In-Depth Notes on Plant Diversity and Life Cycle

Plant Life Cycle

• The plant life cycle involves alternating generations of sporophyte and gametophyte stages.
• Sporophyte (2n) is the diploid phase that produces spores through meiosis.
• Gametophyte (n) is the haploid phase that produces gametes through mitosis.
• The dominance of these generations has shifted over evolutionary time:
  • Non-vascular plants like bryophytes have gametophyte as the dominant stage.
  • In contrast, vascular plants predominantly exhibit the sporophyte generation.

Adaptations to Terrestrial Life

• Adaptations were necessary for plants transitioning from aquatic to terrestrial environments:
  • Water is limited in abundance on land; adaptations were made for water retention and transport.
  • UV radiation is more intense in terrestrial environments, requiring protective mechanisms.
  • Structures were developed for stability against wind and gravity.
  • Mechanisms to protect and effectively distribute reproductive cells were necessary.

Vascular Tissues and Complex Structures

• Vascular plants evolved to have specialized tissues:
  • Xylem: transports water and minerals from roots to shoots.
  • Phloem: transports products of photosynthesis from shoots to roots.
  • Ground tissues support the plant structure and facilitate storage and transport.

Types of Roots

• Roots play essential roles in plants:
  • Anchor and support the above-ground plant structures.
  • Absorb and transport water and nutrients.
  • Types of roots include:
  • Taproots: Deep, single roots that store food and anchor plants.
  • Fibrous Roots: A network of thin roots for stability and nutrient absorption.

Seed Dispersal Mechanisms

• Seeds can be dispersed by various methods:
  • Wind, water, animals, or physical mechanisms. For example:
  • Dandelions use wind for dispersal.
  • Coconut is dispersed by water.

Bryophytes

• Non-vascular plants such as mosses, liverworts, and hornworts:
  • Dominated by the gametophyte generation, which is photosynthetic and larger.
  • Require water for sexual reproduction due to the need for flagellated sperm.
  • Rhizoids anchor them and assist in water absorption.

Vascular Plants

• Vascular plants include ferns and seed plants:
  • Lycophytes and ferns have evolved vascular tissues for support and nutrient transport.
  • Seed plants include gymnosperms (naked seeds) and angiosperms (fruit-bearing).

Gymnosperms and Angiosperms

• Gymnosperms: Cone-bearing plants, include cycads and ginkgo.
• Angiosperms: Flowering plants, most diverse group with adaptations such as:
  • Flowers for efficient pollination.
  • Carpels and fruits for ovule protection and seed dispersal.

Angiosperm Life Cycle

• Adult sporophytes produce:
  • Microspores (pollens) produced by microspore mother cells, leading to pollen grains.
  • Megaspores produced by megaspore mother cells leading to ovum formation.
• Double Fertilization occurs when one sperm fertilizes the ovum and the other fuses with polar nuclei to form the endosperm.

Characteristics of Monocots versus Dicots

• Monocots: One cotyledon, parallel leaf venation, fibrous root system.
• Dicots: Two cotyledons, branched leaf venation, tap root system.

Plant Growth Patterns

• Annuals: Complete lifecycle in one growing season.
• Biennials: Require two years for completion of lifespan.
• Perennials: Live for multiple years and can reproduce several times.