of the simplest forms of algae, Chlorella has been widely used in the study of respiration and photosynthesis. Chlorella is used for the purification of air in space capsule. Besides this, Chlorella has a great economic importance for it has been reported to yield an antibiotic, known as Chlorellin.
The alga reacts remarkably under cultural condition with or without the supply of light. The cells of Chlorella are small, spherical, rarely ellipsoid. Each cell has a thin wall enclosing a single cup-shaped parietal chloroplast with or without pyrenoids, and a nucleus which is centrally placed (Fig. 36A).
Reproduction is effected by the production of autospores which are developed within a cell by the successive division of the protoplast (Fig. 36B).-Two or even sixteen autospores may be formed in each cell which are liberated out by the rupture of the parent cell wall (Fig. 36C). These immobile spores form new individuals.
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Some Indian species:
Chlorella conglomerata; C. ellipsoidea; C. parasitica; C. vulgaris.
Special features:
1. Unicellular non-flagellate plant body—solitary or aggregated in groups.
2. Cup-shaped chloroplast.
3. Reproduction by autospores.
Family # 2. Coelastraceae:
In this family the vegetative cells of the coenobia are grouped in one plane with the long axes of the cells parallel to one another. Each cell possesses laminate chloroplast. Reproduction is by division of cells of the coenobium.
Genus Scenedesmus:
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Coenobia occur abundantly in fresh-water pools as a part of planktonic flora. Each coenobium is a flat plate of usually four to eight elongated cells, arranged with their long axes parallel to each other. The two end cells of a coenobium often develop processes which are elaborations of the mucilaginous layer of the cell membrane.
Cells of the coenobium contain a single nucleus and a single, longitudinal laminate chloroplast with a prominent pyrenoid (Fig. 37A). The only method of reproduction is by the division of a protoplast leading to form a daughter coenobium, which may or may not have the same number of cells as that in the parent coenobium (Fig. 37B to E). Globular resting spores are known in a few species (Fig. 37F to I).
Some Indian Species:
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Scenedesmus acuminatus (Lagherheim) Chodat.; S. bijugatus (Turpin) Kutz.; S. obliquous (Turpin) Kutz.; S. perforatus Zemmerman.; S. quadricauda (Turpin) Brebisson.
Special Features:
1. Flat plate-like coenobium of elongated cells arranged with their long axes parallel to each other.
2. The two end cells of a coenobium often develop processes.
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3. Laminate chloroplast.
4. Reproduction by the division of a protoplast leading to form a daughter coenobium.
Family # 3. Hydrodictyaceae:
The members of this family occur in the form of colonies in fresh-water ponds and ditches. Sexual reproduction is Isogamous.
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Genus Pediastrum:
This jewel-like beautiful alga occurs as a free-floating disc-shaped coenobium in pools, ditches, and forms a component of the plankton flora of lakes. The alga sinks to the bottom of the pool when the water-level recedes. Its abundance is very rare. The coenobia are flat and are formed of as many as 128 plane-faced or lobed cells arranged in a single layer producing a stellate plate-like in appearance (Fig. 38 A).
The outer cells differ from the internal ones for having radiating processes (Fig. 38 B). Young cells have a single parietal chloroplast with one to four pyrenoids and are uninucleate, while the older ones are multinucleate.
Asexual reproduction is by biflagellate zoospores formed by successive nuclear division followed by progressive cleavage of the protoplast (Fig. 38D & E). Every cell in a coenobium is capable of giving rise to zoospores, but simultaneous division of the protoplast of the mother cells is rather rare.
The zoospores produced by a cell are enclosed in a thin membrane and are suddenly liberated into an external vesicle through a slit in the wall (Fig. 38 C).
They swarm around inside the vesicle for a short time, then arrange themselves in a position like that of the cells of the parent coenobium, becoming motionless (Fig. 38F to H). The cells then acquire mature characteristics before liberation. Thus, this alga lends some support to the suggestion that this non-motile line of green alga descended from flagellated ancestors.
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Pediastrum reproduces sexually by spindle-shaped biflagellate isogametes produced like zoospores (Fig. 38 I). The isogametes fuse in pairs to form a zygote which increases remarkably in size (Fig. 38 J to L). Zygote on germination produces a number of zoospores which when liberated swim in water (Fig. 38 M & N).
Each zoospore loses its flagella comes to rest, becomes angular in shape, and subsequently turns into a thick-walled polyhedral cell, known as ‘polyeder’ (Fig. 38 O). The polyeder now increases in size. Its protoplast divides to form a number of zoospores which ultimately form a new coenobium in the process described earlier (Fig. 38P & Q). The zygotic nuclear details have not yet been clearly worked out.
Some Indian species:
Pediastrum duplex Meven var. loktakense Bruhl et Biswas; P incavatum Turner.
Special Features:
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1. Free-floating disc-shaped coenobium composed of single layer of cells of which the outer ones having radiating processes.
2. Parietal chloroplast.
3. Asexual reproduction by biflagellate zoospores.
4. Sexual reproduction isogamous.
5. The zygote gives rise to zoospores from which specialized structures—potyeders are developed which again produce zoospores. These zoospores ultimately give rise to new coenobium.
Genus Hydrodictyon:
This is a large macroscopic coenobic alga commonly known as ‘water net’ because of the fact that the vegetative plant body grows as a net-like structure usually floating on the surface of quiet fresh-Water. The coenobium is composed of free-floating network of cylindrical to broadly ovoid cells that are united to form a sac-like reticulum (Fig. 39A).
The meshes of the net, formed by five or six coenocytes, are of variable size being pentagonal or more usually hexagonal reaching a length of 8 to 20 cm. (Fig. 39B). The angles are formed by the junction of three of the cells (Fig. 39C).
Young cells contain a single nucleus and a single pyrenoid, but older cells become multinucleate with cytoplasm containing a large reticulate chloroplast with many pyrenoids forming a lining layer around a large central vacuole (Fig. 39D).
Asexual reproduction is by means of biflagellate uninucleate zoospores that are formed by progressive irregular cleavage of the protoplast of a coenocyte (Fig. 39E). The zoospores after swarming within the parent coenocyte, come to rest and arrange themselves to form a new net. The young net is at first enclosed within the-old mother cell wall, but this soon gelatinizes and the net liberates out (Fig. 39F).
Sexual reproduction is isogamous and the plants are monoecious. The biflagellate isogametes are formed in the same manner as the zoospores, but are larger in number and smaller in size (Fig. 39G). They are liberated out of the parent cell through a hole in the parent cell membrane. The fusion product of the isogametes is a thin-walled spherical zygote.
After a short period of rest the resulting zygote nucleus undergoes reduction division forming four zoospores (Fig. 39H).
These zoospores, after a period of motility, come to rest, and are transformed into non-motile aplanospores, each of which increases remarkably in size and develops into polyhedral cell also known as polyhedron (Fig. 391). When enlargement ceases, the contents of the polyhedron divide to form many zoospores.
These zoospores come together and form a new net in the manner already described (Fig. 39J & K). Thus Hydrodictyon is essentially a collection of a number of individual plants, because it arises from a number of zoospores (Fig. 40).
Indian species:
Hydrodictyon indicum Iyeng.
Special Features:
1. Large net-like macroscopic coenobium.
3. Cells of the net coenocytic.
3. Reticulate chloroplast.
4. Asexual reproduction by biflagellate zoospores.
5. Sexual reproduction isogamous.
6. Development of polyhedral cell and post-sexual stages have similarity with Pediastrum.
