Chapter 2: Plant Kingdom

Introduction

The plant kingdom represents a diverse group of multicellular, eukaryotic organisms that are primarily autotrophic. Plants evolved from aquatic ancestors and gradually adapted to terrestrial habitats through significant evolutionary advancements. The study of the plant kingdom reveals the fascinating journey of plant evolution and adaptation.

Classification of Plant Kingdom

Algae

• Thalloid body structure (undifferentiated into root, stem, and leaves)
• Mostly aquatic, some terrestrial in moist habitats
• Chlorophyll-bearing simple photosynthetic organisms
• Cell walls primarily composed of cellulose
• Reserve food material: starch

Classification of Algae

Chlorophyceae (Green Algae)

• Contain chlorophyll a and b, similar to land plants
• Store starch as reserve food
• Cell wall made of cellulose
• Examples: Chlamydomonas, Spirogyra, Ulothrix, Volvox

Phaeophyceae (Brown Algae)

• Contain fucoxanthin pigment giving brown color
• Store mannitol and laminarin as food reserves
• Cell wall composed of cellulose and alginic acid
• Mostly marine, largest and most complex algae
• Examples: Fucus, Laminaria, Sargassum, Ectocarpus

Rhodophyceae (Red Algae)

• Contain r-phycoerythrin pigment giving red color
• Store floridean starch as food reserve
• Predominantly marine, found in deeper waters
• Examples: Porphyra, Gelidium, Gracilaria

Bryophytes (Amphibians of Plant Kingdom)

• First land plants, evolutionarily between algae and pteridophytes
• Lack true vascular tissues (xylem and phloem)
• Require water for sexual reproduction
• Show alternation of generations with dominant gametophyte
• Examples: Mosses, liverworts, hornworts

Major Groups of Bryophytes

Hepaticae (Liverworts)

• Thalloid or leafy body structure
• Rhizoids present for attachment
• Example: Marchantia, Riccia

Anthocerotae (Hornworts)

• Thalloid gametophyte with horn-like sporophytes
• Stomata present in sporophytes
• Example: Anthoceros

Musci (Mosses)

• Leafy gametophyte with rhizoids
• Complex sporophyte with seta and capsule
• Example: Funaria, Polytrichum, Sphagnum

Pteridophytes (First Vascular Plants)

• Possess vascular tissues (xylem and phloem)
• Dominant sporophyte generation
• Reproduce by spores (seedless vascular plants)
• Still require water for fertilization
• Examples: Ferns, horsetails, club mosses

Major Groups of Pteridophytes

Psilopsida

• Primitive vascular plants
• Lack true roots and leaves
• Example: Psilotum

Lycopsida (Club Mosses)

• Microphyllous leaves (single unbranched vein)
• Example: Selaginella, Lycopodium

Sphenopsida (Horsetails)

• Jointed stems with whorled branches
• Example: Equisetum

Pteropsida (Ferns)

• Most advanced pteridophytes
• Large, compound leaves called fronds
• Example: Pteris, Dryopteris, Nephrolepis

Gymnosperms (Naked Seed Plants)

• First seed-bearing plants
• Seeds not enclosed in fruits (naked seeds)
• Dominant sporophyte with reduced gametophyte
• Adapted to dry environments
• Well-developed vascular system
• Examples: Pines, spruces, cycads, ginkgo

Major Groups of Gymnosperms

Cycadophyta

• Palm-like appearance
• Pinnate leaves forming a crown
• Example: Cycas

Ginkgophyta

• Single living species: Ginkgo biloba
• Fan-shaped leaves with dichotomous venation

Coniferophyta (Conifers)

• Largest group of gymnosperms
• Needle-like or scale-like evergreen leaves
• Examples: Pinus (pine), Cedrus (cedar), Abies (fir)

Gnetophyta

• Most advanced gymnosperms
• Vessel elements in xylem
• Examples: Gnetum, Ephedra, Welwitschia

Angiosperms (Flowering Plants)

• Most advanced plant group
• Seeds enclosed in fruits
• Flowers as reproductive structures
• Dominant plant life on earth today
• Complete vascular system
• Examples: Oak trees, roses, grasses, orchids

Classification of Angiosperms

Dicotyledons (Dicots)

• Two cotyledons in seed
• Reticulate venation in leaves
• Tap root system
• Floral parts in multiples of 4 or 5
• Examples: Beans, roses, oaks

Monocotyledons (Monocots)

• Single cotyledon in seed
• Parallel venation in leaves
• Fibrous root system
• Floral parts in multiples of 3
• Examples: Grasses, lilies, palms

Plant Life Cycles and Alternation of Generations

Haplontic Life Cycle

• Dominant in algae
• Gametophyte (n) is the dominant phase
• Zygote is the only diploid cell
• Zygote undergoes meiosis to form haploid spores

Diplontic Life Cycle

• Dominant in angiosperms and gymnosperms
• Sporophyte (2n) is the dominant phase
• Gametophyte highly reduced
• Gametes are the only haploid cells

Haplo-diplontic Life Cycle

• Found in bryophytes and pteridophytes
• Both gametophyte (n) and sporophyte (2n) phases are multicellular
• Distinct alternation of generations
• Bryophytes: Gametophyte dominant
• Pteridophytes: Sporophyte dominant

Evolutionary Trends in Plants

1. Transition from aquatic to terrestrial habitats
2. Development of vascular tissues for efficient transport
3. Evolution of seeds for protection and dispersal
4. Development of fruits for seed protection and dispersal
5. Reduced gametophyte and dominant sporophyte
6. Evolution of complex reproductive structures (flowers)
7. Adaptations for pollination and seed dispersal
8. Development of efficient water conservation mechanisms