Transcript Gymnosperm - heterospory

Seed Plants

Two groups of seed plants:


Gymnosperms
Angiosperms

Gymnosperms include the conifers and cycads and this
group originated in the Carboniferous period.

Angiosperms are the flowering plants. Oldest accepted
fossils are from about 125mya and are scarce. By
65mya angiosperms were dominant plants on earth.
Heterospory and alternation of
generations
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Recall that in mosses the life cycle is dominated by the
gametophyte generation. The sporophyte is dependent
on the gametophyte. Both are visible with the naked eye.
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In ferns, the sporophyte generation is dominant and the
gametophyte is reduced, but still visible to the naked
eye. Both are independent.

In seed plants the situation from mosses is reversed.
The gametophyte generation is dependent on the
sporophyte. However, the gametophyte generation is
microscopically small.
Alternation of generations in
seedless and seed plants
Origin of gymnosperms
 Gymnosperms
first group to develop
seeds. Probably evolved from a
heterosporous group called the
progymnosperms in the Devonian Period
(408-360 mya).
 First
gymnosperms lacked seeds, but had
developed them by the end of the
Devonian.
Radiation of Gymnosperms

Gymnosperms underwent adaptive radiation
during the Carboniferous and Permian periods
(360-245 mya).

Planet became progressively drier during the
Permian and gymnosperms replaced ferns and
relatives as the dominant plants. Gymnosperms
were the dominant plants during the Age of
Dinosaurs (Mesozoic 245-65 mya).
Advantages of heterospory
 Ancestor
of gymnosperms was
heterosporous.
 Evolution
of heterospory was a significant
development because it led to the
evolution of seeds.
Gymnosperm - heterospory
Independent and
dominant sporophyte
microspores
meiosis
2n
embryo
megaspores
n
megagametophyte
zygote
eggs
sperm
microgametophyte
Advantages of heterospory
 Female
gametophyte greatly reduced in
size, so much so that it could be enclosed
and protected within a layer of sporophyte
tissue (the ovule).
 In
seed plants the female gametophyte is
not dispersed and is protected from drying
out and other hazards.
Advantages of heterospory
 The
male gametophyte (n) is dispersed in
seed plants, but is protected by a layer of
sporophyte tissue (2n).
 The
whole structure is called
a pollen grain.
Pollen grain
Gymnosperm Life Cycle
 In
gymnosperms dominant generation is
the tree (sporophyte),
 Pine
trees (and other gymnosperms)
produce both ovulate and pollen cones.
Gymnosperm Life Cycle
 Pollen
cone contains sporangia that
undergo meiosis to produce haploid
microspores that develop into male
gametophytes (n).
 Pollen
is wind dispersed and some lands
on ovulate cones.
Gymnosperm Life Cycle
 Ovulate
cone contains many scales, each
with two ovules.
 Ovule
contains megaspore mother cell
that undergoes meiosis and eventually
produces the female gametophyte (n) that
produces eggs.
Gymnosperm Life Cycle
 Eggs
are fertilized by sperm from the male
gametophyte. Usually only one fertilized
egg develops into an embryo.
 Embryo
is surrounded and nourished by a
food supply (the female gametophyte
tissue). Both are protected by a seed coat
derived from the parent tree.
Gymnosperm Life Cycle
 Seed



thus consists of
Embryo (2n)
Layer of nutrient (female gametophyte (n))
Seed coat (2n from parent)
 Seed
falls from cone, and if lucky,
develops into a tree.
Advantages of seeds
 Seed
coat provides protection and
gametophyte tissue provides
nourishment for developing embryo.
 Dormancy:
the developing embryo is
protected and can wait a long time to
germinate when conditions are good.
Advantages of having seeds
 Dispersal:
The seed coat can be modified
for dispersal (e.g. by developing wings or
becoming fleshy). Angiosperm seeds are
enclosed in a bribe (fruit) and animals
move them.
Seeds vs spores
 Seeds
are better than spores because
spores have a short lifetime.
 Spores
are thinner walled and more
vulnerable to pathogens and damage.
Modern Gymnosperms
 Gymnosperms
have “naked” seeds that
are not enclosed in an ovary (as
angiosperm seeds are).
 There
are four extant phyla.
Cycads
 130
species in 2
families
 Old World
tropics
 Large palm-like
leaves and large
cones.
Ginkgo
1
species
 Unknown in wild,
previously
widespread
 Seed coat is
fleshy.
 Widely planted
street tree
Conifers





600 species in 7
families
Most important
gymnosperms
Pine, spruce, fir,
cedar, etc.
Complex seed
cones
Needled leaves
reduce water loss
Ephedra
Gnetophytes
 90
species,
3 genera
 Double
fertilization
 Transition to
angiosperms?
Welwitschia
Gnetum