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Multicellular Photosynthetic Autotrophs

Plants

• Multicellular
• Cellulose cell walls
• Chlorophylls a and b
• Develop from embryophyte
• Alternation of generations
• Major food source for terrestrial life
• Atmospheric O2 and CO2 balance
• Coal deposits
• Intimate association with mycorrhizal fungi
• >250,000 species (~500,000?)
• Taxonomy
  • State of flux
• DNA sequencing
• Developmental studies
  • Division (old literature) = phylum (new literature)
  • ~12 phyla, 9 of which are vascular plants

Plant phyla

Phyla are 12 groupings

Plant evolution

• Evolved from green algae, likely related to charophytes
• Evidence
  • DNA sequences
  • homologous chloroplasts: chlorophyll b and beta-carotene; thylakoids in grana;
  • Cellulose in both groups; also peroxisomes
  • Mitosis and cytokinesis similar
  • Sperm ultrastructure

Terrestrial adaptations

• Stomata: pores in leaves for exchange of gases; prevent desiccation
• Secondary metabolites:
  • cuticle: waxy coating to prevent H2O loss
  • lignin: hardens wood
  • sporopollenin: resistant polymer; coats pollen
  • predator defenses
• Embryonic development
  • gametangia in early plants
  • spores; seeds
• Mycorrhizae
• Water/food conducting systems

Plant life cycles

• Alternation of generations
• Sporophyte
  • diploid
  • produces spores in sporangia
• Gametophyte
  • develops from spore
  • haploid
  • produces gametes in gametangia
• Haplodiplontic life cycle

Vascular plants

• Most have roots
• Aerial shoot systems
• Vascular tissue
  • xylem: water, mineral transport
  • phloem: food transport
• Lignin
• Branched sporophyte is dominant stage
  • amplified production of spores
  • evolution of complex plant bodies
• Dominated Carboniferous (360 my)

Vascular plant life cycles

• Homosporous (single type of spore)
• Heterosporous (two types of spore)

Pterophyta (ferns)

• Non-seed plant
• Sporophyte conspicuous (vascular tissue)
• Rhizome: ground stem, roots
• Fronds: leaves
• Sori: clusters of sporangia
• Motile sperm require external water for fertilization
• Originated in Devonian, 350 my

Tree fern Fern life cycle

Non-seed plants, continued

• Lycophyta: club mosses
  • E.g., Lycopodium (“ground pine”)
  • Many species became extinct 270 my, once dominant (coal formations)
  • Gametophyte non-photosynthetic, nourished by fungi
• Arthrophyta: horsetails
  • Equisitum
  • Some fossil forms (300 my) were tree-size (coal)
  • Photosynthetic stems, no leaves
  • Silica deposits in epidermal cells

Seed plants

• 1st appeared in Devonian, 360 my
• Seed develops from ovule, protects embryo
  • withstands drought
  • dispersal is enhanced
  • no immediate need for water for germination
• Heterosporous
  • male gametophyte: arise from microspores
  • female gametophyte: arise from megaspores in ovule in ovary
• Two groups
  • gymnosperms
  • angiosperms

plant

Megasporangium (nucellus)

• Key to seed development
• Nucellus: solid, fleshy, surrounded by integuments derived from sporophyte (seed coat)
• Entire structure called ovule
• Flower may have many ovules

Pollen

• Develop from microspores, become male gametophyte
• Protected by sporopellenin
• In most plants, sperm lack flagella (loss)
• Many mechanisms to transport pollen
  • wind
  • insects, birds,

Gymnosperms

• “naked seed”
  • ovule not fully enclosed by sporophyte at time of pollination
• Conifers, cycads, gnetophytes, Ginkgo
• Small, inconspicuous plants to giants like sequoia
• Conifers: to carry cones fv
  • male cones, Female conesvv
  • evergreen

Other Coniferophyta

• Cycadophyta: cycads
  • tropical, subtropical
  • flagellated sperm
• Gnetophyta
  • e.g., Ephedra, Mormon Tea
• Ginkgophyta: Ginkgo
  • only one surviving species
  • diocious (separate % and &trees)

Angiosperms

• Flowering plants, Anthophyta
  • monocots- single seed leaf (grasses, lilies, etc.)
  • dicots- two seed leaves (roses, pulses, maples)
• More specialized xylem (water transport)
  • vessel elements
  • fiber cells
• Fossils date to 130 my
• Animal (e.g., insect) coevolution

Earliest angiosperm

• What is earliest angiosperm?
• Recent analysis of nucleotide and amino acid sequences suggests that Amborella, a tropical plant found only on the island of New Caledonia, is closest relative to flowering plants

Angiosperm flower

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Angiosperm life cycle

This text is based on notes very generously donated by Paul Doerder, Ph.D., of the Cleveland State University.