Grasses; family: Poaceae

Here listed separately due to some intriguing characters
suggesting a possible alternate evolution from other Magnoliophyta

characters

• seeds with one reduced cotyledon, large triploid endosperm
  cotyledon does not store nutrients
  but absorbs nutrients from endosperm
• endosperm forms two layers
• embryo has large, distinct mesocotyl (neither root nor stem)
  with branch vascular bundles into cotyledon
  epicotyl has apical meristem, usually lacking embryo stem
• leaf meristem at leaf base, not apical
• leaves long and narrow, somewhat needle-like
  stomata on upper side of leaf, similar to Araucariaceae
  cellular details of stomata match between Araucariaceae and Poaceae
  often with crystals of silicon dioxide (glass) along leaf margin
• megasporophylls and microsporophylls often in separate inflorescences
• megasporophylls frequently in compact, strobilus-like inflorescences
  on short lateral branch, adaxil to leaf
• microsporophylls often in loose strobilus-like structures
  apical on stem, which dies after pollen is released
• even monecious flowers may be borne in strobilis-like inflorescences
• some with monecious flowers in open inflorescences [not like strobili]

These and other comparisons between the Araucariaceae and the Poaceae
suggest the possiblity that Poaceae are not angiosperms
but highly evolved angiosperm-like Araucariacean plants

geologic history of angiosperms

• earliest known angiosperm tree, late Jurassic, 142 Myr BP
  origin believed to be Permian, 270 - 220 Myr BP
  based on angiosperm-like fossils,
  often with few to several angiosperm characters
• majority of angiosperm families, mid Cretaceous, ca 100 Myr BP
• rapid development of modern families in Cretaceous
  attributed to insect pollination
  (Insects also show expansion during Cretaceous)
• earliest known Poaceae, Tertiary, 65 - 2 Myr BP

possible phylogeny of angiosperms

• Gnetalian origin.
  • Gnetalian sporophytes seem transitional from gymnospermous to angiosperms
  • gametophytes show little or no similarity
• Bennettitalian [Cycadophyta, fossil only] origin.
  • some angiosperm-like characters in sporophyte wood
  • highly similar sporophyte leaf structure (cellular detail)
  • strobili of Bennettitales have strong infloresence-like characters
  • sporophylls show little or no similarity
• Caytonialian origin.
  • fossil gymnosperms with seeds enclosed in carpel-like sheaths
    similar to angiosperm pistils

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