seed plants. evolution of the seed seeds represent an extreme form of heterospory seed – mature...
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Evolution of the seed Seeds represent an extreme form of
heterospory Seed – mature ovule with embryo
– Megasporangium surrounded by integuments Events leading to evolution of seed
– Retention of megaspores in megasporangium (nucellus)
– Reduction in number of megaspore mother cells– Only one megaspore survives– Reduced megagametophyte– Young sporophyte developes in
megagametophyte which is in megasporangium– Integument envelopes megasporangium– Apex of megasporangium modified for pollen
Progymnosperms Seedless vascular plants – but likely
progenitors of seed plants Unlike other seedless vascular plants,
progymnosperms had secondary vascular tissue (both xylem and phloem) and its structure is very like that of modern conifers
Some had a eustele and were heterosporous
Extinct Gymnosperms Three groups of extinct gymnosperms Seed ferns Cordaitales Bennettitales – may be ancestors of
angiospems– Had a flower – like structure
Living Gymnosperms Gymnosperm means “naked seed” – ovules
& seeds exposed on surface of sporophylls Most have polyembryony Pollen grain – no water required!
– In Conifers and Gnetophytes, sperm are non-motile
Pollen tube
– In Ginkgo and cycads, transitional. Pollen tube is present, but sperm swim to egg
Coniferophyta Xylem composed of tracheids Non-motile sperm conveyed to egg by
pollen tube Leaves usually needle-like or scale-like
– Fascicles in pines Ovulate and microsporangiate cones on
same plant; ovulate cone compound
Pine life cycle In general, representative of gymnosperms,
but has many peculiarities specific to pines Highlights
– Male gametophyte consists of 4 nuclei when dispersed, 2 sperm produced near pollination
– Female produces several archaegonia, all of which may be fertilized (polyembryony #1)
– Each archaegonium produces 4 embryos (polyembryony #2)
Some “living fossils” Metasequoia – the dawn redwood Wollemia pine – discovered in 1994
near Sydney, Australia
Cycadophyta Xylem of tracheids Produce flagellated motile sperm
– Pollen tube not the main conveyer of sperm to egg
Ovulate and microsporangiate cones on separate plants
Leaves “palm-like” Often contain toxins Pollen may be carried from male to female
by insects
Ginkgophyta Xylem composed of tracheids Motile sperm
– Pollen tube not true conveyer of sperm to egg
Ovulate and microsporangia on separate plants
Ovules fleshy, stinky Leaves fan shaped
Gnetophyta Most like flowering plants Xylem both tracheids and vessel elements Non-motile sperm
– Pollen tube conveys sperm to eggs Ovulate and microsporangiate cones
compound, mostly on separate plants Double-fertilization in Ephedra Pollination often by insects, cones produce
nectar Leaves various