Chapter 25 Seedless Plants

Chapter 25 Seedless Plants

Learning Objectives

  • Explain the evidence linking the ancestors of plants to multicellular algae.

  • 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.

Topics Overview

  1. Evolution of Land Plants: From Water to Land

    • Algal ancestry

    • Shared characteristics in green algae and plants

    • Derived characteristics of plants

  2. Major Divisions of Seedless Plants

    • Seedless nonvascular plants

    • Seedless vascular plants

Algal Ancestry

  • Archaeplastida: Algae and plants share a common ancestor.

  • Green Plants are specifically linked to charophytes (green algae) in the Archaeplastida supergroup of eukaryotes.

Shared Characteristics Between Green Algae and Plants

  • Multicellularity: Both chlorophytes, charophytes, and plants demonstrate this trait.

  • Cell Walls: Composed of cellulose.

  • Chloroplasts: Contain the same pigments: chlorophyll a & b.

  • Storage Molecule: Starch is used for energy storage.

  • Illustrative Examples:

    • Chlorophyte (Ulva)

    • Charophyte (Chara)

    • Plant (Moss)

Transition from Water to Land

  • Selection for Land Life: Between 470 million years ago (my) multicellular green algae began to expand from shallow seas into rivers and lakes; by 450 mya, significant developments were occurring in the Paleozoic Era.

  • Strategies:

    • Green algae populated lake shores for advantages such as:

      • More resources available

      • Less competition for survival.

    • Risks: Periodic dehydration necessitated specific adaptations.

Advantages of Coming onto Land

  • Higher carbon dioxide levels ([CO2])

  • Increased light intensity

  • Greater availability of essential minerals

  • Absence of herbivores initially, reducing immediate survival pressures

Disadvantages of Transitioning from Water to Land

  • Exposure to desiccation (drying out): A constant danger for organisms.

  • Both gametes and zygotes face desiccation risk during their life cycle.

  • Required development of structural support due to less buoyancy in air compared to water.

  • The male gametes (sperm) must find the female gametes (eggs) without the ability to swim, thus necessitating adaptations for fertilization.

Derived Characteristics of Plants

  1. Alternation of Generations: A life cycle that features both haploid and diploid stages to maximize reproductive success.

  2. Walled Haploid Spores: Protected spores for dispersal through air, created in multicellular sporangium.

  3. Multicellular Gametangia: Structures to protect gametes, with distinct male (antheridia) and female (archegonia) structures.

  4. Sporophyte Embryos: Grow protected within female gametophyte structures.

  5. Apical Meristem Tissue: Allowing roots and shoots to grow toward resources (water, minerals, light, CO2).

  6. Waxy Cuticle: A protective layer that reduces water loss through desiccation while allowing gas exchange via controlled pores (stomata).

  7. Secondary Compounds: Chemicals that deter herbivores and pathogens (e.g., caffeine, latex, rubber).

  8. Mycorrhizae: Mutualistic associations with fungi, aiding in nutrient and water absorption; evolved before the existence of true plant roots.

Life Cycle Expressions

  • Haplontic: Life stage where the haploid form is dominant.

  • Diplontic: Characterized by a dominant diploid stage; e.g., human life cycle.

  • Most plants show an alternation of generations, with the gametophyte generation being dominant in lower plants but becoming reduced as evolution progressed.

Walled Haploid Spores

  • Protection: Sporopollenin in spores provides air dispersal resilience.

  • Spores produced in multicellular sporangia during sporophyte stages.

Multicellular Gametangia Descriptions

  • Antheridia: Male structures that protect sperm.

  • Archegonia: Female structures where eggs are protected and fertilization occurs, leading to zygote formation.

Apical Meristems

  • These are regions of continuously dividing cells that enable roots and shoots to grow towards water, minerals, light, and CO2.

Waxy Cuticle Functionality

  • This coating minimizes water loss, featuring controllable stomata for gas exchange (oxygen and carbon dioxide).

Secondary Metabolites Role

  • Chemicals such as caffeine, latex, and rubber are produced as defense mechanisms against herbivores, competitors, and pathogens.

Mycorrhizae Importance

  • This relationship between fungi and plants facilitates enhanced absorption of water and nutrients; believed to be among the first mutualisms formed with terrestrial plants.

Major Divisions of Seedless Plants

Nonvascular Plants: Bryophytes

  • Characteristics: Non-woody, herbaceous, small ground-covering plants requiring water for reproduction.

  • Attachment using rhizoids, which are not true roots.

  • Phyla:

    • Marchantiophyta: Liverworts

    • Anthocerotophyta: Hornworts

    • Bryophyta: Mosses

  • Life Cycle: The gametophyte is dominant, responsible for making eggs and flagellated sperm. The sporophyte remains dependent, growing within the female gametophyte.

Liverworts (Marchantiophyta)

  • Display elevated gametophytes that resemble miniature trees with reduced sporophyte forms; categorized as "thalloid" or "leafy".

Hornworts (Anthocerotophyta)

  • Defined by their horn-like sporophytes and notable symbiotic relationships with nitrogen-fixing cyanobacteria.

Mosses (Bryophyta)

  • The most abundant non-vascular plant, known for inhabiting diverse and extreme environments (mountain tops, tundra, deserts).

  • Ecological roles include serving as pioneer species in nutrient-poor soils, contributing to soil formation via peat, and serving as food and fuel in some cultures.

Moss Life Cycle Specifics

  • Key Initiatives:

    • Gametophyte forms from spores, developing into protonemata and ultimately buds.

    • Fertilization takes place in the archegonium to produce a zygote, which grows into a sporophyte.

    • The sporophyte consists of a capsule that releases spores as peristomes.

Major Divisional Overview of Seedless Vascular Plants (SVPs)

  • Evolutionary milestone marked by the existence of elongated, branched sporophytes that are independent from the gametophyte.

  • Features include:

    • Dominance of the diploid sporophyte in most life cycles

    • Development of xylem for water transport and phloem for nutrient transport, enabling true roots and leaves.

Microphylls vs. Megaphylls

  • Microphylls: Small leaves with single strands of vascular tissue, characteristic of Lycophytes.

  • Megaphylls: Broad leaves with branched vascular systems found in most other vascular plants, illustrating increased photosynthetic capability.

Sporophyll Variants

  • Leaves that adapt to bear sporangia. Different seed plants demonstrate:

    • Homosporous: Production of one type of spore developing into bisexual gametophytes.

    • Heterosporous: Producing distinct megaspores and microspores that form separate male and female gametophytes.

Specific Groups Within SVPs

  1. Lycophytes (Club Mosses and Relatives)

    • Composition of approximately 1,200 species, generally being small. Examples include Selaginella and Isoetes.

  2. Monilophytes (Ferns and Relatives)

    • Like horsetails and whisk ferns, these plants encompass a diverse group of vascular plants characterized by homosporous reproduction and various leaf forms (e.g., ferns with megaphylls).

Importance of Seedless Plants

  • Certain plants like mosses serve as biological indicators of environmental health, particularly pollution levels.

  • Seedless vascular plants play significant ecological roles and natural processes, such as weathering of rocks and soil conditioning.

  • Historically, some extinct seedless vascular plants contributed to coal formation and modern energy resources.