BIO UNIT 2 NOTES

Bryophytes and Seedless Plants

• Bryophytes: Nonvascular plants including liverworts, hornworts, and mosses.

• Seedless Vascular Plants:

  • Lycophytes: Club mosses, spike mosses, quillworts.

  • Pterophytes: Ferns, horsetails, whisk ferns.

• Seedless plants represent a small fraction of over 300,000 species of plants; more than 260,000 are seed plants.

• Dominated land approximately 300 million years ago.

• Current theory states that land plants are monophyletic, descending from a single common ancestor:

  • Charophyceans being the ancestral green algae from which

  • Bryophytes evolved (liverworts, hornworts, mosses).

Challenges of Transitioning to Land

• Moving from aquatic to terrestrial environments posed several challenges:

  • Desiccation: Risk of drying out.

  • Support: Loss of buoyancy in water affects structural stability.

  • Reproduction: Need for gamete dispersal in air.

  • Nutrient Absorption: Transition from water uptake to soil-based methods.

  • Gas Exchange: Adaptation for CO₂ absorption from air instead of water.

  • UV Radiation: Increased exposure to radiation requires protective adaptations.

  • Water Transport: Development of vascular tissues for effective hydration.

Advantages of Early Land Plants

• Sunlight is abundant on land, leading to the potential for increased photosynthesis.

• Carbon dioxide is more readily available in air than in water.

• Initial lack of predators allowed early land plants to thrive.

Evolutionary Milestones

• Origin of Vascular Plants: About 420 million years ago.

• Origin of Land Plants: About 475 million years ago.

• Seed Plants: Gymnosperms (~360 mya), leading to angiosperms.

Alternation of Generations

• Definition: A life cycle pattern where plants alternate between two distinct forms:

  1. Sporophyte (2n):

     • Diploid, containing two sets of chromosomes.

     • Produces spores through meiosis.

  2. Gametophyte (n):

     • Haploid, contains one set of chromosomes.

     • Produces gametes through mitosis.

     • In fertilization, sperm and egg unite to form a new zygote, restarting the cycle.

• Dominance of life cycle stages varies among plant groups:

  • Mosses & Liverworts: Gametophyte is dominant.

  • Ferns: Sporophyte is dominant, with a small gametophyte.

  • Flowering Plants and Conifers: Sporophyte is dominant, with gametophyte existing inside flowers as pollen and ovules.

Mosses vs. Pine Trees

• Moss: Gametophyte dominant stage (n). Produces gametes in antheridia and archegonia.

• Pine Tree: Sporophyte dominant stage (2n). Large, with cones and needles; involves seed formation.

Apical Meristems

• Regions in roots responsible for growth and elongation.

• Root Cap: Protects apical meristems during root extension.

Structural Adaptations for Land

1. Early land plants: Low-growing with shoots for light capture.

2. Vascular Tissue:

   • Xylem: Transports water and minerals.

   • Phloem: Transports nutrients.

3. Lignin: Provides structural support, allowing vertical growth.

4. Roots: Help anchor and absorb nutrients/water.

Water Loss Protection

• Waxy Cuticle: Prevents water loss but limits gas exchange.

• Stomata: Openings to regulate gas exchange and moisture.

Comparison of Plant Structural Characteristics

Major Groups of Green Plants

• Streptophytes:

  • Nonvascular: Bryophytes

  • Seedless Vascular: Lycophytes, Pterophytes

  • Seed Plants: Gymnosperms, Angiosperms

Importance of Vascular Tissue

• Xylem and Phloem: Crucial for nutrient transport and structural integrity.

  • Xylem provides mechanical support and helps achieve great heights, critical for competing for sunlight.

Evolution of Seedless Vascular Plants

• Phylum Lycopodiophyta (Club Mosses):

  • Small, evergreen plants with branching stems and microphyll leaves.

  • Examples include club mosses, spike mosses, quillworts.

Horsetails and Ferns

• Phylum Monilophyta: Includes horsetails and ferns.

  • Horsetails: Thin leaves at each joint; once used as scrubbing brushes.

  • Ferns: Recognizable seedless vascular plants with large compound leaves (fronds). Dominant sporophyte stage.

Whisk Ferns

• Psilotum nudum: Lacks true roots and leaves; closely related to ferns.

Importance of Seedless Vascular Plants

• Fossil Fuel Formation: Ancient seedless plants contributed to coal deposits.

• Ecosystem Support: Prevent soil erosion, recycle nutrients, provide habitats.

• Economic & Cultural Uses: Used in horticulture, food, and traditional medicine.

• Environmental Impact: Aid in carbon dioxide absorption, influential for climate regulation.

Features of the Animal Kingdom

• Animal: Multicellular, heterotrophic eukaryotes.

• Characteristics: Lack of cell walls, specialized tissues (muscle, nerve).

• Movement and Digestion: Internal digestion and ability to respond to external stimuli.

Reproduction and Development

• Sexual Reproduction: Predominant mode, leading to genetic diversity.

• Cleavage patterns and development stages are critical in embryonic development.

Classification

• Divided based on symmetry, tissue layers, and mouth/anus development.

  • Eumetazoa: Animals with distinct tissues.

  • Bilateria: Animals with bilateral symmetry (most).

Mass Extinctions

• Defined as a loss of more than 75% of species within a short geologic time.

• Major events include:

  1. End Ordovician (444 Mya)

  2. Late Devonian (360 Mya)

  3. End Permian (250 Mya)

  4. End Triassic (200 Mya)

  5. End Cretaceous (65 Mya)

• Natural background extinction rates observed over geological time.

Cambrian Explosion

• A rapid diversification of species around 542-488 million years ago.

• Reasons for diversification include rising oxygen levels, shallow seas, and predator-prey dynamics.

Evolution and Classification of Chordates

• Phylum Chordata includes invertebrates (tunicates, lancelets) and vertebrates.

• Key Features: All chordates possess notochord, dorsal hollow nerve cord, pharyngeal slits, and post-anal tail at some developmental stage.

Vertebrate Traits

• Endoskeletal structure made of bone or cartilage.

• Distinct organ systems including muscular, circulatory, excretory, immune, and nervous systems.

  • Various vertebrate groups include jawless (Hagfish, Lampreys), jawed (Cartilaginous, Bony Fish), and tetrapods (Amphibians, Reptiles, Birds, Mammals).

Evolution of Mammals

• Modern mammals classified into monotremes, marsupials, and eutherians.

• Distinctive features include mammary glands and hair.

• Human Evolution: Defined by lineage branching leading to Homo sapiens.

Bryophytes and Seedless Plants

Bryophytes

• Nonvascular plants that include dominant groups like liverworts, hornworts, and mosses.

• Typically found in moist environments, they rely on diffusion for nutrient uptake and lack true roots, stems, and leaves.

Seedless Vascular Plants

Lycophytes

• Comprising club mosses, spike mosses, and quillworts.

• Typically characterized by small leaves called microphylls and have a unique life cycle involving sporophytes that produce spores in specialized structures.

Pterophytes

• Consist of ferns, horsetails, and whisk ferns.

• Ferns are notable for their large, compound leaves (fronds) that are typically divided into smaller leaflets, contributing to their effective photosynthesis. They also reproduce via spores produced in clusters called sori located on the underside of fronds.

• Horsetails, identifiable by their jointed stems and fine leaves, used historically for scrubbing surfaces due to their high silica content.

Overview of Seedless Plants

• Seedless plants comprise only a small fraction of the more than 300,000 species of plants, with over 260,000 categorized as seed plants.

• They dominated terrestrial ecosystems approximately 300 million years ago during the Carboniferous period.

• Current phylogenetic theories suggest that land plants are monophyletic, all originating from a single common ancestral lineage from Charophyceans, a group of green algae.

Challenges of Transitioning to Land

1. Desiccation: The risk of drying out in aerial environments necessitated the development of protective structures.

2. Support: As buoyancy decreased, plants evolved structural adaptations such as lignin in cell walls to support upright growth.

3. Reproduction: The need for effective gamete dispersal in air led to adaptations like pollen and seed development.

4. Nutrient Absorption: Transitioning from aquatic nutrient absorption to soil-based methods required root development.

5. Gas Exchange: The ability to absorb CO₂ from the atmosphere rather than water through stomata became vital.

6. UV Radiation: Increased exposure to solar radiation posed risks; plants adapted through protective pigments.

7. Water Transport: The evolution of vascular tissues (xylem and phloem) was crucial for efficient water and nutrient transport.

Advantages of Early Land Plants

• Sunlight: Abundant in terrestrial settings, pushing evolutionary advantages for photosynthesis.

• Carbon Dioxide: Readily available in the air compared to water, improving photosynthesis efficiency.

• Lack of Predators: Early stages allowed plants to flourish without significant herbivory.

Evolutionary Milestones

• Origin of Vascular Plants: Circa 420 million years ago with significant adaptations enabling life on land.

• Origin of Land Plants: This emergence dates back approximately 475 million years.

• Seed Plants: Gymnosperms arose around 360 million years ago and later gave rise to angiosperms, marking a substantial evolutionary advancement in reproduction and survival.

Alternation of Generations

• Plants exhibit a unique life cycle where two distinct forms alternate:

  • Sporophyte (2n): The diploid phase that produces spores through meiosis for reproduction.

  • Gametophyte (n): The haploid phase that develops gametes via mitosis and produces the zygote upon fertilization.

• Dominance of the life cycle stage varies: mosses and liverworts primarily exhibit a gametophyte-dominant stage, while ferns and seed plants favor the sporophyte phase.

Comparison of Plant Structural Characteristics

Major Groups of Green Plants

• Streptophytes include nonvascular (bryophytes), seedless vascular plants (lycophytes and pterophytes), and seed plants (gymnosperms and angiosperms).

Importance of Vascular Tissue

• Xylem and phloem are indispensable for nutrient transport and providing structural integrity for plants, enabling them to reach great heights to compete effectively for sunlight.

Environmental Significance of Seedless Vascular Plants

• They played a critical role in ancient biomass and contributed to the formation of fossil fuels, particularly coal.

• Their presence in ecosystems prevents soil erosion, recycles nutrients and provides habitats for various organisms.

• Economically, they have uses in horticulture, food production, and traditional medicine, and play a role in climate regulation by absorbing carbon dioxide.

Alternation of Generations Diagram

Definition:

A life cycle pattern observed in plants where they alternate between two distinct stages:

• Sporophyte (2n):

  • Diploid stage with two sets of chromosomes.

  • Produces spores through meiosis.

• Gametophyte (n):

  • Haploid stage containing one set of chromosomes.

  • Produces gametes through mitosis.

Diagram:

               Fertilization
                      |
                      v
           +------------------+
           |      Zygote       | (2n)
           +------------------+
                      |
                      v
            +------------------+
            |   Sporophyte      | (2n)
            +------------------+
                      |
                      v
             Meiosis (spores)
                      |
                      v
           +------------------+
           |    Spores (n)     | 
           +------------------+
                      |
                      v
       +---------------------------+
       |                           |
 Meiosis (n)                 Germination
       |                           |
       v                           v
+-----------------+        +-----------------+
|   Gametophyte   | (n)   |   Gametophyte   | (n)
+-----------------+        +-----------------+
       |                           |
       | (n)                     | (n)
       |                          | 
       |       Gametes          |     Gametes
       +-----------------+        +-----------------+
                      |                  |
                      |                  |
                      v                  v
                    Fertilization
                      
                           

Mosses vs. Pine Trees

Mosses:

• Life Cycle: Mosses exhibit a gametophyte-dominant life cycle (haploid, n). The dominant stage is the gametophyte, which produces gametes in specialized structures called antheridia (for sperm) and archegonia (for eggs).

• Structure: Mosses are typically small, nonvascular plants that thrive in moist environments. They lack true roots, stems, and leaves, relying on diffusion for water and nutrient uptake. Their structure allows them to absorb water and nutrients directly from the environment.

• Habitat: They often grow in shady, damp locations and can form dense green mats on the ground or rocks, playing a critical role in retaining moisture in their ecosystems.

• Reproduction: Mosses reproduce via spores, which are produced in sporangia on the sporophyte (2n) generation, often seen as stalks rising from the gametophyte.

Pine Trees:

• Life Cycle: Pine trees are characterized by a sporophyte-dominant life cycle (diploid, 2n). The large, tree-like structure represents the sporophyte stage, which produces cones where gametes are formed.

• Structure: Pines have true roots, stems, and leaves, making them vascular plants. They feature needle-like leaves adapted to reduce water loss and cones that house reproductive structures – pollen cones (male) and seed cones (female).

• Habitat: Typically found in a variety of ecosystems, pine trees are more adaptable to different soil types and can endure harsher environments compared to mosses.

• Reproduction: Pine trees reproduce via seeds, which are formed after fertilization from the ovules contained within seed cones. Their seeds are often adapted for wind dispersal, allowing for colonization of new areas.

Xylem and Phloem

Xylem:

• Function: Xylem is responsible for the transport of water and dissolved minerals from the roots to the rest of the plant. It plays a critical role in maintaining water balance and ensuring that all cells receive the necessary nutrients for growth and development.

• Structure: Xylem tissues are composed of various cell types including tracheids and vessel elements, which are specialized for efficient water conduction. These cells are often lignified, providing support and rigidity to the plant, allowing it to grow taller and compete effectively for sunlight.

• Direction of Transport: Xylem primarily transports fluids unidirectionally, from the roots upward through the stem and into the leaves. This process is driven by capillary action, root pressure, and transpiration pull.

Phloem:

• Function: Phloem is responsible for transporting the products of photosynthesis, primarily sugars, from the leaves (where they are produced) to other parts of the plant, including roots, stems, and fruits. This process is essential for energy distribution and storage within the plant.

• Structure: Phloem is made up of sieve tube elements and companion cells. Sieve tube elements facilitate the transport of nutrients, while companion cells provide metabolic support. Unlike xylem, phloem is living tissue that allows for the bidirectional transport of materials.

• Direction of Transport: Phloem can transport nutrients both upward and downward, depending on the needs of the plant. The flow in phloem is driven by processes like osmosis and pressure flow, which allow for efficient nutrient distribution even to non-photosynthetic parts of the plant.