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SEED EVOLUTION, image, image - Coggle Diagram
SEED EVOLUTION
MICROGAMETOPHYTES
Gymnosperms & Angiosperms: Pollen replaces water transport
Exine layer protects against drying
Some pollen have air bladders for wind transport
Pollen Tube:
Haustorial type feeds on nucellus
Direct delivery to megagametophyte (angiosperms)
Reduced male gametophyte in angiosperms:
3-celled pollen grain (tube cell + 2 sperm)
OVULE DIVERSITY
Gymnosperm Ovule
1 integument
Nucellus = megasporangium
Archegonia + eggs
Angiosperm Ovule
2 integuments
Megasporangium = nucellus
Megagametophyte = embryo sac
3 antipodals
2 polar nuclei → form endosperm after fertilization
2 synergids → guide pollen tube
1 egg → forms zygote
ORIGINS OF SEED PLANTS
Progymnosperms (extinct)
Gymnosperms
Angiosperms
Earliest known seed: Elkinsia polymorpha
Seed Ferns (extinct)
EVOLUTION OF THE OVULE
Heterospory — separate microspores and megaspores
Megasporangium enclosed in integuments (protective layers)
Retention of megaspores within megasporangium
Reduction to one megasporocyte per megasporangium
Only one megaspore survives (others degenerate)
Endosporic megagametophyte — develops inside the megaspore wall
Modification of megasporangium apex — formation of pollen chamber and micropyle
Embryo development inside megagametophyte
