chapter 25

Chapter 25: Seedless Plants

Learning Objectives

• Plants share a common ancestor with charophytes, a group of green algae, within the archaeplastida supergroup of eukaryotes.

• Key plant adaptations for the transition to land include:
    1. Alternation of Generations: Life cycle that has both multicellular diploid sporophyte and multicellular haploid gametophyte stages.
    2. Walled Haploid Spores: Spores are resistant to desiccation, aiding in dispersal through air, produced in multicellular sporangia.
    3. Multicellular Gametangia: Gametes are produced in protective structures (antheridia for sperm and archegonia for eggs).
    4. Sporophyte Embryos: Embryos develop within the female gametophyte, providing protection and nutrition.
    5. Apical Meristems: Continuously dividing cells located in roots and shoots, allowing growth towards light and water.
    6. Waxy Cuticle: Protective layer that reduces water loss, with pores for gas exchange.
    7. Secondary Metabolites: Chemicals that deter herbivores and parasites.
    8. Mycorrhizae: Symbiotic relationships with fungi that enhance water and nutrient absorption.

• Bryophytes (Seedless, Nonvascular Plants):
    - Characterized as non-woody, small, ground-covering plants that depend on water for reproduction, utilizing rhizoids for attachment.
    - Dominant haploid gametophyte form produces eggs and flagellated sperm, with the diploid sporophyte dependent on the gametophyte for nutrition.
    - Includes three phyla: Marchantiophyta (Liverworts), Anthocerotophyta (Hornworts), and Bryophyta (Mosses).

• Evolution of Early Vascular Plants:
    - Key adaptations include the evolution of vascular tissue, enabling taller growth and resource transport.
    - Traits are characterized by independent branched sporophytes and a dominant diploid sporophyte life cycle.

• Seedless Vascular Plants (SVP):
    - Features include evolved vascular tissue, true roots, and leaves.
    - The oldest SVP fossils date back to approximately 425 million years ago and include club mosses and ferns.
    - They can be further classified into lycophytes (e.g., club mosses) and monilophytes (e.g., ferns and horsetails).

• Describe plant adaptations key to the transition to land life.

• Describe the characteristics and diversity of extant Bryophytes.

• Describe the evolution of key adaptations of early vascular plants.

• Describe the characteristics and diversity of extant seedless vascular plants.

Chapter 25 - Topics

• Evolution of land plants: from water to land

• Algal ancestry

• Shared characteristics in green algae and plants

• Derived characteristics of plants

• Major divisions of the seedless plants

• Seedless nonvascular plants

• Seedless vascular plants

Algal Ancestry

• Plants share a common ancestor with charophytes, a group of green algae, within the archaeplastida supergroup of eukaryotes.

Green Algae and Plants: Shared Characteristics

• The following characteristics are shared among chlorophytes, charophytes, and plants:
    - Multicellularity
    - Cell walls composed of cellulose
    - Chloroplasts containing the same pigments
      - Chlorophyll a and b
    - Storage molecule is starch

Transition from Water to Land

• Selection for Land Life
    - By 470 mya, multicellular green algae expanded from shallow seas into rivers and lakes.
    - Marine life existed around 450 mya during the Paleozoic Era.

• Advantages of Land:
    - Higher concentrations of CO₂
    - Increased light intensity
    - More available minerals
    - Absence of herbivores and competition at the time

• Disadvantages of Transitioning to Land:
    - Constant risk of desiccation, necessitating protection for gametes and zygotes.
    - Need for structural support in an air medium which does not provide buoyancy.
    - Need for new strategies for male gametes to reach female gametes, as swimming is no longer viable.

Derived Characteristics of Plants

1. Alternation of Generations:
      - Life cycle includes both a multicellular diploid sporophyte and a multicellular haploid gametophyte.

2. Walled Haploid Spores:
      - Spores are resistant to desiccation, aiding in dispersal through air and made in multicellular sporangia.

3. Multicellular Gametangia:
      - Sperm is produced in antheridia, and eggs are produced in archegonia, protecting the gametes.

4. Sporophyte Embryos:
      - Embryos develop within the female gametophyte for protection.

5. Apical Meristems:
      - Continuously dividing cells located in roots and shoots allowing growth toward resources.

6. Waxy Cuticle:
      - A protective layer that reduces desiccation; pores regulate gas exchange.

7. Secondary Metabolites:
      - Chemicals like caffeine, latex, and rubber that deter herbivores and parasites.

8. Mycorrhizae:
      - Symbiotic relationships with fungi that enhance water and nutrient absorption, important before true roots evolved.

Alternation of Generations

• Haplontic: Life cycle dominated by the haploid stage.

• Diplontic: A life cycle where the diploid stage is dominant (e.g., humans).

• Most plants exhibit an alternation of generations where the gametophyte (haploid) is dominant in lower plants, and, with evolution, the sporophyte (diploid) becomes more dominant.

• Comparison with Charophyte Lifecycle:
    - Charophytes display no alternation of generations with only a multicellular haploid being present and undergo meiosis in the zygote to produce spores.

Plants' Lifecycles and Structures

1. Walled Haploid Spores:
      - Protection: Sporopollenin protects spores during dispersal.
      - Produced in sporangia within sporophyte structures.

2. Multicellular Gametangia:
      - Sperm produced in antheridia; eggs produced within archegonia, providing protection during fertilization.

3. Apical Meristems:
      - Enable root and shoot growth by dividing continuously towards resources like light, water, and minerals.

4. Waxy Cuticle:
      - Prevents dehydration while allowing for gas exchange through stomata which regulate the intake of CO₂ and O₂, and release of H₂O vapor.

5. Secondary Metabolites:
      - Chemicals that serve defensive functions against competitors, herbivores, and pests such as caffeine and latex.

6. Mycorrhizae:
      - Symbiotic relationships with fungi essential for early land plants aiding in nutrient absorption.

Major Divisions of Land Plants

Bryophytes (Seedless, Nonvascular Plants)

• Characteristics:
    - Non-woody, small, ground-covering plants dependent on water for reproduction.
    - Use rhizoids for attachment (not true roots).
    - Dominant haploid gametophyte form that produces eggs and flagellated sperm.
    - Diploid sporophyte is dependent on the gametophyte for nutrition.

• Phyla of Bryophytes:
    - Marchantiophyta (Liverworts)
    - Anthocerotophyta (Hornworts)
    - Bryophyta (Mosses)

Liverworts (Marchantiophyta)

• Characteristics:
    - Elevated gametophytes resembling miniature trees (e.g., Marchantia).
    - Sporophytes are reduced; may be thalloid or leafy.

• Sporophytes:
    - Thallus structure with foot, seta, and capsule (sporangium).

Hornworts (Anthocerotophyta)

• Characteristics:
    - Sporophyte exhibits a horn-like shape, good for colonizing moist soils.
    - Has a symbiotic relationship with nitrogen-fixing cyanobacteria.

Mosses (Bryophyta)

• Characteristics:
    - Most numerous of nonvascular plants; found in extreme environments.
    - Sporophyte grows upward from the female gametophyte for spore dispersal.

• Ecological Importance:
    - Serve as pioneer species in nutrient-poor soils, crucial producers in cold or high-altitude regions, and often form ‘peat bogs’ that are significant wetlands.

Moss Life Cycle

• Key Stages:
    - Haploid (n) Stage: Sperm and egg formation occurs in male and female gametophytes; driven via antheridia and archegonia.
    - Diploid (2n) Stage: Fertilization results in the zygote developing into a mature sporophyte within archegonia.

• Process:
    - Fertilization: Zygote formation within the archegonium.
    - Meiosis: Formation of spores that begin the cycle anew when dispersed.
    - Germination: Spores develop into protonema from which buds form gametophytes.

Major Divisions of Land Plants

Seedless Vascular Plants (SVP)

• Characteristics:
    - Evolved vascular tissue; previously, plants were low-growing and lacked true roots and leaves.

• Evolutionary Timeline:
    - The oldest seedless vascular plant fossils date back to approximately 425 million years ago, particularly seen in club mosses and ferns.

Seedless Vascular Plant (SVP) Traits

• Characters include independent branched sporophytes that do not rely on the gametophyte for nutrition, and a dominant diploid sporophyte life cycle.

• Additional adaptations include:
    - Xylem: Specialized for transporting water and minerals.
    - Phloem: Specialized for moving sugars and organic products.
    - Development of true roots and true leaves.

Microphylls and Megaphylls

• Microphylls:
    - Small, spine-like leaves supported by a single vascular strand, typically found in Lycophytes.

• Megaphylls:
    - Found in most vascular plants, these leaves have extensive branching in their vascular systems, providing greater photosynthesis efficiency than microphylls.

Sporophylls

• Definition:
    - Leaves modified to bear sporangia; examples include:
    - Fern sporophylls appear like normal leaves but possess sori underneath for spore generation.
    - Lycophyte sporophylls modified into structures called strobili.

Spore Production

• Homosporous Production:
    - Most seedless vascular plants produce a single type of spore from the sporangium, resulting in usually bisexual gametophytes.

• Heterosporous Production:
    - Seen in all seed plants and some seedless vascular plants; involves producing different sizes of spores:
      - Megasporangium for megaspores (female gametophyte).
      - Microsporangium for microspores (male gametophyte).

Divisions of Seedless Vascular Plants

Lycophytes (Club Mosses and Relatives)

• Characteristics:
    - Currently, all living species are relatively small (around 1,200 species).
    - Common members include Selaginella (spike moss), Isoetes (quillwort), and Diphasiastrum (club moss).

Monilophytes (Ferns and Relatives)

• Components:
    - Includes horsetails, whisk ferns, and true ferns.

• Horsetails:
    - Characterized by jointed stems with tiny leaves.
    - Photosynthesis occurs in the stem, producing strobili for spore formation.

Ferns

• Characteristics:
    - Most widespread and diverse group of Monilophytes, known for their large megaphylls and the presence of sori on the underside of leaves.

• Life Cycle Details:
    - Highlight the transition from zygote to sporophyte and the formation of antheridia and archegonia in the gametophyte stage.

Importance of Seedless Plants

• Ecological Indicators:
    - Disappearance of mosses can indicate environmental pollution.

• Role in Weathering:
    - Ferns contribute to the weathering of rocks, promoting soil formation and providing habitat.

• Utilization of Peat Moss (Sphagnum):
    - Used as fuel (renewable resource), as a soil conditioner, and for food.

• Extinct SVPs:
    - Historically, they contributed to coal formation, serving as a crucial energy source for human consumption and accelerating topsoil development.