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Plant Kingdom

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Chapter 3 : Plant Kingdom

3.1 Algae arrow_upward

  • Algae comprise several different groups of organisms which produce energy through photosynthesis.
  • Algae are chlorophyll-bearing, simple, thalloid, autotrophic and largely aquatic (both fresh water and marine) organisms.
  • They occur in a variety of other habitats: moist stones, soils and wood.
  • Some of them also occur in association with fungi (lichen) and animals (e.g., on sloth bear).
  • The form and size of algae is highly variable.
  • The size ranges from the microscopic unicellular forms like Chlamydomonas, to colonial forms like Volvox.
  • The algae reproduce by vegetative, asexual and sexual methods.
  • Vegetative reproduction is by fragmentation. Each fragment develops into a thallus.
  • Algae are useful to man in a variety of ways.
  • At least a half of the total carbon dioxide fixation on earth is carried out by algae through photosynthesis.
  • The algae are divided into three main classes:
    • Chlorophyceae: The members of chlorophyceae are commonly called green algae.
    • Phaeophyceae: The members of phaeophyceae or brown algae
    • Rhodophyceae: Rhodophyceae are commonly called red algae because of the predominance of the red pigment, r-phycoerythrin in their body.

    3.2 Bryophytes arrow_upward

  • Bryophytes include the various mosses and liverworts that are found commonly growing in moist shaded areas in the hills
  • Bryophytes are also called amphibians of the plant kingdom because these plants can live in soil but are dependent on water for sexual reproduction
  • The plant body of bryophytes is more differentiated than that of algae.
  • It is thallus-like and prostrate or erect, and attached to the substratum by unicellular or multicellular rhizoids.
  • They lack true roots, stem or leaves. They may possess root-like, leaf-like or stem-like structures.
  • The main plant body of the bryophyte is haploid. It produces gametes, hence is called a gametophyte.
  • The bryophytes are divided into following categories:
    • Liverworts
    • Mosses

    3.2.1 Liverworts

  • The liverworts grow usually in moist, shady habitats such as banks of streams, marshy ground, damp soil, bark of trees, and deep in the woods.

  • 3.2.2 Mosses

  • They consist of upright, slender axes bearing spirally arranged leaves. They are attached to the soil through multicellular and branched rhizoids.

  • 3.3 Pteridophytes arrow_upward

  • The pteridophytes are vascular plants (plants with xylem and phloem) that produce neither flowers nor seeds, and are hence called vascular cryptogams.
  • The Pteridophytes include horsetails and ferns. Pteridophytes are used for medicinal purposes and as soil-binders.
  • They are also frequently grown as ornamentals.
  • Evolutionarily, they are the first terrestrial plants to possess vascular tissues – xylem and phloem.
  • In pteridophytes, the main plant body is a sporophyte which is differentiated into true root, stem and leaves.
  • The sporophytes bear sporangia that are subtended by leaf-like appendages called sporophylls.

  • 3.4 Gymnosperms arrow_upward

  • The gymnosperms are plants in which the ovules are not enclosed by any ovary wall and remain exposed both before and after fertilisation.
  • Gymnosperms include medium-sized trees or tall trees and shrubs
  • The roots are generally tap roots.
  • The gymnosperms are heterosporous; they produce haploid microspores and megaspores.
  • The two kinds of spores are produced within sporangia that are borne on sporophylls which are arranged spirally along an axis to form lax or compact strobili or cones.

  • 3.5 Angiosperms arrow_upward

  • Angiosperms are flowering plants.
  • They are the most widespread and diverse group of land plants.
  • Their seeds are found in a closed ovary.
  • Unlike the gymnosperms where the ovules are naked, in the angiosperms or flowering plants, the pollen grains and ovules are developed in specialized structures called flowers.
  • In angiosperms, the seeds are enclosed by fruits.
  • The angiosperms are an exceptionally large group of plants occurring in wide range of habitat
  • The life cycle of an angiosperm is shown below:

  • 3.6 Plant Life Cycles and Alternation of Generations arrow_upward

  • In plants, both haploid and diploid cells can divide by mitosis.
  • This ability leads to the formation of different plant bodies - haploid and diploid.
  • The haploid plant body produces gametes by mitosis. This plant body represents a gametophyte.
  • The plant life cycle, unlike that of animals, consists of alternating generations of individual organisms that are haploid (the gametophyte) and diploid (the sporophyte).
  • Specialized diploid cells in the sporophyte undergo meiosis to produce haploid spores (hence the name "sporophyte").
  • Each spore grows mitotically to become the new gametophyte, which then produces gametes (hence, the name "gametophyte") which fuse to form a zygote.

  • 3.6.1 Haplontic Life Cycle

  • In haplontic life cycle, haploid individual forms haploid gamete by mitosis during gametogenesis.
  • Then, this gamete (N) fuses and forms zygote that is diploid.
  • This zygote undergoes meiosis to form haploid adult. Hence, adult is haploid and exhibits zygotic meiosis.
  • It takes place in Spirogyra, Chlamydomonas, and Volvox.

  • 3.6.2 Diplontic Life Cycle

  • In diplontic life cycle, diploid individual forms haploid gamete by meiosis during gametogenesis.
  • Then, these gametes (N) fuse and form zygote that is diploid, which by mitosis forms diploid adult.
  • Hence, adult is diploid and exhibits gametic meiosis.
  • It takes place in all seed-bearing plants such as gymnosperms and angiosperms.
  • 3.6.3 Haplo-diplontic Life Cycle

  • Both haplontic and diplontic type of life cycle alternate with each other.
  • It takes place in bryophytes and pteridophytes.

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