1. Systems of Classification

Three major types of classification systems have been used:

1.1 Artificial System

• Given by Carolus Linnaeus.
• Based on gross superficial morphological characters (habit, color, number/shape of leaves) or androecium structure.
• Drawback: Separated closely related species; gave equal weightage to vegetative and sexual characteristics (vegetative characters are easily affected by environment).

1.2 Natural System

• Given by George Bentham and Joseph Dalton Hooker.
• Based on natural affinities and considers external + internal features (ultrastructure, anatomy, embryology, phytochemistry).

1.3 Phylogenetic System

• Given by Engler & Prantl, Hutchinson.
• Based on evolutionary relationships.
• Assumes organisms in the same taxa have a common ancestor.

2. Algae

Chlorophyll-bearing, simple, thalloid, autotrophic, and largely aquatic (fresh & marine) organisms.

Common Characteristics

• Habit: Variable size/form. Colonical (Volvox), Filamentous (Ulothrix, Spirogyra), Massive kelps.
• Reproduction:
  • Vegetative: Fragmentation.
  • Asexual: Spores (commonly Zooospores - flagellated).
  • Sexual: Fusion of gametes.
    • Isogamous: Similar size (flagellated: Ulothrix, non-flagellated: Spirogyra).
    • Anisogamous: Dissimilar size (Eudorina).
    • Oogamous: Large static female + Small motile male (Volvox, Fucus).

Economic Importance

• Algae fix half of Earth's CO₂ via photosynthesis.
• Primary producers in aquatic food cycles.
• Food: Porphyra, Laminaria, Sargassum (70 marine species used).
• Hydrocolloids (Water holding): Algin (Brown algae), Carrageen (Red algae).
• Agar: From Gelidium, Gracilaria (for ice-creams, jellies, microbe cultures).
• Space Food: Chlorella, Spirullina (protein-rich unicellular algae).

Comparison of Algae Classes

3. Bryophytes

Known as the Amphibians of the Plant Kingdom because they live in soil but depend on water for sexual reproduction.

Key Features

• Thallus: More differentiated than algae. Prostrate or erect. Attached to substratum by rhizoids (unicellular or multicellular).
• Dominant Phase: Gametophyte (Haploid, n). It produces gametes.
• Sex Organs: Multicellular and jacketed.
  • Male: Antheridium (produces biflagellate antherozoids).
  • Female: Archegonium (Flask-shaped, produces single egg).
• Fertilization: Water needed for antherozoids to reach archegonium.
• Sporophyte: Not free-living; attached to and dependent on gametophyte for nutrition. Produces spores (via meiosis) that germinate into gametophyte.

3.1 Liverworts

• Example: Marchantia.
• Body: Dorsiventral thallus, appressed to substrate.
• Asexual Reproduction: Fragmentation or Gemmae (green, multicellular, asexual buds in gemma cups).

3.2 Mosses

• Examples: Funaria, Polytrichum, Sphagnum.
• Stages:
  1. Protonema: Creeping, green, branched, filamentous (from spore).
  2. Leafy Stage: Developed from secondary protonema, bears sex organs.
• Economic Importance: Sphagnum (Peat moss) provides peat (fuel, packing material due to water holding). Mosses + Lichens = First colonizers on rocks.

4. Pteridophytes

First terrestrial plants to possess vascular tissues (Xylem & Phloem).

Key Features

• Dominant Phase: Sporophyte (Diploid, 2n). Differentiated into true root, stem, and leaves.
• Leaves: Microphylls (small, e.g., Selaginella) or Macrophylls (large, e.g., Ferns).
• Sporophylls: Leaves bearing sporangia. Sometimes form distinct cones/strobili (e.g., Selaginella, Equisetum).
• Prothallus: Spores germinate to form small, multicellular, free-living, photosynthetic, thalloid gametophyte. Needs cool/damp places (restricted distribution).

Classification

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

5. Gymnosperms

Plants where ovules are not enclosed by ovary wall and remain exposed (Naked seeds), both before and after fertilization.

Features

• Size: Medium trees, tall trees (Sequoia is giant redwood), or shrubs.
• Roots: Tap roots.
  • Pinus: Roots have fungal association (Mycorrhiza).
  • Cycas: Small specialized roots (Coralloid roots) associated with N₂-fixing Cyanobacteria.
• Stem: Unbranched (Cycas) or Branched (Pinus, Cedrus).
• Leaves: Needle-like in conifers (reduce surface area), thick cuticle, sunken stomata (xerophytic adaptations).

Reproduction

• Heterosporous: Haploid microspores and megaspores.
• Cones:
  • Male Strobili: Bear microsporophylls -> Microsporangia -> Pollen grain (reduced male gametophyte).
  • Female Strobili: Bear megasporophylls -> Megasporangia (Ovules).
• Gametes: Pollen carried by wind -> Pollen tube -> Fertilisation -> Zygote -> Embryo.
• Seed: Ovules develop into seeds (naked).

6. Angiosperms

Flowering plants where pollen and ovules are developed in specialised structures called flowers. Seeds are enclosed by fruits.

• Classes:
  • Dicotyledons: Two cotyledons, reticulate venation, tetramerous/pentamerous flowers.
  • Monocotyledons: Single cotyledon, parallel venation, trimerous flowers.
• Double Fertilization (Unique):
  • Syngamy: Male gamete + Egg -> Zygote (2n).
  • Triple Fusion: Male gamete + 2 Polar nuclei -> PEN (3n).
  • PEN develops into Endosperm (nutritive tissue).

7. Plant Life Cycles

a. Haplontic

• Sporophytic generation represented ONLY by one-celled Zygote. No free-living sporophyte.
• Meiosis in zygote -> Haploid spores -> Gametophyte (Dominant).
• Examples: Volvox, Spirogyra, Chlamydomonas.

b. Diplontic

• Diploid sporophyte is dominant, photosynthetic, independent.
• Gametophyte is single-to-few celled haploid.
• Examples: All Seed-bearing plants (Gymnosperms, Angiosperms). Algae Exception: Fucus.

c. Haplo-diplontic

• Both phases are multicellular and often free-living.
• Bryophytes: Dominant Gametophyte (Haploid).
• Pteridophytes: Dominant Sporophyte (Diploid).
• Algae Exceptions: Ectocarpus, Polysiphonia, Kelps are haplo-diplontic.