Flowers

• The evolution of seeds

  1. Homospory -> heterospory

  2. Retaining the megaspore

  3. An outer coat (integument)

  4. Becoming a seed
• Lycopodium


  A homosporous clubmoss
• Selaginella


  A heterosporous clubmoss
• Spore sizes


  • Megaspores 150-200 μm
  • Microspores <50 μm
• Seed


  A heterosporous means of reproduction involving a single megaspore that germinates within an indehiscent megasporangium (nucellus) retained on the maternal sporophyte, enclosure of the nucellus within an integument, and the capture of pollen before seed dispersal
• Integumentation


  Megasporangium fully enclosed within integument
• Five groups of seed plants today

  • Cycads
  • Ginkgo
  • Gnetales
  • Conifers
  • Angiosperms
• Cycads


  • More than 300 species
  • Tropics & subtropics
  • Slow-growing
  • Slow to reproduce
  • Palm-like morphology
• Cycads


  • Cycas revoluta
• Cycas revoluta - Male


  Pollen-producing cone
• Cycas revoluta - Female


  Seed-producing cone
• Cycads


  • A group of sago palms, Cycas revoluta, in Audubon Park, New Orleans
• Ginkgo biloba


  • Single extant species
  • Native to China - endangered
  • Living fossil
  • Dioecious
  • Motile sperm from pollen
  • Ovules develop into fruit-like organs containing seeds
  • Edible - contain high levels of butyric acid
• Gnetales


  • Ephedra - 70 species worldwide in deserts, mostly shrubs
  • Pharmacological compounds
  • Some grown commercially in China
  • Welwitschia mirabilis - single species endemic to Namib desert
  • Gnetum - 50 species, tropical evergreens
• Conifers


  • Today: ~7 families, ~70 genera, ~600 species
  • Araucariaceae - Araucaria araucana (monkey puzzle)
  • Pinaceae - Pinus
  • Deep time survivors: Dawn redwood, Metasequoia, Wollemi pine, Wollemia, Umbrella pine, Sciadopitys
  • Conifer forests now largely restricted to higher latitudes/altitudes
  • Northern hemisphere: Pines
  • Southern hemisphere: Podocarps
  • Cypresses (Cupressaceae): everywhere!