Chlorophyceae or the Green Algae | Botany

In this article we will discuss about:- 1. Salient Features of Chlorophyceae (Green Algae) 2. General Characters of Chlorophyceae (Green Algae) 3. Reproduction 4. Classification 5. Alternation of Generations.

Contents:

1. Salient Features of Chlorophyceae (Green Algae)
2. General Characters of Chlorophyceae (Green Algae)
3. Reproduction in Chlorophyceae (Green Algae)
4. Classification of Chlorophyceae (Green Algae)
5. Alternation of Generations of Chlorophyceae (Green Algae)

1. Salient Features of Chlorophyceae (Green Algae):

The Chlorophyceae or the green algae are characterized in having grassgreen-coloured chloroplasts containing the photosynthetic pigments, chlorophyll a, chloro­phyll b, carotenes and xanthophylls of which the first two predominate over the others.

There are several xanthophylls in the chloroplasts which are not found in other algae, and of these, lutin is found in abundance. Food reserves are stored in the form of starch, whose formation is intimately associated with pyrenoids. Motile reproductive bodies, zoospores and gametes, are usually associated with two or four whip-like flagella of equal length.

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2. General Characters of Chlorophyceae (Green Algae):

The members of this group are found both in fresh and salt waters. The majority of them occur as submerged, freshwater plants being attached to submerged rocks, wood-pieces and similar other objects, but may also float on the surface of stagnant water forming green scums.

Quite a large number of species grow on moist soil, rocks and cliffs, damp wood-work, barks of trees and even on snow and ice. A few species are internal parasites on land plants, or may occur as epiphytes upon land animals.

In general, the members of Chlorophyceae show great varia­tions in form and structure of their vegetative body, ranging from simple unicellular types to typical multicellular filaments through intermediate colonial forms.

When multicellular, the number of cells may be definite or indefinite. But when indefinite, the cells of the thallus may be arranged in irregular masses, in filaments, in sheets, or in solid or hollow cylinders. The unicellular forms may be non-motile or motile by flagella. Typically, the plant body is filamentous and it occupies a prominent place among most of the Orders of the green algae.

With the exception of a few primitive unicellular genera, the protoplast of Chlorophyceae is surrounded by a distinct cell wall which often becomes considerably thick.

The cell wall consists of at least two concentric portions:

(a) An inner homogeneous or con­centrically stratified layer, mostly or wholly made up of cellulose, and

(b) An outer pectose layer which in some genera may be chitinized or impregnated with lime.

The protoplast is well-differentiated into cytoplasm, nuclei and chloroplasts. It mostly occurs as the primordial utricle surrounding a conspicuous central vacuole. But, in some cases, several vacuoles may be present instead. When the cells are flagellate, as in the uni­cellular and some of the colonial forms, there are usually two small contractile vacuoles near the base of the flagella. They expand and contract alternately and are regarded as excretory organs. In some cases, the sap of the vacuole becomes coloured due to anthocyanin.

There may be one or more nuclei in each protoplast, and these are found either in the primordial utricle or are suspended in a mass of cytoplasm at the centre of the vacuole by means of cytoplasmic threads. The nucleus possesses a distinct nuclear membrane, one or more nucleoli and a characteristic chromatin reticulum.

The nu­clear division is mitotic. The chloroplasts are most characteristic and are of various forms. They may be cup-shaped, laminate, en­tire or perforate, disciform, peripheral in position, and are mostly with one or more pyrenoids. Each pyrenoid has a central proteinaceous portion which is surrounded by plates of starch. The food reserves are usually in the form of starch, but fats and oils are found in some cases.

Within the chloroplasts the photosynthetic (chlorophyll a and chlorophyll b) and other pigments (carotenes- ∝-carotene and β-carotene; xanthophylls- lutin, zeaxanthine, viola-xanthine and neoxanthine) become associated together, but the chlorophylls predominate over the others.

The amount of the pigment also va­ries from species to species, and according to their proportions typically grass-green to almost light-green Chlorophyceae are found in nature. In some genera, the vegetative cells and the zygotes also contain a red pigment, haematochrome, which masks other pig­ments. Haematochrome is a carotenoid pigment. Sometimes the colour of the cells in certain species is modified by the presence of anthocyanin pigments of the cell sap.

The motile vegetative and reproductive cells are provided with flagella inserted at the anterior end of the cell. They are usually of equal length. Located near the insertion of the flagella there is an orange-red to reddish-brown circular or oval spot, called the eye-spot or stigma, which is regarded as a photo-receptive organelle and is supposed to control the directive movement of the flagella.

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3. Reproduction in Chlorophyceae (Green Algae):

The methods of reproduction in green algae are well-advanced and show great variations. All the three principal methods of re­production, as detailed below, are found in this group. Vegetative reproduction takes place usually by the process of fragmentation.

Among filamentous forms, fragmentation is either due to accidental breaking of a filament or certain cells of the fila­ment may produce spores or gametes, so that short sections of the Filament become ultimately separate from one another resulting in the increase of the number of filaments.

The colonial genera reproduce vegetatively by accidental fragmentation of the colony. Among unicellular forms, vegetative reproduction always takes place by cell division, which, in multicellular forms, becomes a me­thod for the growth of the thallus.

Asexual reproduction usually takes place by the production of spores within metamorphosed vegetative cells. The spores may vary in number, from one to many in each cell and may be either naked and motile or provided with a distinct wall. The motile spores are known as zoospores, which are provided with two, four or more fla­gella of equal length at the anterior end.

The liberation of zoos­pores usually takes place by the formation of a pore in the wall of the mother cell, or by the breaking or gelatinization of the sporan­gial wall. The non-motile spores are known as aplanospores, whose walls are quite distinct from the wall of the mother cell. These are in reality modified zoospores which have secreted wall around them prior to the liberation from the mother cell.

The aplanospores are liberated by the rupture or decay of the wall of the mother cell. The mother cell producing these spores is termed a sporangium, which, in most cases, is not morphologically different from the vegetative cells. Sometimes, the wall of the aplanospore becomes greatly thickened and forms a hypnospore. Asexual reproduction also takes place by the formation of akinetes. 

In such cases, a single non-motile spore is formed within the sporangium whose wall is not distinct from that of the spore. But, the vegetative-cell often become very much thick-walled containing abundant food reserve to form akinete-like structures for tiding the alga over unfavourable periods.

The sexual reproduction in Chlorophyceae ranges in complexity from the simplest to the most complex sex structures. The methods of gametic union may be arranged in an evolutionary series and are noticeable from the most primitive to the most highly advanced forms with specialized and differentiated sex structures.

In the simplest case, sexual union is by isogamy, i.e. gametic union takes place between morphologically identical flagellate or non-flagellate gametes. Ulothrix is a familiar example in which there is a fusion of a flagellate gamete with another gamete of identical size and structure. This is known as planogamic union. Sometimes (as in Spirogyra, Zygnema, etc.) there is union of game­tes without flagella, and this type of isogamy is known as aplanogamy. In some cases, the plant reproduces by anisogamy, in which gametic union is effected between two flagellate gametes of un­equal sizes, as in Caulerpa.

The most advanced method of repro­duction is oogamy. In this case, gametic union takes place between a small flagellate, motile, active male gamete and a large non-fla­gellate, non-motile, passive female gamete as in Vaucheria, Oedogonium, etc. The male gametangium producing the antherozoids is known as the antheridium, and is usually a distinct structure easily recognizable from the vegetative cells. Each antheridium produces one or more antherozoids.

In most cases the oosphere or egg is produced singly within the female gametangium, known as the oogo­nium, which differs morphologically from the vegetative cells The zygote formed as a result of sexual union, usually after a period of rest, gives rise to one or more new plants. Many species of Chloro­phyceae are homothallic, i.e. monoecious. In such cases, gametes derived from a single parent plant unite in pairs. Other species are heterothallic, i.e. dioecious. Here gametes of different parentage fuse in pairs.

There are various instances in which parthenospores (or azygospores or aboospores or azygotes) are formed directly from the gametes in the absence of sexual union.

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4. Classification of Chlorophyceae (Green Algae):

Chlorophyceae are divided into nine Orders which may be distinguished as follows:

1. Volvocales:

Unicellular or colonial; motile throughout life or form sedentary colonies which readily revert to a motile con­dition; reproduce asexually as well as sexually; mainly freshwater.

Examples:

Chlamydomonas, Sphaerella, Pandorina, Eudorina Volvox, etc.

2. Chlorococcales:

Unicellular or colonial; non-motile in the vegetative condition, reproduced by zoospores or aplanospore; almost exclusive fresh water.

Examples:

Hydrodictyon, Pediastrum, etc.

3. Ulotrichales:

Filamentous, simple and unbranched or cellular expanse with small cells; chloroplasts parietal, axial or stel­late; sometimes filaments simple and with large multinucleate cell; mostly isogamous; mostly species freshwater, some are marine.

Examples:

Ulothrix, Ulva, etc.

4. Cladophorales:

Simple or branched, filamentous with cells containing two to many nuclei and usually with elaborate large chloroplasts; mostly isogamous; freshwater and marine.

Exam­ple:

Cladophora.

5. Chaetophorales:

Filamentous and sharply differentiated into prostrate and erect portions (heterotrichous); erect portion often reduced and the prostrate portion often forming discoid ex­panse; hairs of diverse type are often present; mostly isogamous and freshwater.

Examples:

Chaetophora, Enteromorpha, Draparnaldiopsis, Trentepohlia. Coleophaete, Protococcus, etc.

6. Oedogoniales:

Simple or branched, filamentous; zoo­spores multiflagellate; oogamous; cell divisions characterized by intercalation of strips of membrane between two parts of the mo­ther cell; entirely freshwater.

Example:

Oedogonium.

7. Conjugales:

Unicellular or colonial (generally filamen­tous) with elaborate chloroplasts; motile gametes unknown; re­production by vegetative cell division or by conjugation of amoe­boid gametes; exclusively freshwater.

Examples:

Spirogyra, Zygnema, Cosmarium, Closterium, etc.

8. Siphonales:

Filamentous; without septa or elaborately differentiated; all parts coenocytic; chloroplasts numerous and discoid; sexual reproduction mainly isogamous, sometimes ooga­mous, unknown in many cases; mostly marine.

Examples:

Vaucheria, Caulerpa, Bryopsis, etc.

9. Charales:

Thallus well-differentiated into nodes and internodes; internodes sometimes corticated; branches of limited growth in whorls; chloroplasts numerous and discoid; cells usually uninucleate; reproduction vegetative and sexual with elaborate oogonia and antheridia; germination of zygote indirect; fresh water and brackish water.

Examples:

Chara, Nitella, etc.

5. Alternation of Generations of Chlorophyceae:

Some Chlorophyceae show regular alternation of similar hap­loid and diploid generations, but in great majority the plant body is haploid with the zygote representing the only diploid phase. In the simplest case, as illustrated by Chlamydomonas, the life cycle consists of alternation of unicellular haploid and diploid phases.

This alternation is, however, not obligatory in the sense that the haploid phase will always give rise to the diploid phase, because there may be a formation of haploid individuals in succession be­fore the formation of a diploid phase.

On the contrary, it is obligaloid one. This represents, therefore, the most primitive type of life cycle. Starting from this primitive condition, one finds an evolu­tion of multicellular thalli in either phases (haploid, as in Spirogyra, Oedogonium, etc., or diploid, as in Codium) of the life cycle. In such cases, there is an alternation of a multicellular haploid generation with an unicellular diploid phase, or a multicellular dip­loid generation with an unicellular haploid phase.

Lastly, there may be an alternation between multicellular haploid and diploid gene­rations (as in Cladophoraceae, Ulvaceae, Chaetophoraceae). The two generations may be quite identical (isomorphic) with each other, or they are morphologically different (heteromorphic).