NCERT Biology: Kingdom Plantae Notes

Classification of the Plant Kingdom

• Historical Context:

  • Robert Whittaker (1969) proposed the Five Kingdom classification: Monera, Protista, Fungi, Animalia, and Plantae.
  • Over time, the definition of Kingdom Plantae has changed. Fungi and members of Monera and Protista that possess cell walls have been excluded, despite being included in earlier systems.
  • Cyanobacteria, previously called blue-green algae, are no longer classified as 'algae' but belong to Monera.
  • Current Kingdom Plantae includes: Algae, Bryophytes, Pteridophytes, Gymnosperms, and Angiosperms.

• Classification Systems for Angiosperms:

  • Artificial Systems: Used by Linnaeus. Based on gross superficial morphological characters (habit, colour, leaf number and shape) or androecium structure.
    • Weaknesses: Separated closely related species based on few characteristics; gave equal weight to vegetative and sexual characteristics, which is problematic as vegetative traits are easily affected by the environment.
  • Natural Classification Systems: Developed by George Bentham and Joseph Dalton Hooker. Based on natural affinities and considered both external and internal features (ultra-structure, anatomy, embryology, and phytochemistry).
  • Phylogenetic Classification Systems: Currently acceptable systems based on evolutionary relationships. Assumes organisms in the same taxa share a common ancestor.
  • Numerical Taxonomy: Uses computers and all observable characteristics. Numbers and codes are assigned to characters, allowing hundreds of factors to be weighted equally.
  • Cytotaxonomy: Based on cytological information including chromosome number, structure, and behaviour.
  • Chemotaxonomy: Uses the chemical constituents of plants to resolve taxonomic confusions.

Algae

• General Characteristics:

  • Chlorophyll-bearing, simple, thalloid, autotrophic, and largely aquatic (freshwater and marine).
  • Habitats: Moist stones, soils, wood, and symbiotic associations (lichens with fungi, or on animals like the sloth bear).
  • Size and Form: Highly variable.
    • Colonial: Volvox.
    • Filamentous: Ulothrix and Spirogyra.
    • Massive bodies: Kelps (marine).

• Reproduction:

  • Vegetative: Via fragmentation; each fragment develops into a thallus.
  • Asexual: Via spores, most commonly flagellated (motile) zoospores.
  • Sexual: Fusion of two gametes.
    • Isogamous: Gametes are similar in size. Can be flagellated (Ulothrix) or non-flagellated (Spirogyra).
    • Anisogamous: Fusion of two gametes dissimilar in size (e.g., species of Eudorina).
    • Oogamous: Fusion between one large, non-motile (static) female gamete and a smaller, motile male gamete (e.g., Volvox, Fucus).

• Economic Importance:

  • Carbon Dioxide ($CO_2$) Fixation: Algae perform at least half of the total $CO_2$ fixation on Earth through photosynthesis.
  • Oxygen Production: Increase dissolved oxygen levels in aquatic environments.
  • Primary Producers: Form the basis of food cycles for all aquatic animals.
  • Food Source: Approximately $70$ species of marine algae are used as food, including Porphyra, Laminaria, and Sargassum.
  • Hydrocolloids: Water-holding substances produced by marine brown algae (algin) and red algae (carrageen).
  • Agar: Obtained from Gelidium and Gracilaria; used for microbe culture and in ice creams and jellies.
  • Space Food: Chlorella, a unicellular protein-rich alga, is used as a food supplement by space travellers.

Classes of Algae

• Chlorophyceae (Green Algae):

  • Pigments: Dominance of chlorophyll $a$ and $b$, giving them a grass-green colour. Pigments are in defined chloroplasts (discoid, plate-like, reticulate, cup-shaped, spiral, or ribbon-shaped).
  • Storage: Pyrenoids (located in chloroplasts) contain protein besides starch. Some store food as oil droplets.
  • Cell Wall: Rigid; inner layer of cellulose and outer layer of pectose.
  • Reproduction: Vegetative (fragmentation), asexual (flagellated zoospores in zoosporangia), and sexual (isogamous, anisogamous, or oogamous).
  • Examples: Chlamydomonas, Volvox, Ulothrix, Spirogyra, and Chara.

• Phaeophyceae (Brown Algae):

  • Habitat: Primarily marine. Range from simple branched filamentous forms (Ectocarpus) to massive kelps (up to $100\,m$ in height).
  • Pigments: Chlorophyll $a, c$, carotenoids, and xanthophylls (specifically fucoxanthin which determines the shade of brown).
  • Storage: Complex carbohydrates like laminarin or mannitol.
  • Structure: Cellulosic wall with a gelatinous coating of algin. Plant body consists of a holdfast (attachment), stipe (stalk), and frond (leaf-like photosynthetic organ).
  • Reproduction: Asexual via biflagellate, pear-shaped zoospores with unequal lateral flagella. Sexual can be isogamous, anisogamous, or oogamous; gametes are pyriform with lateral flagella.
  • Examples: Ectocarpus, Dictyota, Laminaria, Sargassum, and Fucus.

• Rhodophyceae (Red Algae):

  • Pigments: Predominance of red pigment $r$-phycoerythrin. Also contain chlorophyll $a, d$.
  • Habitat: Mostly marine, especially in warmer areas. Found at surface well-lighted regions and great depths.
  • Storage: Floridean starch, similar to amylopectin and glycogen in structure.
  • Reproduction: Vegetative (fragmentation), asexual (non-motile spores), and sexual (non-motile gametes, oogamous with complex post-fertilisation developments).
  • Examples: Polysiphonia, Porphyra, Gracilaria, and Gelidium.

Bryophytes

• General Characteristics:

  • Includes mosses and liverworts. Known as the "amphibians of the plant kingdom" because they live in soil but require water for sexual reproduction.
  • Role: Important for plant succession on bare rocks/soil.
  • Structure: Thallus-like (prostrate or erect) attached by unicellular or multicellular rhizoids. Lack true roots, stems, or leaves, but possess similar structures.

• Life Cycle:

  • Main plant body is haploid ($n$), called a gametophyte.
  • Sex Organs: Multicellular. Male is the antheridium (produces biflagellate antherozoids); female is the archegonium (flask-shaped, produces one egg).
  • Fertilisation: Antherozoids swim in water to reach the archegonium. Fusion creates a zygote ($2n$).
  • Sporophyte: Zygote produces a multicellular sporophyte which is not free-living but remains attached to the gametophyte for nourishment. Cells in the sporophyte undergo meiosis to produce haploid spores, which germinate back into gametophytes.

• Economic and Ecological Importance:

  • Sphagnum: A moss providing peat used as fuel and packing material for shipping living material (due to water-holding capacity).
  • Succession: Pioneer organisms on rocks (along with lichens), decomposing rock for higher plants.
  • Soil protection: Form dense mats that reduce the impact of rain and prevent soil erosion.

• Liverworts:

  • Example: Marchantia. Thallus is dorsiventral.
  • Asexual reproduction: Via fragmentation or gemmae (green, multicellular buds in gemma cups).
  • Sporophyte consists of foot, seta, and capsule.

• Mosses:

  • Two stages of gametophyte:
    • Protonema: Creeping, green, branched, filamentous stage from spore.
    • Leafy Stage: From secondary protonema; consists of upright axes with spiral leaves and multicellular rhizoids.
  • Examples: Funaria, Polytrichum, Sphagnum.

Pteridophytes

• General Characteristics:

  • Includes horsetails and ferns. Evolutionarily the first terrestrial plants to possess vascular tissues (xylem and phloem).
  • Habitat: Cool, damp, shady places.
  • Structure: Dominant phase is the sporophyte ($2n$), differentiated into true roots, stem, and leaves.
    • Microphylls: Small leaves (Selaginella).
    • Macrophylls: Large leaves (Ferns).

• Reproduction:

  • Sporophylls are leaf-like appendages subtending sporangia. Some form compact strobili/cones (Selaginella, Equisetum).
  • Spores ($n$) produced via meiosis germinate into a prothallus (small, multicellular, photosynthetic, free-living gametophyte).
  • Water is required for the transfer of antherozoids to the archegonium.
  • Heterospory: Some genera like Selaginella and Salvinia produce two types of spores: macro (large) and micro (small). This is a precursor to the seed habit.

• Classification:

  • Psilopsida (Psilotum)
  • Lycopsida (Selaginella, Lycopodium)
  • Sphenopsida (Equisetum)
  • Pteropsida (Dryopteris, Pteris, Adiantum)

Gymnosperms

• General Characteristics:

  • Plants with naked seeds; ovules are not enclosed by an ovary wall.
  • Size: Range from shrubs to giant trees like Sequoia (Giant Redwood).
  • Roots: Generally tap roots.
    • Pinus: Mycorrhizal association.
    • Cycas: Coralloid roots associated with $N_2$-fixing cyanobacteria.
  • Leaves: Adapted for extremes—needle-like in conifers to reduce surface area, with thick cuticle and sunken stomata to reduce water loss.

• Reproduction:

  • Heterosporous: Produce microspores and megaspores within strobili (cones).
  • Male Strobili: Bear microsporophylls. Microspores develop into pollen grains (highly reduced male gametophyte).
  • Female Strobili: Bear megasporophylls with ovules. Megaspore mother cell divides meiotically to form four megaspores; one develops into the multicellular female gametophyte containing two or more archegonia.
  • Unlike bryophytes/pteridophytes, gametophytes are not free-living; they remain on the sporophyte.
  • Pollination: Via air currents.

Angiosperms

• General Characteristics:

  • Flowering plants where seeds are enclosed in fruits. Exceptionally large group from tiny Wolffia to tall Eucalyptus (over $100\,m$).
  • Dicotyledons: Two cotyledons, reticulate venation, tetramerous or pentamerous flowers.
  • Monocotyledons: One cotyledon, parallel venation, trimerous flowers.

• Double Fertilisation (Unique to Angiosperms):

  • Stamen consists of filament and anther (produces pollen).
  • Pistil consists of ovary, style, and stigma. Ovules contain the embryo sac ($7$-celled, $8$-nucleate).
  • Two male gametes enter the embryo sac:
    • Syngamy: One male gamete ($n$) + Egg ($n$) $\rightarrow$ Zygote ($2n$).
    • Triple Fusion: One male gamete ($n$) + Diploid secondary nucleus ($2n$) $\rightarrow$ Primary Endosperm Nucleus (PEN) ($3n$).
  • Post-fertilisation: Zygote becomes embryo; PEN becomes endosperm (nourishment); ovules become seeds; ovaries become fruit.

Plant Life Cycles and Alternation of Generations

• Patterns:
  1. Haplontic: Sporophytic generation is represented only by the one-celled zygote. Dominant phase is the free-living gametophyte. Examples: Volvox, Spirogyra, some Chlamydomonas.
  2. Diplontic: Diploid sporophyte is dominant and independent. Gametophytic phase is single to few-celled. Examples: Fucus, Gymnosperms, and Angiosperms.
  3. Haplo-diplontic: Intermediate condition where both phases are multicellular.
     • Bryophytes: Gametophyte is dominant; sporophyte is dependent.
     • Pteridophytes: Sporophyte is dominant; gametophyte is independent but short-lived.
     • Algal exceptions: Ectocarpus, Polysiphonia, and kelps are haplo-diplontic.

Questions & Discussion

• Q: What is the basis of classification of algae?

  • A: Algae are classified into three classes—Chlorophyceae, Phaeophyceae, and Rhodophyceae—based on the type of pigments possessed and the type of stored food.

• Q: When and where does reduction division take place in the life cycle of various plants?

  • A: In liverworts and mosses, it occurs in the capsule of the sporophyte to produce spores. In ferns, it occurs in the sporangia. In gymnosperms and angiosperms, it occurs in the microsporangia (to produce pollen) and megasporangia (to produce the embryo sac/megaspores).

• Q: Mention the ploidy of various structures.

  • A: Protonemal cell of moss ($n$); PEN in dicot ($3n$); leaf cell of moss ($n$); prothallus cell of fern ($n$); gemma cell in Marchantia ($n$); meristem cell of monocot ($2n$); ovum of liverwort ($n$); zygote of fern ($2n$).