Plant Biology and Adaptations

Learning Objectives

  • Understand morphological traits of green plants and relationships among groups.
  • Evaluate molecular phylogeny of Plantae.
  • Compare traits of green algae, nonvascular plants, seedless vascular plants, gymnosperms, and angiosperms.
  • Explain adaptations for land living.
  • Map evolutionary changes in land plant phylogeny.
  • Identify challenges angiosperms face, including pollination and seed dispersal.

Key Plant Adaptations

1. Embryo Protection

  • Retention of multicellular embryo in female plant (distinguishes land plants from algae).
  • Embryos depend on parent plant for nourishment (Embryophyta: "embryo-plants").

2. Vascular Tissue

  • Vascular plants have xylem (transports water) and phloem (transports sugars).
  • Xylem has lignin-rich walls for structure; Phloem consists of sieve-tube elements and companion cells.

3. Seeds

  • Seeds facilitate dispersal and contain embryo plus nutrients (improved compared to spores).
  • Portable and allow colonization of new environments.

4. Flowers and Fruits

  • Flowers attract pollinators, leading to efficient pollination and reproductive success.
  • Fruits aid in seed dispersal through hooks, flotation, or being tasty to animals.

Plant Transport Mechanisms

Water Transport

  • Water moves from roots to shoots via transpiration; driven by evaporation and cohesive properties of water (Cohesion–tension theory).
  • Xylem's upward movement resists gravity due to structural support of lignin.

Sugar Transport

  • Sugars move from high concentration areas (sources like leaves) to low concentration areas (sinks like roots) via phloem.
  • Pressure-flow hypothesis explains movement driven by turgor pressure differences.

Major Plant Groups

  1. Non-vascular plants (Bryophytes)
    • Simplest forms, reliant on diffusion, water for reproduction.
  2. Seedless vascular plants (e.g., ferns)
    • Evolved vascular tissue, larger than bryophytes.
  3. Gymnosperms
    • Seed-producing plants, primarily cone-bearing.
  4. Angiosperms
    • Flowering plants with seeds enclosed in fruits; highest diversity and adaptive radiation.