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Seeded Plants
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Lecture notes on Seeded plants (gymnosperms and angiosperms)
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Lecture 1: GymnospermsSpermatophyta (seed plants) can be divided into 2 groups
Gymnosperms - cone-bearing plants with unenclosed ovules (naked seeds)
Angiosperms - flowering plants with seeds enclosed in carpels (vessel seeds)
For both of them, the gametophytes are microscopic
Why have seed plants become dominant through evolution?
Embryo and food supply (endosperm) is surrounded by a protective coat, protecting it from UV
radiation and damage. This creates a greater chance that the seed will successfully be transferred
further.
Seeds can also retain viability for centuries - cereal and wheat - 200 year old seed
Lifecycle modifications in seed bearing plants
Heterospory (ensures cross fertilisation)
Gametophyte reduction and protection (protected within sporophytic plant)
Retention of female gametophyte on sporophyte
Pollination (gives independence from free water)
Seed habit: embryo, endosperm and testa
Evolution towards gametophyte reduction
Gymnosperms have a pollen cone (which produces pollen) and the ovulate cone (which produces
the ovules)
Angiosperms have ovules
What are the advantages of gametophyte miniaturisation?
Gametophytes develop within sporophytes
Moist environment prevents desiccation
Protection from UV
Nutrients supplied by sporophyte
Seed plants are heterosporous. Seedless plants have one kind of spore which gives rise to a bisexual
gametophyte.
Megasporangia form megaspores - female gametophytes
Microsporangia form microspores - male
Closest relative of seed plants are homosporous suggesting that heterospory evolved from
homosporous ancestors
Production of eggs by seed plants
Protecting the egg is critical for the plants reproduction.
, 8 nuclei and 7 cells.
Integuments are sporophyte tissue that envelopes the megasporangium
Gymno - 1 integument
Angio - 2
Formation of microsporangium
Seedless plants use diff mechanism to seeded ones
Mosses and fern produce flagellated sperm, requires an aqueous env-- vulnerable to drought
In seed plants, the microspore develops into a pollen grain.
Seed plants:
Non flagellated sperm
Use wind/animals to move
Travel vast distances
Survive for long periods
Seedless plants:
Flagellated sperms
Require aqueous env
Travel a few cm
Sensitive to desiccation
Ginko and cycads retain flagellated sperms but most gymnosperms are non flagellated.
Advantages of seeds over spores
Spores:
Survive is local env isn't favourable
Single cell so susceptible
No nutrient supply
Seeds
Survive for long periods
Multicellular with protective coat
Nutrient supply
Evolution of gymnosperms - late Devonian period - some plants acquired characteristics of seed
bearing plants (Archaeopteris)
They were heterosporous, wood forming, fern like leaves, up to 30m tall but no seeds.
Therefore these were referred to a progymnosperm - transitional species (interesting from an
evolutionary perspective)
Seed plant evolution
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