PLANT GROUPS

PLANT GROUPS (3)

General characteristic of the Chlorophyta

1 Habitat Aquatic (mainly fresh water), terrestrial in moist situations, a few Epiphytic

2 Pigments Chlorophyll a & b, β-carotene, α-carotene less prominent

3 Food Reserves Starch, rarely inulin, Oils & Fats

4 Cell Wall Component Cellulose, Crystalline glycoproteins

5 Reproduction Asexual & Sexual (Isogamy, An isogamy & Oogamy

6 Growth Forms Flagellate coccoid, Filamentous rarely Foliaceous or siphonaceous

7 Flagella Two or Four Occasionally numerous

Flagella are present in all classes of algae except Cyanophyceae & Rhodophyceae. There number, nature and mode of Orientation are important. There are two types of flagella.

1) Acronematic or Whiplash types which are smooth & do not have hairs (Mastigonemes) on its surface. 2) Pantonematic or tinsel type which have hairs on its surface.

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Flagella hairs are of two types

a) Fibrous solid hairs b) Tubular hairs

Flagella in Algae & other plant except bacteria are composed of nine peripheral doubled microtubules & two central single microtubules. This is commonly known as (9 + 2 Pattern) of fibrils of flagellum these are produced in a matrix surrounded by Flagellar membrane & Flagellar sheath.

Membrane

Single central tubules

Peripheral doublet tubules

Slide arm

Radial spokes

Cross Section Showing (9 + 2) Pattern ppppPPattern Pattern Pattern) Pattern

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Cell Structure of Chlorophyta

Cellular organization in Chlorophyta as in all other Algae except Cyanophyta is Eukaryotic the cells possess cell organelles

 Cell Wall: The mature cell of possesses a definite cell wall. Usually outer layer is pectic & inner cellulosic.  Chloroplast: Present in many type Reticulate, Peripheral, Stellate, Small discoid & Spiral.  Pyrenoid: In most green Algae the plastids contain one or more specially differentiated regions called Pyrenoids these are the major sites of starch formantion in green Algae.  Eye Spot (Stigma): Most of the flagellated cells that show phototatic movement have an Eye spot. In green Algae the Eye spot is always in the Chloroplast near the flagellar bases in anterior portion.  Mitochondria, Golgi bodies, Endoplasmic reticulum, Ribosomes & Contractile vacuole are present in the cell.

Reproduction

1) Vegetative Reproduction: Portion of the plant body get separated to form new individuals without any obvious change in the protoplast, cell division occur. 2) Asexual Reproduction a) Zoospores are motile & naked spore produced within the Zoosporangium. These frequently move with the help of flagella which maybe two or more. In Chlamydomonas the Zoospore Exactly similar to parent cell & are formed in variable numbers

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(4, 8, and 16). In Filamentous forms, in Ulothrix a cell produces up to (8 or 16) biflagellate or quadri flagellate. In Oedogonium a single multi flagellate Zoospore is formed from each cell. b) Aplanospores are nonmotile & are produced in Aplanosporangia. They are common occurrence in Chlorella. c) Hypnospore these are thick walled non motile. Spores that undergo a long resting period in Sphaerella. d) Akinetes are the direct modification of vegetative cell which becomes converted into a thick wall non motile resting spores is formed in Chladophora. e) Autospores they are non-motile spores resembling the parent cell in shape & structure in Scendesmus. f) Cysts these are resting cells enclosed in a thick envelope is known in Vaucheria. g) Palmella stage occasionally during unfavorable condition.

Sexual Reproduction

1) Isogamy 2) An isogamy 3) Oogamy

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Life Cycle

There are different types of life cycle in Chlorophyta classified into four categories

Haplontic life cycle: The main plant body is haploid & the Zygote being the only diploid stage, in unicellular & filamentous forms.

Main Plant (1n)

Spore (1n) Gamete (1n) Gamete (1n)

Meiosis Fusion

Zygote (2n)

Haplontic Life Cycle

e

Diplontic life cycle: The main plant body is diploid which forms haploid gametes meiotically. Gametes fusion & form diploid Zygote which directly germinates into a new plant in Cladophora.

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Main Plant (2n)

Gametangia

Meiosis

Gamete (+) Gamete (-)

Fusion

Zygote (2n)

Diplontic Life Cycle

Isomorphic alternation of generation: Both haploid & diploid phases are similar in external appearance, a haploid plant produces gametes & a diploid plant produces Zoospores. The Zygote develops into a new diploid plant without undergoing reduction division in Ulva.

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Plant (2n)

Zygote Meiosis

Fusion Spore (1n) Gamete (1n)

Gamete (1n) Plant (1n)

Isomorphic alternation of generation Diplohaplontic Life Cycle

Heteromorphic alternation of generation: The two phases of life cycles are unequal in size different in external appearance; the Zygote does not undergo a reduction division but develops into short diploid filament.

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Classification of Chlorophyta

Chlorophyta divided into (14) orders:

1 Order Volvocales 2 Order Chlorosarcinales 3 Order Tetrasporales 4 Order Chlorococcales 5 Order Chlorellales 6 Order Zygnematales 7 Order Odeogoniales 8 Order Ulotricales 9 Order Chaetophorales 10 Order Chladophorales 11 Order Acrosiphoniales 12 Order Ulvales 13 Order Siphonales 14 Order Caulerpales

Order: Volvocales

Member of this order are Unicellular & Coenobial which are normally motile by means of flagella.

Coenobium a Colony of cells firmly allached to each other in which there is some degree of co-ordination

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In life cycle of these members temporary resting stage, Palmella stage may occurs.

Member of this order represent most primitive, plant life which are widely distributed in fresh water & soil. Some members are found in brackish & marine waters. Volvox is known to cause water blooms.

Unicellular Volvocales Family Chlamydomonadaceae Genus Chlamydomonas

The Chlamydomonas cell represents most primitive member of living green . It’s widely distributed in variety of habitats mainly in fresh water or on moist water few species are found in brackish waters. Chlamydomonas halophile & Chlamydomonas nivalis represent snow algae is found in the Polar Regions imparting red colour. The red colour is due to red pigment Haematochrome.

Solitary free swimming cell maybe ellipsoidal, sub cylindrical, spherical or pyriform, two equal Acronematic flagella are located in the anterior region of the cell which is responsible for motility. Cells have delicate to massive cup shaped chloroplast. There is usually one Pyrenoid within a chloroplast. Usually there are two contractile vacuoles near the base of flagella. A single nucleus lies in the colourless cytoplasm.

The Stigma or red eye spot is embedded at one side of chloroplast & is composed of carotenoid subunits about 75nm in diameter it maybe circular or oval. Positive photo taxis.

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Chlamydomonas

In asexual reproduction the cells of most species become non motile & their nucleus undergo one or more Mitosis which are followed by Cytokinesis. In this way (2, 4) or more young cells are formed within parental wall from which they ultimately escape by rupture of the wall. Released young cells similar than the parent which gradually enlarge to normal size & repeat the asexual cycle. Chlamydomonas may also form Aplanospores, Hypnospores & Akinetes.

Sexual reproduction involves fusion of two haploid sexual units forming diploid cell or Zygote. Some species are Homothallic (Self-compatible) & others are Heterothallic (Self-incompatible) Gametic union maybe Isogamous, An Isogamous or Oogamous.

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Life cycle of Chlamydomonas

Coenobial Volvocales Family Volvocaceae Genus Volvox

Is to be most evolved of Coenobial form the colony is spherical & appears as small pin head on the surface of water. The colonies of most of the species are visible to naked eyes.

The colonies are composed of many small vegetative or somatic cells, & a few larger flagellate reproductive cells (Gonidia). Cells number of colonies in different species ranges from (500-50000) cell per colony.

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Colonies are spherical to ovoid & the biflagellate cells lie in single layer within gelatinous colonial matrix. Each cell is surrounded by gelatinous sheath of its own. In some species cells are joined to one another by conspicuous or delicate cytoplasmic strands. The whole Coenobium moves & rotates by means of flagella of individual cells. Most of the cells in a coenobium are vegetative & do not have the capacity to form new individuals. Each vegetative cell is biflagellate with to contractile vacuoles near the base of flagella. Other features of cells are cup shape to laminate chloroplast, single Pyrenoid centrally located single nucleus & anteriorly arranged eye spot.

For asexual reproduction few cells of Volvox coenobium differentiated as asexual reproductive cells (Gonidia) by growing larger with disappearance of flagella & gradually thickening, a number of daughter cell are produced by the division & re division of gonidium which are held together as a new colony. Since the number of Gonidia divide at the same time many new colonies are formed simultaneously.

Sexual reproduction is strictly Oogamous. Depending on the species & strains coenobia may contain ether eggs or sperms (Dioecious) or they may contain both (Monoecious).

Both kinds of gametes are development from cells of the posterior half of the coenobia. The egg cells are usually dense & darker in appearance than Gonidia they enlarge without undergoing any division & loss their flagella during development. Cells forming Antherozoids divide successively giving arise to numerous spindle shaped biflagellate Antherozoids.

Non motile female gametes are much larger than the male gametes & surrounded by a gelatinous sheath. The groups of sperms are liberated from their

12 PLANT GROUPS CHLOROPHYTA parent colonies & presumably by chemotactic stimulation swim to the surface of female colonies, finally one sperm penetrate female colony & fertilize the egg. The resulting zygote very soon develop thick wall & become red, zygote divides by meiosis & a new coenobium is formed through the formation of single zoospore.

Coenobium of Volvox

Life Cycle of Volvox

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Order: Chlorococcales

This order contains non motile unicellular & colonial green Algae. Cell wall is usually thin & there is no vegetative cell division. Cells lack flagella, contractile vacuoles & eye spot. Asexual reproduction is achieved by Zoospores, Aplanospores, Autospores & daughter coenobia. Sexual reproduction is by Iso or anisogametes. These Algae are mainly present in fresh water reservoirs & in terrestrial situations.

Division Chlorophyta Class Order Chlorococcales Family Genus Chlorella

This genus is found in various habitats. It is mainly present as plankton form in fresh water, bodies & on terrestrial habitats rich in organic matter. Some species are common on sub aerial habitats, Symbiotic association of Chlorella species with Lichens & Animals (Hydra) are also found.

Cells are usually solitary or sometimes aggregated in groups being enveloped by mucilage. The cells are small, spherical or ellipsoidal. Each cell possesses a single parietal & usually Cup shaped chloroplast with one Pyrenoid. Cells have thin cellulosic wall, centrally placed small nucleus & other Eukaryotic organelles.

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Chlorella is an important tool for physiological experiments. It has widely been used in the study of respiration & photosynthesis. It is also used for purification of air in space capsules. An antibiotic Chlorellin is extracted from this Alga. Chlorella is the first Alga to have been grown extensively in axenic culture.

Chlorella

Cells have very high food value with about 50% proteins, 20% fats & 20% carbohydrates & rich in amino acids & vitamins. Because of this, Scientists in different countries are trying to cultivate it on mass scale. It may act as food of future.

Reproduction is exclusively Asexual which is achieved by the production of Autospores, 4, 8, 16 Autospores are produced by the division of mature cells & are liberated by rupture of the parent cell wall. These non-motile spores develop into new individuals.

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Colonial Chlorococcales

Division Chlorophyta Class Chlorophyceae Order Chlorococcales Family Chlorellaceae Genus Scenedesmus

Usually consists of a linear series of 4 or 8 cells (Sometimes 2-32) coenobium. Cells joined together by the localised production of mucilage. This genus is widely used in physiological studies outer cells of which frequently have spines or projections.

Scenedesmus quadricauda

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Division Chlorophyta Class Chlorophyceae Order Chlorococcales Family Chlorellaceae Genus Pediastrum

Pediastrum is a common planktonic genus which consists of flat plate of cells varying from 2-128 depending upon the species. The outer cells have projections on the surface.

Pediastrum

A number of biflagellate Zoospores are produced during asexual reproduction. These are liberated in a vesicle within which they lose their flagella & become arranged in the form characteristic of the present colony. The biflagellate gametes are produced in the same way as Zoospores but are liberated from the vesicle before fusing.

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Different species of Scenedesmus

Different species of Pediastrum

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