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.