Plant Kingdom Class 11 NCERT Biology Notes - Systems of Classification, Algae, Bryophytes, Pteridophytes, Gymnosperms, Angiosperms and Plant Life Cycles

Systems of Biological Classification

• Earliest systems: Artificial Classification Systems.
• Based on vegetative characters or superficial morphological characters like habit, colour, number, and shape of leaves.
• Linnaeus’s artificial system based on androecium structure.

• Drawbacks:

  • Separated closely related species due to reliance on few characteristics.
  • Equal weightage to vegetative and sexual characteristics, which is not ideal as vegetative characters are more environment-dependent.
• Natural Classification Systems: Based on natural affinities among organisms.
• Consider external and internal features (ultrastructure, anatomy, embryology, phytochemistry).
• E.g., Classification for flowering plants by George Bentham & Joseph Dalton Hooker.
• Phylogenetic Classification Systems: Based on evolutionary relationships, assuming organisms in the same taxa share a common ancestor.

• Other sources to resolve classification problems:

  • Numerical Taxonomy: Uses all observable characteristics, computer-processed with assigned numbers and codes for equal consideration of hundreds of characters.
  • Cytotaxonomy: Based on cytological information like chromosome number, structure, behaviour.
  • Chemotaxonomy: Uses chemical constituents of plants.

Algae

• Simple, thalloid, autotrophic, chlorophyll-bearing, and aquatic (fresh water & marine) organisms.
• Also occur in moist stones, soils, and wood.
• Some in association with fungi (lichens) and animals (e.g., on sloth bear).
• Highly variable form and size:
  • Microscopic unicellular: E.g., Chlamydomonas.
  • Colonial: E.g., Volvox.
  • Filamentous: E.g., Ulothrix, Spirogyra.

Reproduction

• Vegetative reproduction: By fragmentation, each fragment develops into a thallus.
• Asexual reproduction: By spores, e.g., zoospores (most common, flagellated, motile, germinate into new plants).
• Sexual reproduction: Fusion of two gametes, of types:
  • Isogamous: Fusion of similar-sized gametes, flagellated (e.g., Ulothrix) or non-flagellated (e.g., Spirogyra).
  • Anisogamous: Fusion of dissimilar-sized gametes, e.g., some Eudorina species.
  • Oogamous: Fusion of large, non-motile female gamete and smaller, motile male gamete, e.g., Volvox, Fucus.

Benefits of Algae

• Fix half of Earth’s CO₂ through photosynthesis, increasing dissolved oxygen levels.
• Primary producers, forming the basis of aquatic food cycles.
• 70 marine algae species used as food, e.g., Porphyra, Laminaria, Sargassum.
• Agar (from Gelidium, Gracilaria) used for microbial growth, ice creams, and jellies.
• Marine brown & red algae produce hydrocolloids (water-holding substances), e.g., algin (brown algae), carrageen (red algae), used commercially.
• Protein-rich unicellular algae like Chlorella, Spirulina used as food supplements by space travellers.

Algae include three classes: Chlorophyceae, Phaeophyceae, and Rhodophyceae.

1. Chlorophyceae (Green Algae)

• Unicellular, colonial, or filamentous.
• Grass green due to chlorophyll a and b in chloroplasts.
• Chloroplasts vary: discoid, plate-like, reticulate, cup-shaped, spiral, or ribbon-shaped.
• Most have pyrenoids (storage bodies with protein and starch) in chloroplasts.
• Some store food as oil droplets.
• Rigid cell wall: inner cellulose, outer pectose.
• E.g., Chlamydomonas, Volvox, Ulothrix, Spirogyra, Chara.

Reproduction

• Vegetative reproduction: By fragmentation or different spore types.
• Asexual reproduction: By flagellated zoospores in zoosporangia.
• Sexual reproduction: Isogamous, anisogamous, or oogamous.

2. Phaeophyceae (Brown Algae)

• Mostly marine, varying from simple branched, filamentous forms (e.g., Ectocarpus) to profusely branched forms (e.g., kelps, up to 100 m).
• Contain chlorophyll a, c, carotenoids, and xanthophylls.
• Colour varies from olive green to brown due to fucoxanthin.
• Food stored as complex carbohydrates: laminarin or mannitol.
• Vegetative cells have a cellulosic wall with a gelatinous algin coating.
• Protoplast contains plastids, central vacuole, and nucleus.
• Plant body attached by a holdfast, with a stalk (stipe) and leaf-like frond.
• E.g., Ectocarpus, Dictyota, Laminaria, Sargassum, Fucus.

Reproduction

• Vegetative reproduction: By fragmentation.
• Asexual reproduction: By pear-shaped biflagellate zoospores with two unequal, laterally attached flagella.
• Sexual reproduction: Isogamous, anisogamous, or oogamous. Gamete union in water or within the oogonium (oogamous species). Gametes are pear-shaped with two laterally attached flagella.

3. Rhodophyceae (Red Algae)

• Contain r-phycoerythrin (red pigment).
• Mostly marine, especially in warmer areas.
• Occur in well-lighted surface waters and deep oceans with low light penetration.
• Red thalli are mostly multicellular, some with complex body organisation.
• Food stored as floridean starch, similar to amylopectin and glycogen.
• E.g., Polysiphonia, Porphyra, Gracilaria, Gelidium.

Reproduction

• Vegetative reproduction: By fragmentation.
• Asexual reproduction: By non-motile spores.
• Sexual reproduction: Oogamous, by non-motile gametes with complex post-fertilisation developments.

Bryophytes

• Called amphibians of the plant kingdom because they live in soil but need water for sexual reproduction.
• Occur in damp, humid, and shaded localities.
• Body more differentiated than algae, thallus-like, prostrate or erect, attached to the substratum by unicellular or multicellular rhizoids.
• Lack true roots, stem, or leaves but may have root-like, leaf-like, or stem-like structures.
• Main plant body is haploid, produces gametes, called a gametophyte.
• Sex organs are multicellular.
• Male sex organ (antheridium) produces biflagellate antherozoids. Female sex organ (archegonium) is flask-shaped, produces a single egg.
• Antherozoids are released to water, meet archegonium, and fuse with the egg to form a zygote.
• Zygotes do not undergo meiosis immediately, produce a multicellular sporophyte.
• Sporophyte is not free-living, attached to the photosynthetic gametophyte, derives nourishment from it. Some sporophyte cells undergo meiosis to form haploid spores, which germinate to form gametophyte.

Importance of Bryophytes

• Some mosses provide food for herbaceous mammals, birds, and other animals.
• Species of Sphagnum (a moss) provide peat, used as fuel and as packing material for trans-shipment of living material due to its water-holding capacity.
• Ecologically important in plant succession on bare rocks/soil. Mosses and lichens decompose rocks, making the substrate suitable for higher plants.
• Form dense mats on soil, preventing soil erosion.

Bryophytes are divided into liverworts and mosses.

Liverworts

• Grow in moist, shady habitats like banks of streams, marshy ground, damp soil, bark of trees, and deep woods.
• Plant body is thalloid, e.g., Marchantia. Thallus is dorsi-ventral, closely appressed to the substrate. Leafy members have tiny leaf-like appendages in two rows on stem-like structures.

• Asexual Reproduction:

  • By fragmentation of thalli or formation of gemmae (sing. gemma).
  • Gemmae are green, multicellular, asexual buds in gemma cups on thalli. They detach and germinate to form new individuals.

• Sexual Reproduction:

  • Male and female sex organs on same or different thalli.
  • Sporophyte is differentiated into foot, seta, and capsule.
  • After meiosis, spores are produced in the capsule, germinate to form free-living gametophytes.

Mosses

• Predominant stage is the gametophyte, with two stages:
  • Protonema stage: Develops from a spore, creeping, green, branched, often filamentous.
  • Leafy stage: Develops from secondary protonema as a lateral bud, with upright, slender axes, spirally arranged leaves, attached to soil by multicellular, branched rhizoids. Bears sex organs.

• Vegetative Reproduction:

  • By fragmentation and budding in the secondary protonema.

• Sexual Reproduction:

  • Antheridia and archegonia produced at the apex of leafy shoots.
  • After fertilisation, the zygote develops into a sporophyte with foot, seta, and capsule.
  • Sporophyte in mosses is more elaborate than in liverworts. Capsule contains spores formed after meiosis.
  • Mosses have an elaborate spore dispersal mechanism.
• E.g., Funaria, Polytrichum, Sphagnum.

Pteridophytes

• They include horsetails and ferns.
• They are found in cool, damp, shady places. Some flourish well in sandy-soil conditions.
• Evolutionarily, they are the first terrestrial plants to possess vascular tissues (xylem & phloem).
• In bryophytes, the dominant phase in the life cycle is the gametophyte. In pteridophytes, the dominant phase (main plant body) is a sporophyte. It is differentiated to true root, stem & leaves. These organs have well-differentiated vascular tissues.
• The leaves in pteridophyta are small (microphylls) as in Selaginella or large (macrophylls) as in ferns.

• Economic importance:

  • They are used for medicinal purposes and as soil-binders and ornamentals.

Reproduction:

• The sporophytes bear sporangia that are subtended by leaf-like appendages called sporophylls. In some cases, sporophylls may form distinct compact structures called strobili or cones (E.g. Selaginella, Equisetum).
• Sporangia produce spores by meiosis in spore mother cells.
• The spores germinate to give inconspicuous, small, multicellular, free-living, mostly photosynthetic thalloid gametophytes called prothallus.
• Prothallus requires cool, damp, shady places to grow. Also, it needs water for fertilization. So, the spread of pteridophytes is limited and restricted to narrow geographical regions.
• The gametophytes (prothallus) bear male and female sex organs called antheridia and archegonia, respectively.
• Water is needed for transfer of antherozoids (male gametes from antheridia) to the mouth of archegonium.
• Antherozoid fuses with the egg in the archegonium to form zygote. Zygote develops to a multicellular well-differentiated sporophyte.
• Most of the pteridophytes produce similar kinds of spores (homosporous plants). Others produce two kinds of spores, macro (mega) & micro spores. They are heterosporous. E.g. Selaginella & Salvinia.
• The megaspores & microspores germinate and give rise to female and male gametophytes, respectively. The female gametophytes are retained on the parent sporophytes for variable periods.
• Within female gametophytes, zygotes develop into young embryos. This event is a precursor to the seed habit. It is considered as an important step in evolution.
• The pteridophytes have 4 classes:
  • Psilopsida: E.g. Psilotum
  • Lycopsida: E.g. Selaginella, Lycopodium
  • Sphenopsida: E.g. Equisetum
  • Pteropsida: E.g. Dryopteris, Pteris, Adiantum

Gymnosperms

• Gymnosperms (gymnos: naked, sperma: seeds) are plants in which the ovules are not enclosed by ovary wall and remain exposed before and after fertilization. Seeds that develop post-fertilization are not covered (naked).
• They include medium-sized trees or tall trees and shrubs. Sequoia (giant redwood) is the tallest tree species.
• The roots are generally tap roots.
• Roots in some genera have fungal association in the form of mycorrhiza (E.g. Pinus).
• In plants like Cycas, small specialized roots (coralloid roots) are associated with N₂-fixing cyanobacteria.
• Stems are unbranched (Cycas) or branched (Pinus, Cedrus).
• Leaves are simple or compound. They are well-adapted to withstand extreme temperature, humidity, and wind.
• In Cycas, the pinnate leaves persist for a few years.
• In conifers (Pinus, Cedrus etc.), the needle-like leaves reduce the surface area. Their thick cuticle and sunken stomata also help to reduce water loss.

Reproduction:

• Gymnosperms are heterosporous. They produce haploid microspores and megaspores.
• Some leaves are modified into sporophylls. They are compactly and spirally arranged along an axis to form lax or strobili or cones.
• Sporophylls bear sporangia in which spores are produced.
• Sporophylls are 2 types:
  • Microsporophylls: They are arranged to male strobili (microsporangiate). They bear microsporangia. The microspores develop into male gametophytes. It is highly reduced and confined to only a limited number of cells. This gametophyte is called a pollen grain. The pollen grains are developed within the microsporangia.
  • Megasporophylls: They are arranged to female strobili (macrosporangiate). They bear megasporangia (ovules). Megasporangium mainly consists of a body called nucellus. It is protected by envelopes. The megaspore mother cell is differentiated from a cell of the nucellus. Megaspore mother cell undergoes meiosis to form four megaspores. One of the megaspores enclosed within the megasporangium (nucellus) develops into a multicellular female gametophyte that bears two or more archegonia. The multicellular female gametophyte is also retained within megasporangium.
• The male or female cones may be borne on the same tree (Pinus) or on different trees (Cycas).
• Unlike bryophytes and pteridophytes, in gymnosperms, the male and the female gametophytes do not have an independent free-living existence. They remain within the sporangia retained on the sporophytes.
• The pollen grain released from the microsporangium are carried in air currents and meet the opening of the ovules. The pollen tube carrying the male gametes grows towards archegonia in the ovules and discharges their contents near the mouth of the archegonia.
• After fertilization, zygote develops into an embryo and the ovules into seeds.

Angiosperms (Flowering Plants)

• They are an exceptionally large group of plants.
• They range in size from tiny, almost microscopic Wolffia to tall trees of Eucalyptus (over 100 metres).
• They include 2 classes:
  • Dicotyledons: Have 2 cotyledons in seeds, reticulate venations in leaves and tetramerous or pentamerous flowers (4 or 5 members in each floral whorl).
  • Monocotyledons: Have only one cotyledon, parallel venation in leaves and trimerous flowers (3 members in each floral whorl).

Reproduction:

• Flower is the reproductive structure.
• Male sex organ in a flower is the stamen. Each stamen consists of a filament with an anther at the tip. Within the anthers, the pollen mother cell divides by meiosis to produce microspores which mature into pollen grains.
• Female sex organ in a flower is the pistil. It consists of a swollen ovary at its base, a long slender style & stigma. Ovary contains ovules. An ovule has a megaspore mother cell that undergoes meiosis to form 4 haploid megaspores. 3 of them degenerate and one divides to form embryo sac.
• Each embryo-sac has a 3-celled egg apparatus (one egg cell & two synergids), 3 antipodal cells & 2 polar nuclei. The polar nuclei eventually fuse to produce a diploid secondary nucleus.
• Pollen grains dispersed from anthers are carried by wind or other agencies to the stigma of pistil. It is called pollination.
• Pollen grains germinate on the stigma and the resulting pollen tubes grow through the tissues of stigma and style and reach the ovule.
• Pollen tubes enter the embryo-sac where 2 male gametes are discharged. One male gamete fuses with egg cell to form zygote (syngamy). The other male gamete fuses with diploid secondary nucleus to produce triploid primary endosperm nucleus (PEN). Because of the involvement of two fusions, this event is called double fertilisation. It is an event unique to angiosperms.
• The zygote develops into an embryo (with one or two cotyledons). The PEN develops into endosperm which provides nourishment to the developing embryo.
• Synergids & antipodals degenerate after fertilization.
• During these events, the ovules develop into seeds and the ovaries develop into fruit.
• The seeds are enclosed by fruits.

Plant Life Cycles and Alternation of Generations

• In plants, both haploid and diploid cells can divide by mitosis. This forms haploid and diploid plant bodies.
• Haploid plant body (gametophyte) produces gametes by mitosis.
• After fertilization, the zygote also divides by mitosis to produce a diploid plant body (sporophyte). This produces haploid spores by meiosis.
• Spores divide by mitosis to form a haploid plant body.
• Thus, during the life cycle of any sexually reproducing plant, there is an alternation of generations between gametophyte (n) and sporophyte (2n).

Patterns of Plant Life Cycles

• Haplontic: In this, sporophytic generation is represented only by the zygote. There are no free-living sporophytes. Zygote undergoes meiosis to form haploid spores. They divide mitotically to form gametophyte. The dominant, photosynthetic phase is the free-living gametophyte. E.g., Algae such as Volvox, Spirogyra and some species of Chlamydomonas.

• Diplontic: In this, diploid sporophyte is the dominant, photosynthetic, independent phase. Gametophytic phase is represented by the single to few-celled haploid gametophyte. E.g., An alga, Fucus sp., and all seed-bearing plants (gymnosperms and angiosperms - the gametophytic phase is few to multi-celled).

• Haplo-diplontic: It is the intermediate condition between haplontic and diplontic. Both gametophyte and sporophyte are multicellular and often free-living. But they have different dominant phases. E.g., Bryophytes and Pteridophytes.