bio 201 Chapter 29

Plant Diversity and Evolution

Major Developments in Plant Evolution

• Origin of Plants

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

  • By 425 million years ago, key traits for terrestrial life emerged:

    • Reproductive structures

    • Photosynthetic branches

    • Structures for anchoring plants to soil

  • This diversification led to nonvascular, seedless vascular, and seed plants.

Concepts of Plant Evolution

• Current Plant Diversity

  • More than 325,000 known plant species, predominantly terrestrial.

  • Plants provide oxygen, food, and habitat for other organisms.

  • Charophytes (green algae) are the closest relatives of plants.

Evidence of Algal Ancestry

• Key similarities between plants and algae:

  • Both are multicellular, eukaryotic, and photosynthetic autotrophs.

  • Cellulose in cell walls; chloroplasts contain chlorophyll a and b.

  • Plants share unique traits with charophytes:

    • Cellulose-synthesizing proteins arranged in rings.

    • Specific structure of flagellated sperm.

    • DNA sequence similarities (nuclear, chloroplast, mitochondrial).

Genetic Analysis

• Genetic studies confirm charophytes, particularly in the Zygnematophyceae clade, as the closest living relatives of plants, indicating a shared common ancestor rather than a direct descent.

Adaptations to Terrestrial Life

• Sporopollenin:

  • A protective coating in charophytes and plant spores preventing desiccation.

• Advantages of Terrestrial Life:

  • Access to sunlight, CO2, and nutrients in soil.

• Challenges:

  • Limited water and structural support against gravity

  • Evolution of adaptations allowed for successful land colonization.

Boundaries Between Algae and Plants

• Ongoing discussions about the classification boundaries between plants and algae, traditionally at the embryophytes (plants with embryos).

Derived Traits of Plants

• Common traits in nearly all plants but absent in charophytes:

  • Alternation of Generations: Plant life cycle alternates between multicellular haploid (gametophyte) and diploid (sporophyte) stages.

  • Multicellular Dependent Embryos: Retained and protected by maternal tissue, enabled through placental transfer cells.

  • Walled Spores: Produced in sporangia; spores are resistant to environmental factors.

  • Apical Meristems: Sites of continuous cell division, facilitating growth of roots and shoots for resource acquisition.

Life Cycle of Plants

• Alternation of Generations Overview:

  • Life cycles involve multicellular forms, where gametophytes produce gametes via mitosis and sporophytes produce spores via meiosis.

  • Outcomes include fertilization creating a zygote that develops into a sporophyte, completing the cycle.

Dependent Embryos

• Multicellular, Dependent Embryos:

  • Embryos are formed inside female gametophytes, receiving nutrients from the parent plant.

Spore Production

• Walled Spores in Sporangia:

  • Spores are produced in sporangia, equipped with sporopollenin for harsh environments.

Apical Meristems

• Definition and Function:

  • Localized regions at root and shoot tips allowing for growth and nutrient uptake.

Additional Features of Plants

• Cuticle: Waxy layer reducing water loss.

• Stomata: Pores aiding gas exchange.

Early Plant Challenges

• Primordial plants lacked true roots, necessitating symbiotic relationships with fungi for nutrient absorption.

Origin and Diversification of Plants

• Fossils indicate plants colonized land about 470 million years ago, with evidence of larger structures emerging around 425 million years ago.

Plant Classification

• Nonvascular Plants (Bryophytes):

  • Comprised of liverworts, mosses, and hornworts, with varying species counts.

• Vascular Plants:

  • Seedless vascular plants (ferns) and seed plants (gymnosperms and angiosperms).

Importance of Vascular Tissue

• Vascular tissue enabled taller plant growth, facilitating competition for sunlight and resource acquisition.

Reproductive Strategies

• Seedless Vascular Plants: Homosporous or heterosporous reproduction with distinct gametophyte and sporophyte phases and adaptations for dispersal.

Economic & Ecological Importance

• Bryophytes stabilize soils, retain moisture, and contribute to nutrient cycling.

• Peat moss serves economic roles in fuel and soil enhancement, playing a part in carbon storage and climate regulation.

STUDY GUIDE:

• Alternation of generations: A life cycle pattern occurring in plants, where the organism alternates between a multicellular haploid gametophyte generation and a multicellular diploid sporophyte generation.

• Angiosperm: Flowering plants that produce seeds enclosed within a fruit. They represent the largest group of plants with over 250,000 species.

• Antheridia: Male gametangia that produce sperm in non-flowering plants.

• Apical meristem: Regions of actively dividing cells located at the tips of roots and shoots enabling growth and resource acquisition.

• Archegonia: Female gametangia that produce eggs in non-flowering plants.

• Bryophytes: Nonvascular plants including liverworts, mosses, and hornworts that require water for reproduction and typically inhabit moist environments.

• Capsule: A structure found in mosses that contains spores, formed at the tip of the sporophyte.

• Charophyte: A clade of green algae that closely relates to land plants, sharing several key characteristics.

• Diploid: A cell or organism with two complete sets of chromosomes, one from each parent (2n).

• Gametangia: Structures in which gametes are produced, including both antheridia and archegonia.

• Gametophore: The stalk that bears the gametangia in mosses.

• Gametophyte: The haploid phase of the plant life cycle that produces gametes.

• Gymnosperm: Seed-producing plants that do not form flowers or fruits, including conifers.

• Haploid: A cell or organism with a single set of chromosomes (n).

• Heterosporous: Refers to plants that produce two different types of spores: microspores (male) and megaspores (female).

• Homosporous: Refers to plants that produce one type of spore that typically develops into a bisexual gametophyte.

• Lignin: A complex organic polymer found in the secondary cell walls of many plants, providing rigidity and resistance to decay.

• Lycophyte: A group of vascular plants that includes clubmosses and their relatives.

• Megaphyll: A type of leaf with a complex structure and a network of veins, characteristic of most vascular plants.

• Megaspore: A larger type of spore that develops into a female gametophyte.

• Microphyll: A type of leaf with a simple structure and a single strand of vascular tissue, characteristic of lycophytes.

• Microspore: A smaller type of spore that develops into a male gametophyte.

• Monilophyte: A group of vascular plants that includes ferns and their relatives.

• Peristome: The region surrounding the opening of a bryophyte capsule, aiding in spore dispersal.

• Phloem: Vascular tissue responsible for the transport of sugars and nutrients throughout the plant.

• Protonema: A filamentous stage in the life cycle of mosses after spore germination, leading to the development of the gametophyte.

• Rhizoids: Hair-like structures that anchor bryophytes to the substrate but do not absorb water and nutrients like roots.

• Seta: The stalk of a moss sporophyte that elevates the capsule.

• Sorus (sori, pl): Clusters of sporangia on the underside of fern fronds responsible for spore production.

• Sporangia: Structures where spores are formed and housed.

• Spore: A reproductive unit of non-flowering plants, capable of developing into a new organism.

• Sporophyll: A leaf that bears sporangia, often found in ferns and seed plants.

• Sporophyte: The diploid phase of the plant life cycle that produces spores through meiosis.

• Sporopollenin: A durable polymer that makes up the outer walls of spores and pollen grains, providing protection.

• Strobili: Cone-like structures that produce spores, commonly found in gymnosperms and some ferns.

• Vascular tissue: Specialized tissue in plants responsible for the transport of fluids, including xylem and phloem.

• Xylem: Vascular tissue responsible for the conduction of water and minerals from the roots to the rest of the plant.