Ch 29 30 Plant Diversity for students

Chapter 29: Seedless Plants

Colonization of the Land

• Timeframe: Around 488 million years ago, there was no soil on land, only rocks, beaches, ponds, and oceans.

• Initial Life Forms: Land was devoid of insects and other animals; only fungi, bacteria, archaea, and protists existed.

• Role of Fungi: Fungi were likely present and contributed to making nutrients and water available to plants.

Origin of Land Plants

• Challenges Overcome by Early Plants:

  • Water Loss: Development of structures to retain moisture.

  • UV Protection: Need for protection against the harmful effects of solar UV rays which cause DNA damage.

  • Gamete Dissemination: Mechanisms to effectively spread gametes for reproduction.

• Tracheophytes:

  • Possess specialized vascular tissue for long-distance transport, facilitating growth and size advancement.

• Bryophytes (e.g., mosses): Limited in size due to absence of vascular tissue.

2. Haplodiplontic Life Cycle

• Definition: Life cycle featuring both multicellular haploid (gametophyte) and diploid (sporophyte) stages.

• Significance: All land plants exhibit this alternation of generations.

Haplodiplontic Life Cycle Details

Multicellular Diploid Stage (Sporophyte)

• Inside sporangia, diploid spore mother cells (sporocytes) undergo meiosis to produce four haploid spores, the precursors to the gametophyte generation.

Multicellular Haploid Stage (Gametophyte)

• Within gametangia, gametes are generated by mitosis.

• Fusion of gametes from different plants results in the formation of a diploid zygote, marking the beginning of the sporophyte generation.

Germination and Mitotic Cell Division

• Breakdown of stages:

  • Spores (n) undergo meiosis to give rise to gametophytes.

  • Gametophyte (n) produces gametes (sperm and egg) by mitosis, leading to fertilization and formation of the diploid zygote (2n)

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Protection Against Desiccation and UV Radiation

• Physical Adaptations:

  • Development of a waxy cuticle and stomata to regulate water loss and protect against UV rays.

• Evolutionary Shift: Dominance of the diploid sporophyte generation helps mask deleterious recessive mutations.

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Variation in Generational Size by Phyla

• Mosses: Large gametophytes with small, dependent sporophytes.

• Angiosperms: Small, dependent gametophytes with dominant large sporophytes

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Phylogeny of Land Plants

• Common Features:

  • Chlorophyll a and b, plasmodesmata, cuticle, antheridia and archegonia.

  • Development of multicellular embryos, stomata, euphylls, seeds, flowers, fruits, and vascular tissues

Bryophytes Characteristics

• Moss Structure: Composed of small, leaf-like structures around a central stem, lacking true leaves or vascular tissues.

• Anchored by rhizoids which serve to stabilize but are not true roots.

• Formation of multicellular gametangia at tips: archegonia (female) and antheridia (male).

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Structure of Sporophyte

• Sporophyte (2n): Grows from the female gametophyte through the archegonia.

• Gametophyte (n): The dominant phase of the plant's life cycle, primarily photosynthetic.

• Male Gametangia (Antheridia): Grows on male gametophyte.

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Phylogeny of Tracheophytes

• Characteristics:

  • All features common in land plants with a focus on vascular plants.

Vascular Tissues in Tracheophytes

• Xylem: Conducts water and minerals upward from roots.

• Phloem: Distributes sucrose and hormones throughout the plant body.

• Vascular tissues enable greater height and mass among tracheophytes.

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Tracheophyte Adaptations

• Cuticle & Stomata: Key features in all vascular plants to manage water loss.

• Leaves: Function to increase the surface area for photosynthesis.

• Roots: Essential for transport and structural support; gametophyte reduced in size relative to sporophyte.

Pterophytes (Ferns and Allies)

• Diversity: Includes true ferns, horsetails, and whisk ferns; both sporophyte and gametophyte stages are photosynthetic.

• Reproductive Strategy: Requires water for flagellated sperm to fertilize eggs.

Features of Seed Plants

Importance of Seeds

1. Maintain dormancy under unfavorable conditions.

2. Protect vulnerable young plants.

3. Provide early food sources for embryos.

4. Facilitate the dispersal of embryos.

Angiosperms Evolution and Abundance

• Emergence altered Earth's biology from dominance by ferns and conifers; unique adaptations facilitate copious diversity (flowers, insect pollination, broad leaves).

• Origins Debate: Angiosperms evolved potentially between 145–208 million years ago; earliest fossils date back 122–145 million years.

Fossil Evidence of Angiosperms

• Examples: Strychnos electri fossilized in amber; revealing adaptations and ecological roles during the Pleistocene epoch.