Plant Kingdom

Plant Kingdom

Introduction

• Whittaker’s Five Kingdom Classification: Monera, Protista, Fungi, Animalia, and Plantae.
• This chapter focuses on the diversity and classification of the Plant Kingdom, covering algae, bryophytes, pteridophytes, gymnosperms, angiosperms, and their life cycles.

Changes in Understanding

• Fungi, Monera, and Protista with cell walls are now excluded from Plantae.
• Cyanobacteria (blue-green algae) are no longer considered ‘algae.’

Plant Kingdom Classification

1. Algae
2. Bryophytes
3. Pteridophytes
4. Gymnosperms
5. Angiosperms

Classification Systems

• Early Classification (Artificial):
  • Based on superficial traits like habit, color, leaf number and shape.
  • Linnaeus’ system used vegetative characters and androecium (male parts) structure.
  • Artificial System Drawbacks – it separated closely related species and gave equal weight to vegetative and sexual traits, which is problematic as vegetative traits can be environmentally influenced.
• Natural Classification:
  • Developed by George Bentham and Joseph Dalton Hooker.
  • Based on natural affinities and included both external and internal features.
  • Considers ultra-structure, anatomy, embryology, and phytochemistry.
• Phylogenetic Classification:
  • Based on evolutionary relationships.
  • Assumes common ancestry within taxa.
  • Utilizes multiple sources of information to resolve classification difficulties, especially without fossil evidence.

Modern Taxonomy Methods

1. Numerical Taxonomy:
   • Uses computers to analyze observable characteristics.
   • Assigns numbers and codes to traits, processing data to give equal importance to all characteristics.
2. Cytotaxonomy:
   • Based on cytological information like chromosome number, structure, and behavior.
3. Chemotaxonomy:
   • Uses chemical constituents of plants to resolve classification issues.

This provides an overview of how the plant kingdom is classified, the evolution of classification systems, and the modern methods used in taxonomy.

Algae

Introduction to Algae

• Algae are simple, thalloid body (lack true stems, roots, and leaves.), non-flowering, chlorophyll-bearing, autotrophic organisms.
• Mostly aquatic, found in freshwater and marine environments.
• Also found on moist surfaces, soils, wood, and in symbiosis with fungi and animals.

Forms and Sizes

• Microscopic unicellular forms like Chlamydomonas.
• Colonial forms like Volvox.
• Filamentous forms like Ulothrix and Spirogyra.
• Large marine forms like kelps.

Reproduction in Algae

• Vegetative Reproduction: By fragmentation.
• Asexual Reproduction: By spores, commonly zoospores (motile).
• Sexual Reproduction:
  • Isogamous: Fusion of similar gametes (e.g., Chlamydomonas).
  • Anisogamous: Fusion of dissimilar gametes (e.g., some species of Eudorina & Chlamydomonas).
  • Oogamous: Fusion of large non-motile female and small motile male gametes (e.g., Volvox ,Fucus).

Importance of Algae

• Fix half of the total carbon dioxide on Earth.
• Increase oxygen levels in water.
• Primary producers in aquatic food cycles.
• Some species are used as food and in commercial products (e.g., Porphyra, Laminaria & Sargassum).
• Agar, derived from Gelidium and Gracilaria, is used for growing microbes and in ice creams and jellies.
• Marine brown and red algae produce commercial hydrocolloids like algin and carrageen.
• Protein-rich unicellular algae like Chlorella and Spirulina, are food supplements for space travelers.

Classes of Algae

1. Chlorophyceae (Green Algae)
   • Unicellular, colonial, or filamentous.
   • Grass green due to chlorophyll a and b.
   • Chloroplasts vary in shape: discoid, plate-like, reticulate, cup-shaped, spiral, or ribbon-shaped.
   • They have pyrenoids in chloroplasts, which store protein and starch; some store food as oil droplets.
   • Cell walls are rigid, with an inner cellulose layer and an outer pectose layer.
   • Example species: Chlamydomonas, Volvox, Ulothrix, Spirogyra, Chara.
2. Phaeophyceae (Brown Algae)
   • Mostly marine.
   • Vary in size and form, from simple branched filamentous (e.g., Ectocarpus) to large kelps (up to 100 m).
   • Contain chlorophyll a, c, carotenoids, and fucoxanthin (brown pigment).
   • Store food as laminarin or mannitol.
   • Vegetative cells have a cellulosic wall with a gelatinous algin coating.
   • Protoplast contains plastids, a central vacuole, and a nucleus.
   • Plant body has a holdfast, stipe, and frond.
   • Example species: Ectocarpus, Dictyota, Laminaria, Sargassum, Fucus.
3. Rhodophyceae (Red Algae)
   • Predominantly marine, especially in warmer regions..
   • Found in well-lit regions near the surface and deep ocean areas with little light.
   • Contain red pigment r-phycoerythrin.
   • Red thalli are mostly multicellular.
   • Store food as floridean starch, similar to amylopectin and glycogen.
   • Example species: Polysiphonia, Porphyra, Gracilaria, Gelidium.

Reproduction in Classes

• Chlorophyceae:
  • Vegetative by fragmentation.
  • Asexual by flagellated zoospores.
  • Sexual methods include isogamous, anisogamous, and oogamous.
• Phaeophyceae:
  • Vegetative by fragmentation.
  • Asexual by biflagellate zoospores.
  • Sexual methods include isogamous, anisogamous, and oogamous.
• Rhodophyceae:
  • Vegetative by fragmentation.
  • Asexual by non-motile spores.
  • Sexual is oogamous with complex post-fertilization development.

Classes	Common Name	Major Pigments	Stored Food	Cell Wall	Flagellar Number and Position of Insertions	Habitat
Chlorophyceae	Green algae	Chlorophyll a, b	Starch	Cellulose	2-8, equal, apical	Fresh water, brackish water, salt water
Phaeophyceae	Brown algae	Chlorophyll a, c, fucoxanthin	Mannitol, laminarin	Cellulose and algin	2, unequal, lateral	Fresh water (rare), brackish water, salt water
Rhodophyceae	Red algae	Chlorophyll a, d, phycoerythrin	Floridean starch	Cellulose	Absent	Fresh water (some), brackish water, salt water (most)

Bryophytes

Introduction to Bryophytes

• Includes mosses and liverworts.
• Found in moist, shaded areas.
• Known as amphibians of the plant kingdom because they live in soil but need water for sexual reproduction.
• Important in plant succession on bare rocks/soil.

Plant Body

• More differentiated than algae.
• Thallus-like, prostrate, or erect.
• Attached to the substrate by rhizoids (unicellular or multicellular).
• Lack true roots, stems, or leaves (Thallus), but may have similar structures.
• Main plant body is haploid, called gametophyte and produces gametes..

Reproduction

• Sexual Reproduction:
  • Male organ (antheridium) produces biflagellate antherozoids.
  • Female organ (archegonium) produces a single egg.
  • Antherozoid fuses with the egg to form a zygote.
  • Zygote develops into a sporophyte attached to the gametophyte.
  • Sporophyte produces haploid spores through meiosis, which germinate into gametophytes.

Economic and Ecological Importance

• Limited economic importance but some mosses provide food for animals.
• Sphagnum moss provides peat, used as fuel and packing material for trans-shipment of living material due to its water-holding capacity..
• First colonizers of rocks, important in ecological succession.
• Help decompose rocks and make soil suitable for higher plants.
• Dense mats of mosses prevent soil erosion.

Types of Bryophytes

Liverworts

• Grow in moist, shady habitats.
• Thalloid body (e.g., Marchantia).
• Thallus is dorsiventral flat, leaf-like structure and close to the substrate.
• Asexual Reproduction: By fragmentation or gemmae (small green asexual buds in gemma cups).
• Sexual Reproduction: Male and female organs on the same or different thalli.
• Sporophyte stage has foot, seta, and capsule. Spores formed by meiosis in the capsule germinate into gametophytes.

Mosses

• Gametophyte: The dominant stage has two stages/part life cycle : protonema and leafy stage.
  • Protonema Stage: Creeping, green, filamentous, develops from a spore.
  • Leafy Stage: Upright, slender axes with spirally arranged leaves, develops from the protonema.
    • Attached to soil by multicellular, branched rhizoids; bears sex organs.
• Vegetative Reproduction: By fragmentation and budding in the protonema.
• Sexual Reproduction:
  • Antheridia & Archegonia: Located at the apex of leafy shoots.
  • Fertilization: Zygote develops into a sporophyte with foot, seta, and capsule.
  • Sporophyte: More complex than liverworts; capsule produces spores through meiosis with an advanced spore dispersal system.

Common Examples

• Funaria
• Polytrichum
• Sphagnum

Pteridophytes

Introduction to Pteridophytes

• Include horsetails and ferns.
• Used for medicinal purposes, soil-binding, and as ornamentals.
• First terrestrial plants with vascular tissues (xylem and phloem).
• Found in cool, damp, shady places, and some in sandy soils.

Plant Body

• Main plant body is a sporophyte.
• Differentiated into true roots, stems, and leaves.
• Possess well-differentiated vascular tissues.
• Leaves can be small (microphylls) like in Selaginella or large (macrophylls) like in ferns.

Reproduction

• Sporophytes bear sporangia on leaf-like appendages called sporophylls.
  • Sometimes Sporophylls may form compact structures called strobili or cones (e.g., Selaginella, Equisetum).
  • Sporangia produce spores through meiosis.
  • Spores germinate into small, multicellular, free-living gametophytes called prothallus.
• Gametophytes need cool, damp, shady places to grow and water for fertilization.
  • Gametophytes have male (antheridia) and female (archegonia) sex organs.
  • Water is required for the transfer of male gametes (antherozoids) to female gametes (eggs).
  • Fusion of gametes forms a zygote, developing into a multicellular sporophyte.

Life Cycle:

• Sporophyte: Dominant phase with true roots, stems, and leaves having vascular tissues.
• Gametophyte (prothallus): smaller, requires water for fertilization to produce the next sporophyte generation.

Spore Types

• Most pteridophytes are homosporous (one type of spore).
• Some genera like Selaginella and Salvinia are heterosporous (produce two types of spores: macro and micro).
• Megaspores and microspores develop into female and male gametophytes.
• Female gametophytes are retained on the parent sporophytes for embryo development, a step towards seed evolution.

Classification of Pteridophytes

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

Key Points

• Pteridophytes are the first plants with vascular tissues.
• They need specific conditions (cool, damp, shady places) and water for fertilization.
• Some are homosporous, while others are heterosporous, showing early steps towards seed development.

Gymnosperms

General Characteristics

• Gymnosperms: Plants with exposed (naked) seeds (no ovary wall).
• Include medium-sized trees, tall trees, and shrubs.
• Example: Giant redwood tree (Sequoia) is one of the tallest trees.

Roots and Stems

• Roots: Generally tap roots.
  • Some have mycorrhiza (fungal association) like in Pinus.
  • Some have coralloid roots with N2-fixing cyanobacteria like in Cycas.
• Stems: Can be unbranched (Cycas) or branched (Pinus, Cedrus).

Leaves

• Leaves: Simple or compound.
  • Cycas has pinnate leaves that persist for a few years.
  • Adapted to withstand temperature, humidity, and wind.
  • Conifers have needle-like leaves to reduce surface area, thick cuticle, and sunken stomata to reduce water loss.

Reproduction

• Gymnosperms are heterosporous: Produce two kinds of spores (microspores and megaspores).
• Sporophylls form cones (strobili).
  • Male Cones (Microsporangiate Strobili):
    • Microsporophylls with microsporangia.
    • Microspores develop into pollen grains (male gametophyte).
  • Female Cones (Macrosporangiate Strobili):
    • Megasporophylls with ovules or megasporangia.
    • Megaspore mother cell divides to form four megaspores; one develops into a multicellular female gametophyte within the megasporangium (Archegonium).

Fertilization

• Male and female gametophytes remain within their respective sporangia on sporophytes.
• Pollen grains are carried by air currents to the ovules.
• Pollen tube grows towards the archegonia, discharging male gametes.
• After fertilization, zygote develops into an embryo, and ovules develop into seeds.

Key Points

• Seeds are not covered (naked seeds).
• Example plants include Pinus and Cycas.

Angiosperms

General Characteristics

• Angiosperms are flowering plants.
• Seeds are enclosed by fruits, unlike gymnosperms where seeds are naked.
• They are found in a wide range of habitats and vary greatly in size (e.g., tiny Wolfia to tall Eucalyptus trees).

Importance

• Provide food, fodder, fuel, medicines, and other commercial products.

Classes of Angiosperms

• Dicotyledons: Two cotyledons in seeds.
  • reticulate venation, and tetramerous/pentamerous flowers.
• Monocotyledons: One cotyledon in seeds.
  • parallel venation, and trimerous flowers.

Reproductive Organs

Flower: The reproductive structure for sexual reproduction.

• Male Organ (Stamen):
  • Consists of a filament and anther.
  • Anthers produce pollen grains through meiosis.
• Female Organ (Pistil/Carpel):
  • Consists of an Stigma, Style, Ovary with ovules.
  • Ovule’s megaspore mother cell undergoes meiosis to form four megaspores; Typically, only one of these megaspores survives and further forms the embryo sac.
  • Summary-
    • The ovule contains the megasporangium (a structure) where the megaspore mother cell lives and produces megaspores.
    • One of these megaspores develops into the embryo sac (female gametophyte), which contains the egg cell.

Embryo-Sac Structure

• 3-celled egg apparatus (1 egg cell, 2 synergids),
• 3 antipodal cells,
• 2 polar nuclei that fuse to form a diploid secondary nucleus.

Pollination and Fertilization

• Pollination: Pollen grains are carried from anther to the stigma by wind or other agents.
• Fertilization:
  • Pollen grains germinate, pollen tube grows from stigma to ovule, releasing two male gametes.
  • Two male gametes are released into the embryo-sac.
  • One male gamete fuses with egg cell to form zygote (syngamy).
  • The other fuses with the diploid secondary nucleus to form the triploid primary endosperm nucleus (PEN).
  • This double fertilization is unique to angiosperms

Double Fertilization & Development

• Unique to angiosperms.
• Results in the formation of:
  • Zygote: Develops into an embryo (with one or two cotyledons).
  • Endosperm (PEN): PEN develops into endosperm, provides nourishment to the developing embryo.

Post-Fertilization

• Synergids and antipodals degenerate.
• Ovules develop into seeds.
• Ovaries develop into fruit.

Plant Life Cycles and Alternation of Generations

Alternation of Generations

• Plants have both haploid and diploid stages in their life cycles.
• Haploid (n) plant stage produce gametes by mitosis and are called gametophytes.
  • These gamete fuses together and forms zygote which develop into Sporophyte.
• Diploid (2n) plant stage produce spores by meiosis and are called sporophytes.
• This alternation between haploid gametophytes and diploid sporophytes is called alternation of generations.

Types of Life Cycles

1. Haplontic Life Cycle
   • Sporophyte: Represented only by a one-celled zygote.
   • Gametophyte: Dominant, photosynthetic, and free-living.
   • Example: Many algae like Volvox, Spirogyra, and some species of Chlamydomonas.
   • Characteristics: Zygote undergoes meiosis to produce haploid spores, which grow into gametophytes.
2. Diplontic Life Cycle
   • Sporophyte: Dominant, photosynthetic, and independent.
   • Gametophyte: Small, short-lived, and often just a few cells.
   • Example: All seed-bearing plants like gymnosperms and angiosperms.
   • Characteristics: Gametophyte phase is brief, and the sporophyte is the main life stage.
3. Haplo-diplontic Life Cycle
   • Bryophytes:
     • Gametophyte: Dominant, independent, photosynthetic, and can be thalloid or erect.
     • Sporophyte: Multicellular but short-lived, dependent on gametophyte for nutrients.
     • Example: Mosses and liverworts.
   • Pteridophytes:
     • Sporophyte: Dominant, independent, photosynthetic, vascular plant body.
     • Gametophyte: Multicellular, often free-living but short-lived.
     • Example: Ferns and horsetails.
   • Characteristics: Both phases are multicellular and often free-living; however, they differ in which phase is dominant.

Examples in Algae

• Haplontic: Most algal genera.
• Haplo-diplontic: Algae like Ectocarpus, Polysiphonia, and kelps.
• Diplontic: Alga Fucus.

Chapter Summary:

Plant Kingdom

• Includes: Algae, bryophytes, pteridophytes, gymnosperms, and angiosperms.

Algae

• Chlorophyll-bearing, simple, thalloid, autotrophic, largely aquatic.
• Classified into three classes based on pigment and stored food:
  • Chlorophyceae
  • Phaeophyceae
  • Rhodophyceae
• Reproduction:
  • Vegetative: Fragmentation
  • Asexual: Formation of different types of spores
  • Sexual: Formation of gametes (isogamy, anisogamy, oogamy)

Bryophytes

• Can live in soil but need water for sexual reproduction.
• Plant body is thallus-like, prostrate or erect, with rhizoids.
• Possess root-like, leaf-like, and stem-like structures.
• Divided into:
  • Liverworts: Thalloid and dorsiventral plant body
  • Mosses: Upright, slender axes with spirally arranged leaves
• Main plant body (gametophyte) produces:
  • Male sex organs: Antheridia
  • Female sex organs: Archegonia
• Zygote forms a sporophyte, which produces haploid spores.

Pteridophytes

• Main plant is a sporophyte with true root, stem, and leaves.
• Organs have well-differentiated vascular tissues.
• Sporophytes bear sporangia that produce spores.
• Spores germinate to form gametophytes (need cool, damp places).
• Gametophytes bear antheridia (male) and archegonia (female).
• Water is needed for male gametes to reach archegonium.
• Zygote produces a sporophyte.

Gymnosperms

• Ovules are not enclosed by ovary wall (naked-seeded plants).
• Produce microspores and megaspores in microsporangia and megasporangia on sporophylls.
• Sporophylls form male and female cones.
• Pollen grain germinates, pollen tube releases male gamete into ovule, fuses with egg cell.
• Zygote develops into embryo, ovules into seeds.

Angiosperms

• Male (stamen) and female (pistil) sex organs are in a flower.
• Stamen: Filament and anther (produces pollen grains after meiosis).
• Pistil: Ovary with ovules.
• Ovule contains female gametophyte or embryo sac with egg cell.
• Pollen tube enters embryo-sac, discharges two male gametes:
  • One fuses with egg cell (syngamy)
  • One fuses with diploid secondary nucleus (triple fusion)
• Unique double fertilization.
• Divided into two classes:
  • Dicotyledons
  • Monocotyledons

Life Cycle

• Alternation of generations between:
  • Haploid gametophyte: Produces gametes
  • Diploid sporophyte: Produces spores
• Different patterns of life cycles:
  • Haplontic
  • Diplontic
  • Intermediate