Biol120 - Lecture 12: From Water to Land

Land Plant Identity

• Land plants (Embryophytes) = monophyletic clade within Eukarya
• Closest algal relatives: a lineage of charophyte green algae (not monophyletic)
• Major extant groups
  • Non-vascular (bryophytes): liverworts, mosses, hornworts
  • Seedless vascular: lycophytes, monilophytes (ferns, horsetails, whisk ferns)
  • Seed plants: gymnosperms & angiosperms

Evolutionary Milestones

• Transition to land $\approx 470$ MYA (spore fossils)
• Vascular tissue appears $\approx 425$ MYA; seeds $\approx 360$ MYA
• Expansion of novel gene families enabled terrestrial adaptation (desiccation resistance, body patterning)

Life Cycle & Alternation of Generations

• Two multicellular phases
  • Sporophyte: diploid $2n$, produces haploid spores via meiosis
  • Gametophyte: haploid $n$, produces gametes via mitosis
• Fertilisation $n + n \rightarrow 2n$ restores diploidy
• Balance shifts through evolution:
  • Bryophytes: dominant gametophyte, dependent sporophyte
  • Ferns: both independent, sporophyte larger
  • Seed plants: dominant sporophyte, microscopic dependent gametophyte

Derived Traits of Land Plants (Synapomorphies)

• Alternation of generations with multicellular $n$ and $2n$ phases
• Walled spores in multicellular sporangia (wall contains sporopollenin)
• Multicellular gametangia (antheridia & archegonia)
• Multicellular, dependent embryo (embryophyte)
• Apical meristems (growth at shoot & root tips)

Non-Vascular Plants (Bryophytes)

• No true vascular tissue, seeds, or flowers
• Gametophyte dominant; sporophyte remains attached & nutritionally dependent
• Water needed for sperm motility → moist habitats common
• Each lineage currently treated as monophyletic (debate continues)

Liverworts, Mosses, Hornworts: Key Features

• Reproductive organs
  • Antheridium (♂) → flagellated sperm
  • Archegonium (♀) → single egg
• Liverworts (Marchantia)
  • Thalloid gametophyte with rhizoids; gemma cups for asexual propagation
  • Archegoniophores & antheridiophores elevate gametangia
• Mosses
  • Leafy gametophytes one-cell thick; rhizoids for anchorage
  • Sporophyte = seta + capsule (sporangium) with operculum for spore release
• Hornworts
  • Horn-like, photosynthetic sporophyte grows from gametophyte base

Glossary (Must-Know)

• Ploidy: $n$ (haploid), $2n$ (diploid)
• Mitosis: $2n \rightarrow 2n$ or $n \rightarrow n$ (identical cells)
• Meiosis: $2n \rightarrow n$ (spores)
• Sporophyte / Sporangium / Spore
• Gametophyte / Gametangium / Gamete
• Antheridium (sperm), Archegonium (egg)
• Rhizoid, Seta, Capsule, Sporopollenin, Charophyte

Quick-Check Questions

1. Which plant characteristic is absent in charophytes? ➔ Alternation of multicellular generations.
2. In plants, what does meiosis produce? ➔ Haploid spores.