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Biology: Class 11 — Chapter 3: Plant Kingdom

Classification Systems in Plantae

Our understanding of the plant kingdom has evolved over time. Fungi, Monera, and Protista with cell walls are now excluded from Plantae. Cyanobacteria (blue-green algae) are no longer classified as algae.

Types of Classification Systems

  • Artificial Classification System: Based on gross superficial morphological characters such as habit, colour, number, and shape of leaves, or on the androecium structure (system by Linnaeus). It separated closely related species and gave equal weightage to vegetative and sexual characteristics.
  • Natural Classification System: Based on natural affinities among organisms, considering both external and internal features like ultrastructure, anatomy, embryology, and phytochemistry (system by George Bentham and Joseph Dalton Hooker).
  • Phylogenetic Classification System: Based on evolutionary relationships, assuming that organisms belonging to the same taxa share a common ancestor.

Modern Taxonomic Approaches

  • Numerical Taxonomy: Easily carried out using computers; based on all observable characteristics. Numbers and codes are assigned to all characters and the data are processed so that each character receives equal importance.
  • Cytotaxonomy: Based on cytological information like chromosome number, structure, and behaviour.
  • Chemotaxonomy: Uses the chemical constituents of the plant to resolve taxonomic confusions.
3.1 Algae

Algae are chlorophyll-bearing, simple, thalloid, autotrophic, and largely aquatic (both fresh water and marine) organisms. They also occur in moist stones, soils, wood, and in association with fungi (lichens) or animals (e.g., on sloth bear).

Reproduction in Algae

  • Vegetative reproduction: Occurs by fragmentation; each fragment develops into a thallus.
  • Asexual reproduction: Occurs by the production of different types of spores, most commonly motile, flagellated zoospores.
  • Sexual reproduction: Occurs through the fusion of two gametes:
    • Isogamous: Fusion of gametes similar in size, which can be flagellated (e.g., Ulothrix) or non-flagellated (e.g., 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 of Algae

  • At least a half of the total carbon dioxide ($CO_2$) fixation on earth is carried out by algae through photosynthesis.
  • They are primary producers of energy-rich compounds forming the basis of food cycles for aquatic animals.
  • Species like Porphyra, Laminaria, and Sargassum are used as food.
  • Certain marine brown and red algae produce large amounts of hydrocolloids (water-holding substances), such as algin (brown algae) and carrageen (red algae).
  • Agar, obtained from Gelidium and Gracilaria, is used to grow microbes and in preparations of ice-creams and jellies.
  • Chlorella, a unicellular alga rich in proteins, is used as a food supplement by space travellers.

Classes of Algae

1. Chlorophyceae (Green Algae)

Commonly called green algae. The plant body may be unicellular, colonial, or filamentous.

  • Pigments: Dominance of chlorophyll $a$ and $b$, localized in definite chloroplasts (discoid, plate-like, reticulate, cup-shaped, spiral, or ribbon-shaped).
  • Storage Bodies: Contain one or more pyrenoids in chloroplasts, which store protein besides starch. Food may also be stored as oil droplets.
  • Cell Wall: Rigid wall made of an inner layer of cellulose and an outer layer of pectose.
  • Examples: Chlamydomonas, Volvox, Ulothrix, Spirogyra, and Chara.

2. Phaeophyceae (Brown Algae)

Found primarily in marine habitats. They range from simple branched, filamentous forms (Ectocarpus) to profusely branched kelps which may reach heights of 100 metres.

  • Pigments: Chlorophyll $a$, $c$, carotenoids, and xanthophylls (specifically fucoxanthin, which determines the olive green to brown shades).
  • Storage Food: Complex carbohydrates stored as laminarin or mannitol.
  • Cell Wall: Cellulosic wall covered on the outside by a gelatinous coating of algin.
  • Structure: Plant body is attached to the substratum by a holdfast, and has a stalk called the stipe, and a leaf-like photosynthetic organ called the frond.
  • Reproduction: Asexual reproduction by biflagellate, pear-shaped zoospores with two unequal laterally attached flagella.
  • Examples: Ectocarpus, Dictyota, Laminaria, Sargassum, and Fucus.

3. Rhodophyceae (Red Algae)

Commonly called red algae due to the predominance of the red pigment r-phycoerythrin. Mostly marine, occurring in well-lighted surface waters as well as great depths.

  • Storage Food: Stored as floridean starch, which is structurally very similar to amylopectin and glycogen.
  • Reproduction: Vegetative by fragmentation; asexual by non-motile spores; sexual by non-motile gametes (oogamous with complex post-fertilisation developments).
  • Examples: Polysiphonia, Porphyra, Gracilaria, and Gelidium.
3.2 Bryophytes

Bryophytes include mosses and liverworts commonly found in moist, shaded areas in the hills. They are called the amphibians of the plant kingdom because they live in soil but rely on water for sexual reproduction.

General Characteristics

  • The plant body is more differentiated than that of algae, thallus-like, prostrate or erect, and attached to the substratum by unicellular or multicellular rhizoids. They lack true roots, stems, or leaves.
  • The dominant plant body is haploid and produces gametes, hence called a gametophyte.
  • Sex Organs: Multicellular. Male organ is the antheridium (produces biflagellate antherozoids). Female organ is the flask-shaped archegonium (produces a single egg).
  • Fertilisation: Antherozoids are released into water to reach the archegonium. The fusion produces a zygote, which does not undergo immediate reduction division but develops into a multicellular sporophyte.
  • The sporophyte is dependent on the photosynthetic gametophyte for nourishment. Some cells of the sporophyte undergo meiosis to produce haploid spores, which germinate to form gametophytes.
  • Economic Importance: Species of Sphagnum (a moss) provide peat used as fuel and packing material for trans-shipment of living material due to its water-holding capacity. Mosses are ecological pioneers on bare rocks, preventing soil erosion.

Classification of Bryophytes

1. Liverworts

Grow in moist, shady habitats. The plant body is thalloid (e.g., Marchantia), dorsiventral, and closely appressed to the substrate.

  • Asexual Reproduction: Takes place by fragmentation of thalli, or by the formation of specialized green, multicellular, asexual buds called gemmae, which develop in gemma cups on the thalli.
  • Sexual Reproduction: Sporophyte is differentiated into a foot, seta, and capsule. Spores produced within the capsule germinate to form free-living gametophytes.

2. Mosses

The predominant stage of the life cycle is the gametophyte, consisting of two stages:

  1. Protonema stage: Develops directly from a spore; creeping, green, branched, and filamentous.
  2. Leafy stage: Develops from the secondary protonema as a lateral bud; consists of upright, slender axes bearing spirally arranged leaves, attached by multicellular branched rhizoids. This stage bears the sex organs.
  • Reproduction: Vegetative reproduction by fragmentation and budding in the secondary protonema. Sexual reproduction involves antheridia and archegonia at the apex of leafy shoots.
  • Examples: Funaria, Polytrichum, and Sphagnum.
3.3 Pteridophytes

Pteridophytes include horsetails and ferns. They are used for medicinal purposes, as soil-binders, and grown as ornamentals. Evolutionarily, they are the first terrestrial plants to possess vascular tissues (xylem and phloem).

General Characteristics

  • The dominant phase is a diploid sporophyte differentiated into true roots, stems, and leaves.
  • Leaves may be small (microphylls, as in Selaginella) or large (macrophylls, as in ferns).
  • Sporophytes bear sporangia subtended by leaf-like appendages called sporophylls. In some cases, sporophylls form compact structures called strobili or cones (e.g., Selaginella, Equisetum).
  • Sporangia produce spores by meiosis in spore mother cells. Spores germinate to give rise to prothallus (inconspicuous, small, multicellular, free-living, photosynthetic thalloid gametophytes).
  • Gametophytes require cool, damp, shady places and water for fertilisation, restricting their geographical spread.
  • Spore Types:
    • Homosporous: Plants producing similar kinds of spores (majority of pteridophytes).
    • Heterosporous: Plants producing two kinds of spores, macro (large) and micro (small) spores (e.g., Selaginella and Salvinia). The megaspores and microspores germinate into female and male gametophytes, respectively. The retention of the female gametophyte on the parent sporophyte is a precursor to the seed habit.

Classification of Pteridophytes

  • Psilopsida: e.g., Psilotum
  • Lycopsida: e.g., Selaginella, Lycopodium
  • Sphenopsida: e.g., Equisetum
  • Pteropsida: e.g., Dryopteris, Pteris, Adiantum
3.4 Gymnosperms

Gymnosperms (gymnos: naked, sperma: seeds) are plants in which the ovules are not enclosed by any ovary wall and remain exposed, both before and after fertilisation. The seeds are naked.

Morphological Features

  • Includes medium-sized trees, tall trees, and shrubs. The giant redwood tree Sequoia is one of the tallest tree species.
  • Roots: Generally tap roots. May have fungal associations (mycorrhiza in Pinus) or specialized roots associated with $N_2$-fixing cyanobacteria (coralloid roots in Cycas).
  • Stems: Unbranched (Cycas) or branched (Pinus, Cedrus).
  • Leaves: Simple or compound. Well-adapted to withstand extremes of temperature, humidity, and wind. Conifers have needle-like leaves, thick cuticles, and sunken stomata to reduce water loss.

Reproduction

  • Gymnosperms are heterosporous, producing haploid microspores and megaspores within sporangia borne on spirally arranged sporophylls forming strobili or cones.
  • Male Strobili: Bear microsporophylls and microsporangia. Microspores develop into a highly reduced male gametophyte called a pollen grain.
  • Female Strobili: Bear megasporophylls with ovules or megasporangia. The megaspore mother cell divides meiotically to form four megaspores, one of which develops into a multicellular female gametophyte bearing two or more archegonia.
  • Unlike bryophytes and pteridophytes, male and female gametophytes do not have an independent free-living existence; they remain within the sporangia on the parent sporophyte.
  • Pollen grains are carried by air currents to the ovules, where a pollen tube grows to discharge male gametes near the mouth of the archegonia. Fertilisation results in a zygote, which develops into an embryo, and the ovule becomes a naked seed.
3.5 Angiosperms

Angiosperms (flowering plants) are an exceptionally large group of plants where pollen grains and ovules are developed in specialized structures called flowers, and seeds are enclosed within fruits.