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

Life on Land

Plants had many issues to overcome on land that include:

• Water loss

• Protection from the harmful effects of the sun

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

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.

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

Multicellular haploid stage - gametophyte

• Spores divided by mitosis

• Produces gametes by mitosis

• Gametes fuse to form diploid zygote

  • First cell of next sporophyte generation.

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

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

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

Liverworts (Phylum Hepaticophyta)

• Have flattened gametophytes with liverlike lobes

  • 80% look like mosses

• Form gametangia in umbrella-shaped structures

• Also undergo asexual reproduction

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.

Hornwort (Phylum Anthocerotophyta)

• Origin is puzzling - no fossils until Cretaceous

• Sporophyte is photosynthetic

• Sporophyte embedded in gametophyte tissue

• Cells have a single large chloroplast

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

Tracheophytes

• Vascular plants include seven extant phyla grouped in three clades

  • Lycophytes (club mosses)

  • Pterophytes (ferns, whisk ferns, and horsetails)

  • Seed plants

• Gametophyte has been reduced in size relative to the sporophyte during the evolution of tracheophytes.

• Similar reduction in multicellular gametangia has occured as well.

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

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.

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

Whisk Ferns

• Found in tropics

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

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

Ferns

• Most abundant group of seedless vascular plants

• About 11,000 species

• Conspicuous sporophyte and much smaller gametophyte are both photosynthetic

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