Plant Diversity

Topic Overview

• Topic 8: Plant Diversity

• Relevant Chapters: 29.1-29.3, 30.1-30.3

Learning Outcomes

Upon successful completion of this topic, students will be able to:

1. Describe the basic characteristics of the Archaeplastida.

2. Explain how pigment profiles are used to compare photosynthetic organisms.

3. Explain the evidence of shared ancestry between plants and green algae.

4. Describe the defining characteristics of plants.

5. Identify the major differences in the life cycles of the major plant phyla.

6. Construct a phylogenetic tree of the four plant phyla discussed in the course.

7. Describe the adaptations that allowed plants to move further onto land.

8. Explain the evolutionary impact of heterospory in plants.

9. Describe and illustrate the spore-producing structures in typical gymnosperms.

10. Compare ovules/seeds of gymnosperms and angiosperms.

11. Identify all four whorls of a perfect/complete flower.

12. Describe spore-producing structures and their relation to the seed in typical angiosperms.

13. Compare basic characteristics of monocots and eudicots.

Major Developments in Plant Evolution

• Origin: Plants originated from green algae approximately 470 million years ago.

• Adaptations for Land: By 425 million years ago, early plants developed traits such as:

  • Reproductive structures

  • Structures that anchor the plant to the soil

  • Photosynthetic branches

• Diversity: Early plants have given rise to:

  • Nonvascular plants (mosses)

  • Seedless vascular plants (ferns)

  • Seed plants (cone-bearing plants and flowering plants)

Plants as Part of Archaeplastida

• Form Types:

  • Unicellular

  • Multicellular

  • Colonial

• Clades:

  • Green Algae (paraphyletic)

  • Red Algae (monophyletic)

    • Structures: holdfast, stipe, blade

• Photosynthetic Pigments: Adaptation to different environments, useful for identification.

Plant Evolution from Green Algae

• Key Traits Shared:

  • Multicellularity

  • Photosynthetic pigments (chlorophylls a and b)

  • Cell walls made of cellulose

• Closest Relatives: Charophytes (Supergroup Archaeplastida, green algae).

• Evidence of Relationship:

  • Rings of cellulose-synthesizing proteins in plasma membrane

  • Flagellated sperm resembling those of charophytes

  • Formation of phragmoplast:

    • Microtubules guiding cell plate formation during cytokinesis

  • Molecular analyses support this relationship, including DNA evidence.

Adaptations for Terrestrial Life

• Analysis indicates that plants colonized land around 450 million years ago.

• Sporopollenin:

  • A polymer preventing desiccation, found in charophytic algae at shallow pond edges.

• Advantages of Terrestrial Environments:

  • Access to unfiltered sunlight, increased carbon dioxide, and nutrients in soil.

• Challenges of Terrestrial Life:

  • Limited water availability; difficulty in sperm dispersal; need to grow against gravity.

• Survival Adaptations:

  1. Alternation of Generations Life Cycle:

     • Includes multicellular haploid and diploid stages.

  2. Walled Spores:

     • Spores contain sporopollenin for protection.

  3. Apical Meristems:

     • Increases exposure to environmental resources.

  4. Accessory Pigments:

     • UV protection and antioxidant effects.

  5. Cuticle:

     • Waxy layer on epidermis to prevent desiccation.

  6. Stomata:

     • Pores allowing gas exchange; can close under dry conditions.

  7. Symbiotic Relationships:

     • Partnerships with fungi for enhanced nutrient uptake.

Alternation of Generations

1. Gametophyte produces haploid gametes (1n) via mitosis.

2. Two gametes (1n) fuse during fertilization forming a diploid zygote (2n).

3. Zygote develops into a multicellular sporophyte (2n).

4. Sporophyte (2n) produces unicellular haploid spores (1n) via meiosis.

5. Spores (1n) develop into multicellular haploid gametophytes (1n).

Plant Diversification

• Classification Based on Vascular Tissue Presence:

  • Non-vascular Plants:

    • Liverworts, mosses, hornworts

  • Vascular Plants:

    • Seedless vascular plants

      • Lycophyta (club mosses, spike mosses, quillworts)

      • Monilophytes (ferns, horsetails, whisk ferns)

    • Seed Plants

      • Gymnosperms (ginkgos, cycads, gnetophytes, conifers)

      • Angiosperms (flowering plants)

Bryophytes

• Dominant Stage:

  • Gametophyte (haploid, 1n) is dominant.

• Structure Details:

  • Protonema: Threadlike filament developing into gametophores.

  • Gametophore: Gamete bearer producing gametes via mitosis.

  • Rhizoids: Root-like filaments anchoring gametophytes.

  • Gametangia: produces gametes, including:

    • Archegonia (female, produces eggs)

    • Antheridia (male, produces sperm).

• Sporophyte Structure:

  • Seta: Stalk supporting sporangium.

  • Foot: Absorbs nutrients from the gametophyte and supports the seta.

  • Capsule: Sporangium producing spores via meiosis.

  • Peristome: Teeth regulating spore dispersal.

Ecological Importance of Bryophytes

• Common in moist ecosystems, helping moisture retention.

• Associated with nitrogen-fixing bacteria, improving nitrogen levels in the environment.

• Tolerance to extreme conditions and poikilohydric capabilities (re-establishing after desiccation).

• Some species, such as Sphagnum, contribute to peat formation, essential for carbon storage.

Seedless Vascular Plants

• Key Characteristics:

  • Evolved around 425 million years ago; dominant sporophyte stage.

  • Vascular Tissues:

    • Xylem: Water transport and structural support.

    • Phloem: Nutrient transport.

  • Some need water for fertilization.

• Adaptations:

  • Evolution of roots and leaves for nutrient absorption and increased photosynthesis.

  • Diversity includes lycophytes and monilophytes.

Diversity in Seedless Vascular Plants

• Lycophyta:

  • Club mosses, spike mosses, quillworts.

  • Most ancient group.

• Monilophyta:

  • Ferns, horsetails, whisk ferns.

  • Significant diversity with ferns having around 12,000 species.

Seed Plants Adaptations

• Reduced Gametophytes:

  • Develop within parental sporophytes, protected from environmental stress.

• Pollen:

  • Glacier for male gametophytes; can be transported by wind or animals.

• Seeds:

  • Develop from fertilized ovules, providing protection, nourishment, and dispersal advantages.

Heterospory in Seed Plants

• Definition: Production of two types of spores by megasporophylls (female) and microsporophylls (male), leading to distinct male and female gametophytes.

Ovule Structure and Egg Production

• Ovule Components:

  • Integument (2n), megasporangium (2n), and megaspore (n).

• Female gametophyte develops within the ovule.

Pollen and Sperm Production

• Pollen Grain:

  • Develops from microspores and consists of the male gametophyte and an outer pollen wall.

• Pollination: Transfer of pollen to ovule via the pollen tube.

Evolutionary Advantages of Seeds

• Protection: Survive harsh conditions.

• Food Supply: Aid during germination stages.

• Dispersal Capabilities: Seeds can be spread by various means (wind, water, animals).

Gymnosperms

• Definition: "Naked seed" plants with various types including cycads, ginkgos, gnetophytes, and conifers.

• Characteristics and Adaptations:

  • Survival in dry climates through thicker cuticles and seeds; dominance of sporophyte; heterosporous nature.

Life Cycle of Pine as Example

1. Conifers have ovulate and pollen cones.

2. Microsporocytes create microspores, leading to pollen grains.

3. Pollination and fertilization typically occur over an extended period!

Gymnosperm Diversity

• Cycadophyta: Large cones, palm-like leaves.

• Ginkgophyta: Only Ginkgo biloba species; pollution-tolerant.

• Gnetophyta: Diverse species; tropical and desert habitats.

• Pinophyta: Largest gymnosperm phylum; economic importance in lumber and chemical compounds.

Angiosperms

• Definition: Flowering plants with unique adaptations for dispersal (wind, animals).

• Flower Structure:

  • Composed of sporophylls: sepals, petals, stamens, and carpels.

• Fruit Function: Protects seeds and aids in their dispersal through various adaptations.

Angiosperm Life Cycle

1. Megasporangium develops megaspores; one survives to form female gametophyte.

2. Microsporocytes develop into pollen grains; generative cell forms two sperm.

3. Double fertilization: One sperm forms the zygote; the other forms the endosperm.

Angiosperm Diversity

• Basal Angiosperms: Water lilies and relations.

• Magnoliids: Includes magnolias and black pepper plants.

• Monocots: Orchids and grasses; fibrous root systems.

• Eudicots: Legumes and flowering trees; tap root systems.

Key Takeaways

• Understanding plant evolution and diversity is fundamental to studying biology.

• The adaptations of plants for terrestrial life reflect their evolutionary history and significant ecological roles.

• Knowledge of plant taxonomy and life cycles aids in appreciating the complexity of plant biology.