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Subkingdom Thallophyta

In thallophytes, the plant body is not differentiated into stem, roots and leaves and is called thallus. It includes algae and fungi.
Algae are simple, non-vascular, photosynthetic plants with unicellular or colonial form, filamentous or with large thallus (Fucus, Laminaria, Sargassum). Majority of algae contain green pigment called chlorophyll, which helps in photosynthesis. Some algae have other photosynthetic pigments such as brown (fucoxan-thin), yellow (xanthophyll), red (phycoerythrin), blue (phycocyanin), etc.
Along with the nature of the stored food, pigments form the basis of classification of algae. The study of algae is called phycology.
Some of the examples are as follows:
Unicellular algae
  • Chlamydomonas is a unicellular, microscopic, free-living green alga. It is found in abundance in fresh water ponds, tanks, ditches, lakes, etc. The cell is oval in shape and it is surrounded by a definite cell wall made up of cellulose. Two flagella are present on the anterior side of the cell. The cell has a large cup-shaped chloroplast.
  • Chlorella is used in space research.
Filamentous algae
  • Spirogyra (pond silk) is a long thread-like alga, and each cell has one or more spiral chloroplasts. The nucleus of the cell is embedded in the cytoplasm. The unbranched filaments of this green alga range from a few inches to a foot.
Colonial algae
  • Volvox forms a spherical, motile colony. All the small cells of the colony possess two flagella and a small eyespot. With this, the colony is able to swim towards light.
Pseudoparenchymatous algae
  • Sargassum is a brown alga which grows on the Atlantic Ocean, nearly for about 64,000 sq. km. Hence, this part of the sea is called ‘Sargasso-Sea’.
  • Gelidium: Agar, an extract of Gelidium, is used in the preparation of ice-creams.
Other examples include
  • Desmid is found only in freshwater and is a unicellular organism. It has a spectacular symmetrical shape. Desmid cannot be seen through naked eyes because of its small size.
  • Diatom is a delicate unicellular organism which has a yellow-brown chloroplast that enables it to photosynthesise. It leaves behind large deposits of silica on the ocean floor, after its death which forms diatomaceous earth.
Fungi are heterotrophic, i.e., either as saprophytes or parasites. They use their hyphae to absorb nutrients from dead organisms. Before they absorb nutrients, the hyphae of the fungi secrete enzymes into food substances to digest it. The study of fungi is called mycology.
Some of the examples are as follows:
  • Agaricus (mushroom)
  • Saccharomyces cerevisae—yeast (used in baking and brewing industries)
  • Penicillium (first antibiotic discovered by Alexander Fleming)
The fungi associate with algae and live symbiotically as lichens. Fungal component (mycobiont) absorbs moisture from the atmosphere and affords shelter to the algal component (phycobiont) by maintaining humidity inside the thallus of lichen. Phycobiont manufactures the starch, which is used as food by the fungus. They are bioindicators of pollution, e.g., Rocella tinctoria (litmus is obtained from it).

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