Seedless Plants 

Origin of Land Plants

• All green algae and the land plants shared a common ancestor a little over 1 BYA.
• Supported by DNA sequence data
• Collectively known as green plants
• Not all photoautotrophs are plants
  • Red & brown algae excluded

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Life on Land

Plants had many issues to overcome on land that include:

• Water loss
• Protection from the harmful effects of the sun

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Green Algae & Land Plants

The green algae split into two major clades

• Chloroplasts- Never made it to land
• Charophytes - Sister to all land plants

@@Land plants@@…

• Have multicellular haploid and diploid stages
• Trend toward more diploid embryo protection
• Trend toward smaller haploid stage

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Adaptations to Terrestrial Life

1. Protection from desiccation with waxy cuticles and regulatable stomata.
2. Developing tissues to move water via a vascular system (tracheids).
3. Dealing with UV radiation cause mutations via DNA repair mechanisms.
4. A shift to a dominant haploid generation to protect the genetic information.

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Haplodiplontic Life Cycle

Multicellular diploid stage - sporophyte

• Produces haploid spores by meiosis
• Diploid spore mother cells (sporocytes) undergo meiosis in sporangia.
• Produce 4 haploid spores
• First cells on gametophyte generation

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Multicellular haploid stage - gametophyte

• Spores divided by mitosis
• Produces gametes by mitosis
• Gametes fuse to form diploid zygote
  • First cell of next sporophyte generation.

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Haplodiplontic Land Plants

• All land plants are haplodiplontic
• Relative sizes of generations vary

Bryophytes (Mosses, Hornworts, Liverworts)

• Large Gametophyte
• Small, dependent sporophyte

Tracheophytes

• Small, dependent gametophyte
• Large sporophyte

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Bryophytes

• Closest living descendants of the first land plants
• Called Nontracheophytes
  • They lack tracheids
  • Do have other conducting cells

Mycorrhizal associations important in enhancing water up take

• Symbiotic relationship between fungi and plants

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Bryophyte Characteristics

• Simple, but highly adapted to diverse terrestrial environments
• Approximately 16,000 species in 3 Clades
  • Liverworts
  • Mosses
  • Hornworts
• Gametophyte - conspicuous and photosynthetic
  • Sporophytes - small and dependent
• Require water for sexual reproduction

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Liverworts (Phylum Hepaticophyta)

• Have flattened gametophytes with liverlike lobes
  • 80% look like mosses
• Form gametangia in umbrella-shaped structures
• Also undergo asexual reproduction

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Mosses (Phylum Bryophyta)

• Gametophytes consist of small, leaflike structures around a stemlike axis
  • Not true leaves - no vascular tissue
• Anchored to substrate by rhizoids
• Multicellular gametangia form at the tips of gametophytes
  • Archegonia - Female gametangia
  • Antheridia - Male gametangia
  • Flagellated sperm must swim in water.

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Hornwort (Phylum Anthocerotophyta)

• Origin is puzzling - no fossils until Cretaceous
• Sporophyte is photosynthetic
• Sporophyte embedded in gametophyte tissue
• Cells have a single large chloroplast

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Tracheophytes Plants with Vascular Tissues

Xylem Vascular Tissue

• Conducts water and dissolved minerals upward from the roots

Phloem Vascular Tissue

• Conducts sucrose and hormones throughout the plant

Both enable enhanced height and size in the tracheophytes

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Tracheophytes

• Vascular plants include seven extant phyla grouped in three clades
  • Lycophytes (club mosses)
  • Pterophytes (ferns, whisk ferns, and horsetails)
  • Seed plants

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• Gametophyte has been reduced in size relative to the sporophyte during the evolution of tracheophytes.
• Similar reduction in multicellular gametangia has occured as well.

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Stems, Roots & Leaves

Stems

• Early fossils reveal stems but no roots or leaves.
• Lack of roots limited early tracheophytes.

Roots

• Provide transport and support
• Lycophytes diverged before true roots appeared

Leaves

• Increase surface area for photosynthesis

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Seeds

• Highly resistant
• Contain food supply for young plant
• Lycophytes and Pterophytes do not have seeds
• Fruits in the flowering plants (angiosperms) add a layer of protection to seeds and attract animals that assist in seed dispersal, expanding the potential range of the species.

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Pterophytes

• Phylogenetic relationships among ferns and their relatives is still being sorted out
• Common ancestor gave rise to 2 clades
  • Whisk ferns and horsetails
• All from antheridia and archegonia
• All require free water for flagellated sperm

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Whisk Ferns

• Found in tropics
• Sporophyte consists of evenly forking green stems without true leaves or roots

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Horsetails

• 15 living species
• Constitute a single genus, Equisetum
• Sporophyte consists of ribbed, jointed photosynthetic stems that arise from branching rhizomes with roots at nodes
• Silica deposits in cells - scouring rush

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Ferns

• Most abundant group of seedless vascular plants
• About 11,000 species
• Conspicuous sporophyte and much smaller gametophyte are both photosynthetic

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Fern Life Cycle

• Fern life cycle differs from that of a moss - much greater development, independent, dominance sporophyte
• Gametophyte lacks vascular tissue
• Produce distinctive sporangia in clusters called sori on the back of the fronds
• Diploid spore mother cells in sporangia produce haploid spores by meiosis
• Spores germinate into gametophyte
• Rhizoids but not true roots - no vascular tissue
• Flagellated sperm

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