Plant Kingdom-The Algae

Introduction of Plant Kingdom

Plant and animal kingdoms both include a wide variety of organisms that contribute to biodiversity.

  • we can define Plants as Multicellular, Eukaryotic, Photosynthetic Autotrophs rarely found heterotrophs having cellulosic cell walls. All are embryophytes.
  • Echiler (1833) divided the plant kingdom into two subkingdoms on the basis of the presence or absence of seeds.
    1. Cryptogamae (Criptos-hidden; Gamous-marriage)- These are lower plants in which sex organs are hidden and seeds and flowers are absent termed as seedless plants. These include three divisions- Thallophytes (Algae), bryophytes, and pteridophytes.
    2. Phanerogamae (Phaneros-visible, Gamous-marriage)- Includes higher plants and sex organs are visible, seed-bearing plants. Also called Spermatophyta. This group includes gymnosperms and Angiosperms.
  • Oswald Tippo (1942) classified plants on the basis of the presence or absence of vascular tissue (Xylem and phloem) and also the presence and absence of embryos in the life cycle.

***Some important facts must be known:

  1. Artificial System of Classification – Based on morphological characters like Colour, Number and shape of leaves, habit, etc.
  2. The natural System of Classification Based on natural affinities, considers both external and internal features. George Bentham and Joseph Dalton Hooker were given this type of classification of the Flowering plant.
  3. Phylogenetic System of Classification– Based on Evolutionary Relationship between various Organisms.
  4. Numerical Taxonomy Classification based on Given Numbers and Code For Characters.
  5. Cytotaxonomy– that is based on cytological information like chromosome number, structure, behaviour
  6. ChaemotaxomyChemical constituents of the plant used to resolve confusion, it is also used by Taxonomists.
Thallophyta- The Algae 

The Algas are chlorophyll-bearing, simple, thalloid, autotrophic, and mostly aquatic (both fresh and marine water) organisms.

The branch of botany that deals with algae is termed Phycology or Algology.

In thallophytes simple unicellular non-jacketed sex organs are present. Embryo development is not found after the gametic union.

These lack true roots, stems, and leaves, these are undifferentiated and thallus-like forms. So these are non-embryophytes that lack seed and vascular tissues.

Habitats: Moist places like moist stones, soil, and wood. These also occur in an association with fungi i.e, Lichen, and with animals e.g. on Sloth bear.

Size: Variable in size, ranges from colonial forms like Volvox and the Filamentous forms like Ulothrix and Spirogyra. Few form massive plant bodies like Kelps.

Reproduction in Algae: 

The algae reproduce by the method of vegetative, asexual, and sexual methods.

Vegetative reproduction

Types of vegetative reproduction found in algae:

  • fragmentation, as fragments develop into a thallus. e.g. Spirogyra, Zgnema, and other colonial forms of algae.
  • Fission– Simple cell division of unicellular algae, which devices in two daughter cells. e.g. desmids.
  • Budding: A bud-like outgrowth is from the thallus, which is cut- off by the transverse septum. e.g. Protosiphon.
  • Tubers: in some rhizoids from tuber-like growth that get filled with food materials. e.g. Chara.

Asexual reproduction with the production of different types of spores, zoospores are the most common. These are flagellated (motile) and during germination give rise to new plants.

Sexual reproduction through the fusion of two gametes. Gametes can be flagellated and in a similar size (Ulotheix) or non-flagellated(non-motile)but similar in size, called isogamous.

    • The fusion of two dissimilar-sized gametes is called anisogamous, as in species of Eudorina.
    • The fusion between one large, nonmotile(static) female and a smaller, motile male gamete is called oogamous. e.g., Volvox, Fucus.

Life cycle in Algae:

Various types of life cycles are seen in algae and due to this alternation of generation (AOG) is found in algae.

The life cycle of most algae-like Volvox, Spirogyra and some of Chlamydomonas represents the haplontic kind of life cycle. (Haplontic life cycle–  Dominant photosynthetic phase is free-living gametophyte).

Haplontic life cycle: life cycle of Algae

Haplontic life cycle: life cycle of Algae

Diplontic life cycle: life cycle of Algae

Diplontic life cycle: life cycle of Algae

Some other like Fucus species shows diplontic kind of life cycle. (Diplontic kind of life cycle– Dipoloid sporophyte is dominant, photosynthetic phase independent phase of the plant, and the gametophytic phase shows single to the few-celled haploid gametophyte).

Alternation of generation, i.e. Gametophyte (n)→ Sporophyte (2n) → Gametophyte (n)

Uses of Algae: These are useful in various ways.
  • It is very useful as around half of the total Carbon-di-oxide fixation on earth is carried by Algae through Photosynthesis.
  • They increase the dissolved level of oxygen in the environment through photosynthesis.
  • These are primary producers of energy-rich compounds in the aquatic animals’ food cycle.
  • 70 species of marine algae are used as food. (Laminaria, Sargassum, Porphyra)
  • Some marine brown and red algae produce large amounts of hydrocolloid (water-holding substances), e.g. Algin (brown algae) and carrageen (red algae) also used commercially.
  • Agar is produced by Gelidium, and Gracilaria is used to grow microbes used in the preparation of ice-creams and jellies.
  • Chlorella is a unicellular alga that is rich in proteins and used for supplement by space travelers.
Classification of Algae

 Algae are divided into three main classes:

  1. Chlorophyceae
  2. Phaeophyceae
  3. Rhodophyceae
Classification of algae: green algae, brown algae, red algae

Classification of algae: green algae, brown algae, red algae

1. Chlorophyceae

Habitat: Freshwater marine or terrestrial.

Thallus form: These are commonly known as green algae. The body of the plant may be unicellular, colonial, or filamentous, thalloid.

Chlorophyll Mostly chlorophyll a and b and β Carotene are found, which is responsible for green colour.

Chloroplast- The definite chloroplast is found for pigments, these are maybe of the discoid, plate-like, reticulate, cup-shaped, spiral, or ribbon-shaped found in different species.

  • Pyrenoids found in the chloroplast are storage bodies that contain protein besides starch.

Cell wall- The rigid cell wall of green algae made up of an inner cellulose layer and an outer layer of pectose.

Reserved food material: Starch and fats

Flagella: 2-8 in numbers, equal and apical position.


  • Vegetative reproduction by fragmentation or spore formation.
  • Asexual reproduction is by flagellated zoospores produced in zoosporangia.
  • Sexual reproduction shows varieties as isogamous, anisogamous, or Oogamous.

Examples of some commonly found green algae: Chlamydomonas, Volvox, Ulothrix, Spirogyra, and Chara.

2. Phaeophyceae

Habitat: Commonly called brown algae found in marine habitats some found in fresh water and brackish water.

Thallus form:  They show great variation in size and form, ranges simply from branched,  filamentous forms (Ectocarpus) to profusely branched forms usually heterotrichous. Kelp may reach a height of 100 meters.

Pigmentation Chlorophyll a, c, carotenoids, and xanthophylls. Colour variation found from olive green to various shades of brown depends on the amount of the xanthophyll pigment, fucoxanthin present.

Stored food: Food is stored as a complex carbohydrate, in the form of laminarin or mannitol.

Cell wall: Cell wall containing cellulosic cell wall covered outside by gelatinous coating of align. (in some sources term alginic and fucinic acid are mentioned) 

organelles: Protoplast containing plastids, a centrally located vacuole, and nucleus. 

Plant body: plant body attached to the substratum by holdfast, has a stalk, and leaf-like photosynthetic organs called a frond.

Flagella: two in number, unequal, and found laterally.


  • Vegetative reproduction by fragmentation, adventitious branches
  • Asexual reproduction found in most of the brown algae by biflagellate zoospores that are pear-shaped, two unequal flagella attached laterally with it.
  • Sexual reproduction may be by isogamous, anisogamous, or Oogamous.
    • Union of gametes may be in water or within the oogonium- Oogamous species.
    • Gametes are pyriform (pear-shaped) by which two flagella are attached laterally.

Examples of some Commonly found brown algae: Ectocarpous, Dictyota, Laminaria, Sargassum, and Fucus.

3. Rhodophyceae

Habitat: Commonly called red algae. The majority of red algae are marine and mostly found in warmer areas, few are terrestrial.

Thallus forms: These are unicellular, filamentous, and pseudo-parenchymatous.

Pigmentation: red colour because of red pigment, r-phycoerythrin in the body, chlorophyll a and d is found.

Stored food: Food is stored as Floridian starch which is similar to amylopectin and glycogen in structure.

Cell wall: the inner layer is Cellulosic while pectin is found in the outer layer and polysulphate esters.

Flagella: These are non-flagellate, motile cells are absent.


  • Vegetative reproduction by fragmentation, cell division.
  • Asexually reproduces by non-motile spores and sexually by non-motile gametes.
  • Sexual reproduction is complex and advanced Oogamous, post-fertilization development is found complex.

Examples of common members of the red algae: Polysiphonia, Porphyra, Gracilaria, and Gelidium.

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