4, Seedless Plants and Their Evolution on Land

Seedless Plants

Colonization of the Land

• Around 488 million years ago, land was barren without soil.
• Initial environment was comprised of rocks, beaches, ponds, and oceans.
• Lack of insects and land animals, hence absence of pollinators.
• Presence of fungi, bacteria, archaea, and protists likely assisted in nutrient mobilization for plants.

Origin of Land Plants

• Main challenges for plants in land colonization included:
  • Water loss
  • Protection from UV radiation, which causes DNA damage
  • Mechanisms for the effective dissemination of gametes for reproduction.

Adaptations to Terrestrial Life

• Water Transport in Plants:

  • Tracheophytes possess specialized vascular tissue enabling long-distance transport throughout the plant body.
  • Bryophytes (like mosses) are constrained in size due to lack of vascular systems.

• Haplodiplontic Life Cycle:

  • Characterized by multicellular haploid (gametophyte) and diploid (sporophyte) stages, also known as the alternation of generations.
  • All land plants exhibit this life cycle structure.

Haplodiplontic Life Cycle Explained

• Sporophyte Stage (2n):

  • The multicellular diploid phase.
  • Inside sporangia, sporocytes undergo meiosis to generate four haploid spores.

• Gametophyte Stage (n):

  • The multicellular haploid phase.
  • Gametes produced via mitosis in gametangia.
  • Fusion of gametes leads to the formation of a diploid zygote, starting the sporophyte generation.

• Visual Representation:

  • Meiosis in sporangia produces haploid spores (n).
  • Mitosis leads to gamete formation (egg and sperm).
  • Fertilization results in a diploid zygote (2n), developing into an embryo.

Protective Adaptations

• To safeguard against desiccation and UV radiation:
  • Development of a waxy cuticle and stomata.
  • Shift towards a dominant diploid sporophyte generation, reducing the impact of recessive mutations.

Variability in Generational Sizes

• The size ratio of gametophyte to sporophyte varies across plant phyla, such as:
  • In mosses, the gametophyte is larger and the sporophyte is smaller and dependent on the gametophyte.
  • In angiosperms, this is reversed where the sporophyte is large and gametophytes are reduced and dependent.

Phylogeny of Land Plants

• Key features defining land plants:
  • Presence of chlorophylls a and b.
  • Formation of plasmodesmata and a cuticle.
  • Presence of multicellular embryos and gametangia (antheridia and archegonia).
  • Development of stomata, euphylls, seeds, and flowers.
  • Dominance of sporophyte generation with structural adaptations like stems, roots, and leaves, and vascular tissue.

Bryophytes

• Characteristic of Bryophyta (mosses):
  • Composed of small, leaf-like structures surrounding a stem-like axis.
  • Lacks true leaves due to absence of vascular tissue.
  • Attached to substrates by rhizoids (not true roots).
  • Produces multicellular gametangia:
  • Archegonia: Female gametangia.
  • Antheridia: Male gametangia.

Pterophytes (Ferns and Allies)

• Spores develop into both conspicuous sporophytes and smaller gametophytes, both capable of photosynthesis.
• They all produce gametangia that require free water for flagellated sperm delivery.

Seed Plants: Adaptations and Evolution

• Seeds maintain dormancy, protect young plants, provide sustenance until the plant can photosynthesize, and facilitate dispersal.
• Conifers and angiosperms showed significant evolutionary advances.
• Angiosperms revolutionized terrestrial landscapes with unique flowering and pollination adaptations.

Conclusion on Plant Evolution

• The evolutionary chronicle of angiosperms, initiated in the Mesozoic era, remains partially unclear, with fossil evidence demonstrating their early emergence.
• Notable examples of seed plants: conifers and flowering plants that exhibit diverse adaptations for survival and reproduction in terrestrial ecosystems.