In-depth Notes on Plant Diversity and Evolution

Plant Evolution and Diversity

• Plants colonized land approximately 500 million years ago. By 385 million years ago, the first forests emerged.

Major Events in Plant Diversification

The fossil record documents five key events in the evolution of land plants:

1. Origin of Waxy Cuticle: Evidence includes fossilized spores with sporopollenin material that prevents dehydration.
2. Silurian–Devonian Explosion (416 to 359 mya): Emergence of vascular tissue, roots, and symbiotic fungi.
3. Carboniferous Period (359 to 299 mya): Dominance of seedless vascular plants leading to fossilized coal deposits.
4. Diversification of Gymnosperms (299 to 145 mya): Development of seeds and pollen.
5. Emergence of Angiosperms (150 mya): Appearance of flowering plants.

Plant Phylogenies

• Prior to seeds: Embryophytes grew in proximity to the parent gametophyte.
• With seeds: Plants can disperse over long distances via wind or other means.

Types of Plants

• Bryophytes: Nonvascular plants.
• Ferns: Seedless vascular plants.
• Gymnosperms: Naked-seed plants, e.g., conifers.
• Angiosperms: Flowering plants.

Adaptations of Seed Plants

• Reduced male and female gametophytes:
  • In gymnosperms, gametophytes develop within cones.
  • In angiosperms, they develop within flowers.
• Pollen: Allows for fertilization without water.
• Seeds: Protect the embryo and provide nourishment.

Life Cycle of Seed Plants

• Sporophyte Dominance:
  • Mosses and nonvascular plants have a gametophyte dominant life cycle.
  • Ferns: Independent, free-living sporophyte.
  • Seed Plants (Gymnosperms, Angiosperms): Have a reduced, dependent gametophyte that relies on sporophyte tissue for nutrition.

Terrestrial Adaptations of Gymnosperms

1. Reduction of gametophyte.
2. Development of pollen.
3. Seed formation.

Heterospory

• Production of two types of spores:
  • Microsporangia produce microspores -> Male gametophytes (sperm).
  • Megasporangia produce megaspores -> Female gametophytes (eggs).

Ovules and Eggs

• An ovule comprises a megaspore within a megasporangium, covered by integuments (one in gymnosperms, two in angiosperms).

Pollen Evolution

• Pollen grains are reduced male gametophytes housing sperm cells.

Seed Formation

• Seeds consist of a plant embryo and a food supply encased in a protective coat. They develop from ovules in female cones and can remain dormant.

Advantages of Seeds

1. Seeds are multicellular; spores are unicellular.
2. Seeds can remain dormant for longer and provide nourishment.
3. Seeds can be dispersed over greater distances compared to spores.

Gymnosperm Lifecycle Overview

1. MEIOSIS in cones produces microspores.
2. Microspores undergo MITOSIS to form pollen grains.
3. POLLINATION occurs via wind.
4. FERTILIZATION produces seeds, which can then be dispersed.

Angiosperms: Key Traits

• Angiosperms evolved to maximize pollination efficiency (flowers attract pollinators).
• Double Fertilization: One sperm fertilizes the egg, while another forms triploid endosperm.

Angiosperm Classification

1. Monocots: One cotyledon, parallel venation, scattered vascular tissue.
2. Eudicots: Two cotyledons, net-like venation, ringed vascular tissue.

Reproductive Structures

• Flowers: Contain both male (stamens) and female (carpels) structures, varying form to attract pollinators.
• Fruits: Structure developed from ovary enclosing seeds, facilitating seed dispersal.

Threats to Plant Diversity

• Habitat destruction, especially in tropical forests, threatens global plant species.
• Loss of plant species correlates with the extinction of associated insects and animals, and many undiscovered medicinal plants may be lost.
• Current rates predict 50% of Earth’s species may become extinct in a few centuries.