Download Pollen - male gametophyte, protected by a spore wall

Gymnosperms: gymnos “naked” sperma “seed”, seed plants
with exposed seeds (no fruit). The seeds are single or borne
on the scales of cones.
(Only flowering plants produce fruit)
average = 70.0
In the gymnosperms, the gametophytes are not free living.
The female gametophyte is maintained in the megasporangia.
The male gametophyte is the pollen and is released and
travels to the female gametophyte for fertilization.
These are adaptations to reduce the vulnerability of the
gametophyte.
After fertilization the new sporophyte generation (the embryo
in the seed) is released.
This transition required the evolution of pollen and seeds
found in gymnosperms.
Monilophytes
True ferns
Horsetails heterospory
(in some)
Ophioglossalean
ferns
leptosporangia
heterospory
(in some)
Whisk
ferns
Marattialean ferns
megaphylls
Isoetes
Selaginella (Zosterophyllophyta)
(Lepidodendrids)
heterospory
heterospory
Seed plants
chloroplast DNA inversion
Lycopodium
group
reduced gametophyte
Lycophytes
(Other Rhyniophytes)
(Aglaophyton)
(Rhyniophytes)
lignin
(Rhynia)
strongly lignified vascular tissue
(xylem and phloem)
Evolution of seeds from retained megaspores.
branched, independent sporophyte
Bryophytes
Conifers
Reduction of the gametophyte
Sporophyte
(2n)
Bryophyte
Sporophyte
(2n)
Gametophyte
(n)
(a) Sporophyte dependent
on gametophyte
(mosses and other
bryophytes).
Gametophyte
(n)
(Progymnosperms)
(Seed ferns)
Conifers
Cycads
Gingkos
Gnetophytes
Cephalotaxaceae
Podocarpaceae
Gnetophytes
Welwitschia
small, independent
gametophyte (ferns
and other seedless
vascular plants).
Taxaceae
Microscopic female
gametophytes (n)
inside these parts
of flowers
(dependent)
Gnetum
Pinaceae
Ephedra
Flowering
plants
Cycads
Angiosperm
Sporophyte (2n)
(independent)
(c) Reduced gametophyte dependent on sporophyte
(seed plants: gymnosperms and angiosperms).
Cupressaceae
Araucariaceae Sciadopityaceae
Ginkgo
Microscopic male
gametophytes (n)
inside these parts
of flowers
(dependent)
Microscopic male
gametophytes (n)
in pollen cones
(dependent)
Monilophyte
(b) Large sporophyte and
Microscopic female
gametophytes (n) in
ovulate cones
(dependent)
Gymnosperm
Gymnosperms:
Sporophyte (2n),
the flowering plant
(independent)
(“Seed ferns”)
“Progymnosperms”
(Archaeopteris)
seeds
Monilophytes
wood
Earliest Tracheophytes
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Progymnosperms such as this drawing of
an “Archaeopteris” had secondary stem
growth similar to today’s conifers. They had
vascular bundles surrounding a pith. But
they produced spores on leaves like ferns.
The evolution of pollen is important because previously a film of water was
required for sperm to swim to the archegonia. With pollen the sperm only
need to swim (or move) down the pollen tube.
Pollen - male gametophyte, protected by a spore wall
These existed in the Devonian (around 400
M years ago.
The evolution of wood is important because it provided strength for very
tall and long-lived trees that could out-compete other plants.
25m tall
Evolution of leaves in progymnosperms
What is a seed?
“Baby plant in a box with its lunch”
Dr. David Biesboer, 1978.
Advantages of seeds:
The young sporophyte no longer needs to grow and photosynthesize
immediately (it can go dormant).
The sporophyte can be dispersed (air, animal vector).
The sporophyte is protected.
Disadvantage: very costly to make.
The first seed plants appeared in the Devonian (400 M years ago). These were
small trees with fern-like leaves. The name “seed fern” was invented to describe
them.
Development of the seed
Ovule - this is the megasporangium surrounded by the integument before the ovule
becomes a seed.
Integument - protective covering of the ovule, sporophyte tissue that becomes the
seed coat.
The integument has an opening at one end called the micropyle that allows the
pollen tube entry.
What were the precursors for the development of seeds?
1. Heterospory - this arose independently several times. It allowed for one mobile
spore - microspore and one larger nutrient laden megaspore.
2. Retention of the megaspore within the sporophyte and reduction of megaspore
number from 4 to 1.
3. Development of the protective layer called the integument.
The megasporangium is called a nucellus in seed plants, it is 2n sporophyte tissue
and produces the megasporocyte that undergoes meiosis.
The megaspore is the first product of meiosis (so it is haploid) and it gives rise to
the megagametophyte (egg sac).
integument
Evolution of the seed
in progynmosperms
This leads to another problem - delivery of the male gamete.
3 aborted
megaspores
functional
megaspore
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Pollen is an immature microgametophyte.
In heterosporous species like Selaginella, the microspore is released and then
the microgametophyte grows within the spore wall and produces sperm which
are released.
rays
resin duct
All gymnosperms have secondary xylem
and phloem, rays and bark. The wood is
regular because there are no vessels
(except in Gnetophytes).
The phloem contains sieve
cells (not sieve elements and
companion cells).
summerwood
Pollen is derived from this. Mature pollen leaves the microsporangium
consisting of 2-5 haploid nuclei. When it reaches the ovule, it produces 2 sperm
cells and a pollen tube.
springwood
one annual
growth ring
Many gymnosperms
produce resin that
accumulates in resin ducts
and inhibits insect feeding.
tracheids
Ginkgo - one species left, only found in the wild in China, planted at monasteries
in Japan and China for centuries. Now it is planted as an ornamental tree
throughout the world, and all over the St. Paul campus. They have stinky seeds
(not fruit).
Characteristics of Gingko:
Dioecious (male and female plants).
Large trees with fan shaped leaves.
The microgametophytes produce large flagellated sperm.
Ovules are borne on stalks rather than cones.
Gingko male cones
Leaves with dichotomous venation
In Ginkgo, the pollen enters the micropyle, grows a bit, and
produces large sperm.
Gingko ovules
Gingko embryo
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Cycads - Mesozoic dinosaur chow. There are 125 extant species, mostly tropical
Cycad characteristics:
They are all dioecious, there are male and female
sporophytes. (Monoecious plants produce both male and
female reproductive structures.)
They have compound leaves (like ferns).
They grow slowly and live a long time (1000 years).
The seeds are eaten by birds, and dispersed in that way. The
seeds are poisonous to humans causing a slow degenerative
brain disease.
Pollination can take up to 10 years. Some are pollinated by
insects.
Ovulate cone
Here is a human removing
a male cone from a cycad.
Structure of the cycad ovule.
Male cone w/pollen
sacs
Cycad ovule
Conifers (“cone bearer”) - pines, firs, spruces, hemlocks, 650 species
Woody seed cone (some lack woody cones - yews, junipers the fleshy covering
of their seeds is not a fruit, it is integument or modified scales).
Conifer families:
Pinaceae: pines, firs, spruces
Cupressaceae: junipers, cypresses, redwoods
Taxaceae: Yews
Araucariaceae: monkey puzzle tree, Norfolk Island Pine
Podocarpaceae: southern hemisphere conifers
Wind pollination - lots and lots of pollen produced
Produce cones w/ ovules - two year cycle
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Pinaceae Characteristics
-Needle-like leaves singly or in fascicles (clusters).
-Most are monecious (bisexual plants with male and female cones)
-Two seeds per seed scale
Pines, genus Pinus is the largest genus with 93 species.
-Needles clustered in 2-5 per fascicle (except for pinyon pine which is a
single leaved pine).
-Needles oval to triangular in cross section.
-Cones hang down and vary in size, usually they shed their seeds, some
“closed-cone pines” such as the jack pine require heat to open the cone.
Fire removes the competition.
-Bristlecone pines (Pinus longaeva) are the longest living organism (5000
years).
mesophyll
vascular
bundle
phloem
xylem
resin duct
Cross section of a pine
needle. Showing thick
cuticle, sunken
stomata, no air space,
fiberous epidermis. All
modifications to reduce
desiccation.
Fascicles of pine needles.
Pollen germination leads to pollen tube growth and the
production of two sperm cells within the ovule.
endodermis
Female
gametophyte (n)
guard cells
mesophyll
stomatal
pore
resin duct
Male gametophyte
(within germinating
pollen grain) (n)
Discharged
sperm nucleus (n)
Micropyle
Pollen grain (n)
(b) Fertilized ovule. A megaspore develops into a
multicellular female gametophyte. The micropyle,
the only opening through the integument, allows
entry of a pollen grain. The pollen grain contains a
male gametophyte, which develops a pollen tube
that discharges sperm.
schlerenchyma
epidermis
The seed develops from the ovule. The seed is the sporophyte embryo,
along with its food supply, packaged in a protective coat.
Egg nucleus (n)
Spore wall
Pine life cycle as an example for
gymnosperms
Important features:
-Dominant sporophyte generation
Seed coat
(derived from
Integument)
Food supply
(female
gametophyte
tissue) (n)
Embryo (2n)
(new sporophyte)
(c) Gymnosperm seed. Fertilization initiates
the transformation of the ovule into a seed,
which consists of a sporophyte embryo, a
food supply, and a protective seed coat
derived from the integument.
-Development of seeds from
fertilized ovules
-Pollen delivers the male
gametophyte to the ovule
The megasporangium (nucellus) produces a
megasporocyte that undergoes meiosis.
One megaspore survives to become the
female gametophyte.
In the seed, the embryo is surrounded by
female gamotophyte (1n) tissue.
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Structure of the female cone of pine.
Structure of the male cone of pine.
pollen
tube
integument
archegonium
with egg
micropylar chamber
female
gametophyte
nucellus
Conifers
Pinaceae Characteristics
Cephalotaxaceae
Podocarpaceae
Gnetophytes
Taxaceae
-Needle-like leaves singly or in fascicles (clusters).
Cupressaceae
Araucariaceae Sciadopityaceae
-Most are monecious (bisexual plants with male and female cones)
-Two seeds per seed scale
Welwitschia
Gnetum
Pinaceae
Ephedra
Pines, genus Pinus is the largest genus with 93 species.
Flowering
plants
Ginkgo
-Needles clustered in 2-5 per fascicle (except for pinyon pine which is a
single leaved pine).
Cycads
-Needles oval to triangular in cross section.
(“Seed ferns”)
“Progymnosperms”
(Archaeopteris)
seeds
Monilophytes
wood
-Cones hang down and vary in size, usually they shed their seeds, some
“closed-cone pines” such as the jack pine require heat to open the cone.
Fire removes the competition.
-Bristlecone pines (Pinus longaeva) are the longest living organism (5000
years).
Earliest Tracheophytes
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Pinaceae, the pine family
Firs (genus Abies)
Examples:
Each year’s growth is a whorl of branches.
Pine (genus Pinus)
Firs (genus Abies)
Douglas firs (genus Pseudotsuga)
Spruce (genus Picea)
Hemlocks (genus Tsuga)
Larch and tamaracks (genus Larix)
Cedars (genus Cedrus)
Single flat, blunt needles
Erect cones
Cone shatter at maturity
Spruce (genus Picea)
Hemlocks (genus Tsuga)
Needles are single, angular in cross section and sharply pointed
Needles are flat with a short petiole
cones hang down
Cones are small and hang down
Black spruce has “closed cones”
Tsuga canadensis
Douglas firs (genus Pseudotsuga)
Young hemlock trees
Pseudotsuga menzeiesii is one of the main trees cut for lumber in the Pacific
NW
Single needles resemble spruce but are softer
Cones hang down
Easily identified by its cone with three-lobed bracts that stick out between the
scales
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Larch and tamaracks (genus Larix)
Unusual because they are deciduous (they lose all of their leaves in the Fall)
Older branches have clusters of multiple needles, oneyear old branches have single or spiral needles.
scale
bract
Cones of Pseudotsuga menzeiesii note three lobed bract.
Cedars (genus Cedrus)
needles are single
Female cone at pollination
Male cones
Cones of Deodar
cedar
Seed cones
Families:
Pinaceae: pines, firs, spruces, hemlock, larches, cedars
Cupressaceae: junipers, cypresses, redwoods
Taxaceae: Yews
Araucariaceae: monkey puzzle tree, Norfolk Island Pine
Podocarpaceae: southern hemisphere conifers
Cupressaceae: Junipers, Cypresses and Redwoods
shrubs or trees
small scale-like leaves
monoecious or dioecious
cone scales are woody (as in cypresses) or fleshy (as in juniper)
130 species, distributed world wide
some species are “closed-cone”
Examples:
dawn redwood (Metasequoia glyptostroboides)
California coast redwood (Sequoia sempervirens)
Sierra redwood (Sequoiadendron giganteum)
bald cypress (Taxodium distichum)
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dawn redwood (Metasequoia glyptostroboides)
deciduous, grows well here (can grow up to 200 ft. tall), was thought to be extinct
but rediscovered in 1944 in China.
California coast redwood (Sequoia sempervirens)
Sierra redwood (Sequoiadendron giganteum)
bald cypress (Taxodium distichum)
Taxaceae: Yews
shrubs or trees
dark, linear, pointed leaves
single seeds with a freshy covering (aril)
these are the only conifers to lack cones
Examples
English Yew (Taxus baccata)
English Yew (Taxus baccata).
The wood is famous for its
use in bows.
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Podocarpaceae - Southern Hemisphere Conifers
Araucariaceae - Southern Hemisphere Conifers
shrubs or trees
leaves vary from short needles to long oblong blades
one ovule per cone scale
seeds spread by birds
mostly dioecious
-generally large trees (monkey puzzle tree,
southern pine)
-the tallest trees in the tropics
-leaves vary from needle-like to flat and broad
-cones are large and globe shaped
(disintegrate when ripe)
cones
genus Araucaria
leaves
ovules
genus Podocarpus
branches
Gnetophytes
-three genera: Ephedra, Gnetum, and Welwitschia
-vessel elements in xylem
-ovules surrounded by fleshy layer
-similarities to flowering plants represent convergent
evolution
-convergent evolution: independent evolution of a trait such
as wings in reptiles and mammals.
Ephedra viridis (Mormon tea)
It has some stimulant properties.
Ephedrine and pseudoephedrine
are the main active ingredients.
Gnetum leyboldii - hard
to distinguish from a
flowering plant
Welwitchia mirabilis
Male and female reproductive structures of
Welwitchia mirabillis
pollen strobili
microsporophylls
Native to Namib desert. It has two leaves.
It grows slowly, this plant is about 1000 years old.
microsporangiate
bearing structure
Female cones
megasporophylls
Discovered by Friedrich Welwitsch
(1806-1874) Austrian explorer and
botanist.
seed
wings
bract
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Monocots
Angiosperms “seed within a vessel” developed around 135 M years ago
Please watch this movie about angiosperm lifecycle
http://www.youtube.com/watch?v=H_UyDtaa8Ow
Important developments in Angiosperms.
- ovules enclosed in ovaries (the ovary forms the fruit)
- pollen enclosed in stamen
- double fertilization
- further reduced male and female gametophytes
increases the speed of egg development
and pollination
showy flowered non-showy flowered
commelinids
Lily group
Ginger group
Eudicots
Grass group
Alismatales
Palms
Magnoliids
Caryophyllids
The angiosperms are
the magnoliids, the monocots
and the eudicots.
Laurels
Magnolias
Peperomias
Pipe vines
Star anise group
single cotyledon
tricolpate
pollen
Rosids
Asterids
Proteales
Ranunculales
Water lilies
Amborella
Living gymnosperms
Gnetophytes
Conifers
vessels
double fertilization
fruit
flowers
Ginkgo
Cycads
Magnolia virginiana
Unknown gymnosperms
Next time: angiosperms (flowering plants)
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