GENERAL CLASSIFICATION OF PLANTS

Evolution of Biological Classification

• Initially (until the 20th century), all living things were classified under two kingdoms: Plantae or Animalia.
• Whittaker (1969) proposed a five-kingdom system: Monera, Protista, Fungi, Animalia, and Plantae.
• In Whittaker's system, Fungi, and members of Monera and Protista with cell walls, were excluded from Kingdom Plantae.
• Currently, living things are classified under six kingdoms: Bacteria, Archaea, Protista, Fungi, Plantae, and Animalia.

The Six Kingdoms (Key Characteristics)

1. Archaea

• Single-celled organisms (Unicellular)
• Prokaryotes
• Autotrophs
• Example: Methanogen bacteria

2. Eubacteria

• Single-celled organisms
• Prokaryotes
• Nutrition:
  • Some are heterotrophs (cannot make their own food).
  • Others are autotrophs (make their own food) by getting energy from:
    • Sunlight (photosynthetic autotrophs)
    • Breakdown of inorganic sulfur & nitrogen compounds (chemosynthetic autotrophs)

3. Protista

• All are eukaryotes.
• Can be multicellular & unicellular.
• Vary in size from microscopic to very large.
• Contain both heterotrophs & autotrophs.
• Examples: Slime mould, Amoeba, Euglena, Diatom, Paramecium, etc.

4. Fungi

• All are eukaryotes.
• All are heterotrophs (either parasitic or mutualistic).
• Have a cell wall made of chitin & glycan.
• Most of them are multicellular.
• Examples: Mushroom, yeast.
• Note: Fungi are now regarded as a separate kingdom and are generally not treated as part of the plant kingdom, despite older classifications.

5. Kingdom Plantae

This kingdom consists of multicellular, eukaryotic, autotrophic organisms containing chlorophyll for photosynthesis. Plant characteristics are identified by cellular structure, habitats, adaptations, nutrition, inter-relationship, reproduction, life-cycles, importance, and classification.

General Characteristics of Cryptogams (Non-seed bearing plants, historically including some groups now separate):

• They are green (except for fungi, which are now a separate kingdom).
• They have hidden reproductive organs.
• They lack flowers and seeds.
• They usually occupy the basal portion of almost all natural ecosystems.
• They are commonly found in humid places on soil, decaying wood, or tree trunks.

Sub-divisions of Kingdom Plantae (Modern View)

The plant kingdom is traditionally sub-divided into two main divisions: Cryptogams and Phanerogams (Spermatophytes).

A. Cryptogams (Cryptogamae)

A cryptogam is a plant that reproduces by spores, without flowers or seeds. The term "Cryptogamae" comes from Greek words kryptos ("hidden") + gamos ("to marry"), referring to hidden reproduction due to the absence of seeds. Cryptogams are considered non-seed bearing plants and are opposite to Phanerogamae ("visible" reproduction).

The sub-kingdom cryptogams (in its traditional sense for plants) is divided into three divisions:

1. Thallophyta (Algae)

• Algae are autotrophic organisms with chlorophyll.
• They are oxygen-producing photosynthetic organisms primarily adapted to aquatic environments.
• The plant body shows no differentiation into root, stem, or leaf, and is called a thallus.
• They do not have vascular tissues.
• Examples: Chlamydomonas, Spirogyra, Zygnema etc.

2. Bryophyta (Bryophytes)

• Bryophytes are the simplest group of land plants.
• They are relatively poorly adapted to life on land, confined mainly to damp, shady places (e.g., damp rocks, logs, swamps, marshes).
• These are terrestrial non-vascular plants.
• They are more advanced than algae, developing special organs.
• The male sex organ is called an antheridium, and the female sex organ is called an archegonium.
• Bryophytes show distinct alternation of generation in their life cycles.
• Examples: Mosses, Liverworts, and Hornworts.

3. Pteridophyta (Pteridophytes)

• Pteridophytes are also called ferns or vascular cryptogams.
• They possess leaves, roots, and stems, but do not have flowers.
• Ferns have special stems called rhizomes, which grow sideways at the surface or underground.
• Many leaves (called fronds) grow from rhizomes, forming fern colonies.
• Examples: Lycopodium, Selaginella, Equisetum, Psilotum, Pteris, Dryopteris.

B. Spermatophytes (Phanerogams / Seed Plants)

Spermatophytes comprise two divisions: Gymnosperms and Angiosperms.

General characteristics of Spermatophytes:

• The plant body has roots, stems, leaves, and seed-bearing structures.
• They produce seeds.
• They have chlorophyll, hence are photosynthetic.
• They have highly developed vascular tissue, including xylem.
• They represent the largest vascular plant lineage, with about 270,000 living species.

1. Gymnosperms

• They form an intermediate group, related to higher cryptogams and angiosperms.
• Approximately 700 known species.
• They do not flower and do not bear their seeds enclosed in a fruit; thus, their seeds are naked.
• They bear two types of cones: male and female.
• After fertilization, seeds are not enclosed in a fruit wall.
• They show xerophytic characteristics (e.g., rolled leaves, needle-shaped leaves, sunken stomata, thick waxy leaves).
• Xylem consists mainly of tracheids, and phloem lacks companion cells.

2. Angiosperms

• This is the most advanced plant group, found almost everywhere.
• They include grasses, herbs, shrubs, and trees.
• They are flower-bearing and are usually bisexual.
• Seeds are enclosed in an ovary which develops into a fruit.
• Xylem has tracheids and vessels, and phloem has companion cells.
• They exhibit double fertilization.

Importance of Spermatophytes (and Photosynthesis)

• Spermatophytes perform Photosynthesis, a crucial process for obtaining food.
• Photosynthesis is the basis of life, regulates the concentration of gases in the atmosphere (affecting global climate), and produces essential nutrients for the proper functioning of living beings.