BIOL 108 Introduction to Biological Diversity

Topic 13: Evolution of Land Plants

Geological Setting

• Terrestrial surface of Earth lifeless for over 3 billion years.
• Key events:
  • Cyanobacteria utilized land resources ~1.2 billion years ago.
  • Small plants, fungi, animals appeared on land ~500 million years ago.
• Approx. 250,000 plant species identified; mostly in terrestrial environments.

Characteristics Shared between Land Plants and Protists

• Land plants share traits with algae:
  1. Multicellular, photoautotrophic eukaryotes.
  2. Cell walls with cellulose.
  3. Chloroplasts with chlorophyll a and b.
  4. Store photosynthetic sugars as starch.
  5. Life cycle with alternation of generations.

Evolution of Green Algae Leading to Land Plants

• Closest relatives are charophytes.
• Streptophyta clade includes both charophytes and land plants.
• Shared features:
  1. Plasma membrane with cellulose-synthesizing protein rings.
  2. Flagellated sperm structure.
  3. Phragmoplast formation during mitosis.

Phylogeny of Basal Land Plants

• Closest taxon to land plants includes Viridiplantae, Streptophyta, and Embryophyta.
• Evidence from chloroplast structure and embryonic development.

Benefits of Moving to Land

1. Decreased competition.
2. Increased photosynthesis.
3. Abundant CO2.
4. Rich soil minerals.
5. Few herbivores/pathogens initially.

Challenges of Moving to Land

1. Scarcity of water.
2. Need for structural support against gravity.

Adaptations for Land Colonisation

1. Sporopollenin for durability.
2. Structures to conserve water (waxy cuticle, stomata).
3. Lignified vascular tissues (xylem, phloem).
4. Functional compartmentalisation (roots, stems, shoots).

Shared, Derived Traits of Land Plants

1. Alternation of two multicellular generations.
2. Embryo retention in the female gametophyte.
3. Sporangia producing walled spores.
4. Presence of multicellular gametangia (archegonia, antheridia).
5. Presence of apical meristems.

Origin of Land Plants

• Photosynthetic cyanobacteria ~1.2 billion years ago.
• Fossilised spores ~470 million years ago.
• Molecular clock estimates origin 425-490 million years ago.

Divergence of Land Plants

• Basal land plants classified into non-vascular and vascular.
• Non-vascular: bryophytes (liverworts, mosses, hornworts).

Basic Phylogenetic Framework of Land Plants

• Non-vascular and vascular plants.
• Vascular plants include paraphyletic seedless plants (lycophytes, monilophytes) and seed plants (gymnosperms, angiosperms).

Reflective Questions

• Why are land plants believed to evolve from algae?
• Did plant characteristics appear gradually?
• How are land plants defined?
• What adaptations facilitated transition to land in streptophytes?
• How do terrestrial adaptations address land challenges?
• Can you illustrate land plant characteristics?