Download Reproductive Life Cycles of Vascular Plants

Reproductive Life Cycles of Vascular Plants
Plants
The most primitive nonvascular and vascular
plants sexually reproduce
by spores.
The more advanced
vascular plants produce
seeds.
Non-vascular
Mosses
400 million years ago
Vascular
Seedless plants
Lycopods
Horsetails
Spores
Ferns
Seed plants
Seeds
Cycads
Ginkgo
Conifers
Angiosperms
300 million years ago
200 million years ago
100 million years ago
55 million years ago
Seed plants
Seed plants are separated into gymnosperms and angiosperms.
The seed habit vs. spore production:
(1) Rather than producing a single spore type (homospory),
seed plants produce a separate female megaspore and male
microspore (heterospory).
(2) The female gametophyte is retained on the mother plant
(sporophyte) and is enclosed within a protective maternal
seed coat.
(3) The ovule has an opening designed to receive pollen that
does not depend on water for male gamete transfer.
Seed evolution
Seed Plants - Extinct seed producing plants
The evolution of the seed habit
began during the Devonian period
about 350 to 385 million years ago
in progymnosperms.
Fossil leaf
Progymnosperms were sporeproducing, but showed heterospory.
Heterospory is a first step in the
evolution of a true seed.
Progymnosperms are considered
the common ancestor of all seed
plants.
Archaeopteris
Seed evolution
Seed Plants - Extinct seed producing plants
The “seed ferns” in the late Devonian
were the first plants to produce seedlike structures enclosed in female
tissue called cupules.
The cupules contained a single seed
(megaspore) within protective
coverings.
Cupules
Cupules
Alethopteris
Seed Plants - Gymnosperms
Gymnosperms are the oldest living
seed producing plants.
The term gymnosperm means
“naked seeds”.
Gymnosperms include the cycads,
ginkgo, gnetophytes (Ephedra,
Gnetum) and the conifers (like pine,
fir, and hemlock).
Fir (Abies) cone
Cycads
Cycads appear in the fossil
record 320 million years
ago.
Encephalartos
Cycads
The living cycads are palm-like in character and
mostly tropical and subtropical.
Cycas
Cycads
Cycads produce male and female cone-like sporangia
on separate plants.
Intact
sporophyll
cone
Shattered
cone
Seed
Zamia floridana
Male shedding pollen
Female
Female with seeds
Cycads
Unlike other gymnosperms, cycad
sperm is motile and swims.
Male cone (strobili)
Cycads
Cycas nicely shows that the female is a
modified leaf called a megasporophyll
with attached sporangia.
Cycas
Cycads
Females in other cycads are
more cone-like.
Encephalartos
Lepidozamia
Cycads
Cycad seeds are covered with a
fleshy sarcotesta that resembles
and functions like a true fruit.
Encephalartos
Ginkgo
This is a single genus family. Only
extant member from a family
developed over 120 million years ago.
Ginkgo is a unique gymnosperm
because it has a broad leaf and is
deciduous.
Ginkgo
Female ovule in Ginkgo are produced in pairs at the tips
of reproductive short shoots. There is a small opening
at the tip of the ovule to permit sperm to enter.
Reproductive short shoot
Pair of ovules
Ovule opening
Ginkgo
Males are produced on separate trees and are clusters
of sperm containing microsporangia that release pollen.
Microsporangia
Ginkgo
Ginkgo seeds are produced in autumn and have
an outer fleshy and inner hard seed coat.
Endosperm
Embryo
Intact seed with fleshy
seed coat covering.
Seeds with the outer coat
removed.
Cut seed showing
embryo and
endosperm.
Gnetophytes
Gnetophytes include Gnetum, Ephedra, and Welwitschia.
Gnetophytes appear to be the closest living relatives to the
Angiosperms. Ephedra can have double fertilization.
Ephedra female (left) and male (right) plants.
Gnetophytes
Welwitschia is an unusual desert plant that produces only two
opposite strap-like leaves on either side of a central disc (stem).
Gnetophytes
Reproductive structures are produced
on the margin of the central disc.
Seeds have a papery outer seed coat.
Welwitschia
Conifers
Conifers (cone-bearing) represent the largest group of genera
in the gynmosperms dating back to 290 million years ago.
Abies
Picea
Pseudotsuga
Conifers
Conifers include:
Araucariaceae – Agathis and Araucaria.
Pinaceae – Abies, Cedrus, Larix, Picea, Pinus, Pseudolarix,
Pseudotsuga, Tsuga, and Sciadopitys.
Cupressaceae – Chamaecyparis, Cupressus, Juniperus,
Microbiota,Thuja, Platycladus, and Thujopsis.
Taxodiaceae – Taxodium, Metasequoia, Cryptomeria,
Cunninghamia, Taiwania, Sequoia, and Sequoiadendron.
Taxaceae – Cephalotaxus, Taxus, and Torreya.
Taxaceae
Members of the Taxaceae produce males and females on
separate plants.
The female is cone-like and produces a drop of fluid at the
tip of the ovules to capture air borne pollen.
Taxus
Cephalotaxus
Taxaceae
Males shed wind-borne pollen.
Cephalotaxus
Taxus
Wind-borne pollen
Taxaceae
Unlike many of the other conifers,
members of the Taxaceae produce
seeds with a fleshy outer coat.
In Taxus, the red coat is an aril.
Taxus
Cephalotaxus
Conifer life cycle
Conifer Reproductive Cycle - Pine
Pine reproduction
takes two growing
seasons to complete.
Conifer life cycle
Pollen formation
The male gametophyte is
produced in a staminate cone.
Staminate cone
Conifer life cycle
Pollen formation
Four haploid pollen grains are
produced through meiosis.
Conifer life cycle
Pollen formation
The male gametophyte is a winged
pollen grain spread by the wind.
Pollen grain
Pollen in the staminate cone.
Conifer life cycle
Ovule formation
In most gymnosperms, the female gametophyte is produced in
the axils of the ovulate cone between protective scales.
The ovulate cone
consists of many spirally
arranged ovuliferous
scales subtended by a
cone bract.
Ovulate cone
Each ovuliferous scale
has a pair of ovules on
its surface.
The ovuliferous scale will
form the seed wing that
covers the mature seed.
Spruce
(Picea)
Ovuliferous
scale
Conifer life cycle
Ovule formation
Megaspore mother cell
The haploid female gametophyte
is developed within the nucellus
(megasporangium) from the
megaspore mother cell by meiosis.
Nucellus
Cone bract
Ovuliferous
scale
Nucellus
Megaspore
mother
cell
Ovuliferous
scale
Cone bract
Conifer life cycle
Ovule formation
Within the female gametophyte, several archegonia are
formed each with one haploid egg cell.
Micropyle
Integuments
Archegonia
Ovule
Conifer life cycle
Pollen germination
The pollen grain contains two nuclei – one is
the tube cell and one is the generative cell.
Following pollen germination, the generative
nucleus divides into additional sperm nuclei.
Pollen grain
Germinating pollen
Sperm
nuclei
Pollen
tube
Generative
cell
Tube
cell
Tube nucleus
Conifer life cycle
Fertilization
The pollen germinates and the
pollen tube enters the ovule and
deposits the sperm nuclei.
The sperm nucleus and egg
nucleus fuse to complete
fertilization and form the 2n
zygote.
However, fertilization in most
conifers does not occur until
months after the pollen tube
enters the ovule.
In pines, it can take over a year
between pollination and egg cell
fertilization.
Micropyle
Pollen
Ovule
Egg
nucleus
Conifer life cycle
Fertilization
Fusion of the egg and
sperm cells completes
fertilization and
results in a new zygote.
Pollen
tube
Sperm
nucleus
Egg
nucleus
Archegonium
Conifer life cycle
Fertilization
Following gamete fusion, the
suspensor cells elongate and
several multiple embryos
(polyembryos) are formed
inside a single ovule.
Initially, there can be as many
as 12 developing embryos.
However, it is usual that one
embryo becomes dominate
and continues to develop as
the seed matures.
Suspensors
Embryos
Conifer life cycle
Fertilization
In most conifers, the mature seed is attached to a wing
derived from the ovuliferous scale and the embryo that will
be the next sporophytic generation.
Seed coat
Embryo
A pair of mature
seeds in pine
Cone
bract
Wing
Endosperm (1n)
Female gametophyte
Spruce (Picea) seed
Seed
Wing
Seed
Conifer life cycle
Fertilization
In gymnosperms, there is
only a single fertilization
event between the sperm
and egg nuclei.
Therefore, the endosperm
is the female gametophyte
(megasporangia) that is
haploid.
Endosperm
Female gametophyte (1n)
Seed coat
Embryo
Mature pine seed
Angiosperm life cycle
Angiosperms are true flowering
plants.
The term angiosperm means
“enclosed seeds” and refers to the
female ovary tissue that forms
the fruit surrounding angiosperm
seeds.
Angiosperms are the dominant
plant type on Earth with
approximately 250,000 species,
compared with only about 8,000
living species of gymnosperms.
Akebia quinata
Angiosperm life cycle
Angiosperms
Toward the end of the Mesozoic
era (140 million years ago),
angiosperms began to appear.
They became dominant by 90
million years ago as they replaced
gymnosperms.
The sudden abundance of
angiosperms in the fossil record
was famously called “an abominable
mystery” by Charles Darwin.
Charles Darwin
Angiosperm life cycle
Angiosperms
One reason for angiosperm
success and diversity is the
mutualistic co-evolution of
animals (especially insects) as
pollinators and seed
dispersers.
Charles Darwin
Angiosperm life cycle
Angiosperms present an incredible diversity of
flower forms and colors.
Angiosperm life cycle
Flowers are the sexual organs in angiosperms.
The male organ is the stamen and the female is the pistil.
Stamen
Anther Pollen
Filament
Petals
Stigma
Stigma
Style
Anthers
Pistil
Petal
Ovule
Ovary
Ovary
Sepal
Style
Receptacle
Pedicel
Receptacle
Pedicel
Angiosperm life cycle
Angiosperm life cycle
Angiosperm life cycle
Pollen development (Microsporogenesis)
Male gametes are formed in the pollen
grains (microspores) that are produced
within the stamen of the flower.
Stamen
Anther
Anther
Pollen grains
Pollen
grain
Filament
Angiosperm life cycle
Pollen development (Microsporogenesis)
There are four
pollen sacs in
each lily anther.
Microsporangia
(Pollen sacs)
Microspore
mother cells
Cross-section lily stamen
Angiosperm life cycle
Pollen development (Microsporogenesis)
Microspore mother cells (Microsporocytes) will divide via
meiosis to become the pollen grains. The tapetum is a
layer of nutritive cells surrounding the developing pollen.
Microsporangia
(Pollen sacs)
Microspore
mother cells
initiating
division
Tapetum
Angiosperm life cycle
Pollen development (Microsporogenesis)
Many pollen grains matures within each pollen sac.
Eventually the anther will open to release the pollen.
Pollen sacs
Pollen
Angiosperm life cycle
Pollen development (Microsporogenesis)
The outer layer of the pollen grain
is called the exine. The exine
provides protection for the pollen
grain.
The exine tends to be smooth in
wind-pollinated plants and rough or
spiked in insect-pollinated plants.
Hibiscus pollen with a rough exine indicating
that the pollen is carried by insects.
Angiosperm life cycle
Pollen development (Microsporogenesis)
A mature pollen grain typically contains two nuclei; one generative
nucleus and one tube nucleus.
The outer surface of the pollen grain has an outer exine and inner
intine interrupted by several pores.
Pore
Pore
Tube
nucleus
Tube
nucleus
Intine
Generative
nucleus
Exine
Pore
Generative
nucleus
Angiosperm life cycle
Pollen germination
The pollen tube will exit (germinate) through one of the pores.
Generative
nuclei
Tube
nucleus
Pore
Pollen
tube
Tube
nucleus
Generative
nuclei
Angiosperm life cycle
Pollen germination
The pollen tube moves
down the style towards
the ovule.
The journey may be
short, less than ½ an
inch in beet; or it might
be a long distance (over
5 to 15 inches) as in
lily or corn.
Pollen
Stigma
Style
Pollen
tube
Generative
nuclei
Tube
nucleus
Angiosperm life cycle
Pollen germination
The tube nucleus acts to
guide the pollen tube, while
the generative nuclei will
eventually fuse with female
egg cells.
The pollen tube enters the
micropyle (a natural opening
between the integuments)
releasing the generative
nuclei into the embryo sac.
Pollen
Pollen tube
Micropyle
Integuments
Embryo
sac
Angiosperm life cycle
Ovule formation
Embryo sac development (Megagametogenesis)
In the angiosperm flower, the female gametophyte consists of nucellar
tissue that is surrounded by either a single or a double outer tissue layer
called the integuments. The integuments will become the seed coat.
Integuments
Integuments
Nucellus
Nucellus
Angiosperm life cycle
Ovule formation
Embryo sac development (Megagametogenesis)
In the nucellus, a megaspore mother cell forms that will undergo
meiosis and become the female egg cells within the egg sac.
Outer
Integument
Inner
Integuments
Megaspore
mother cell
Nucellus
Angiosperm life cycle
Ovule formation
Embryo sac development (Megagametogenesis)
Ovule development over time. The integuments grow to cover the
nucellus and continues to enclose the embryo sac creating the ovule.
Funiculus
Nucellus
Integuments
Micropyle
Angiosperm life cycle
Ovule formation
Embryo sac development (Megagametogenesis)
Embryo sac
Integuments
Integuments
Micropyle
Nucellus
Megaspore mother cell
forms in the nucellus.
Funiculus
Megaspore mother cell
undergoes meiosis.
Haploid cells form in the
embryo sac.
Angiosperm life cycle
Ovule formation
Embryo sac development (Megagametogenesis)
Ovules vary in their orientation and shape in the ovary. Three common
types include orthotropous, anatropous, and hemianatropous.
Ovary
Integuments
Ovule
Egg sac
orthotropous
anatropous
hemianatropous
Angiosperm life cycle
Ovule formation
Embryo sac development (Megagametogenesis)
Soon after the
completion of meiosis,
the egg sac is formed.
Funiculus
Ovary
Ovule
A gap is retained
between the enveloping
integuments called the
micropyle.
This is the opening
where the pollen tube
will enter the embryo
sac.
Micropyle
Integuments
Egg sac
Nucellus
Angiosperm life cycle
Ovule formation
Embryo sac development (Megagametogenesis)
Nuclei in the embryo sac from by meiosis. In meiosis I, there is an
initial cell division to give two cells with diploid nuclei.
At the end of meiosis II, there are four linear haploid nuclei formed.
Only one nucleus survives to duplicates to form the archegonia in
gymnosperms or the contents of the embryo sac in angiosperms.
Angiosperm life cycle
Ovule formation
Angiosperm life cycle
Ovule formation
Embryo sac development (Megagametogenesis)
In angiosperms, the most common arrangement of
cells in the embryo sac is called the Polygonum type
and occurs in about two-thirds of flowering plants
Angiosperm life cycle
Ovule formation
Embryo sac development (Megagametogenesis)
The Polygonum type of embryo
sac has seven cells (eight nuclei)
that occupy specific locations
that dictate their function.
Egg sac
Angiosperm life cycle
Ovule formation
Embryo sac development (Megagametogenesis)
Egg sac organization in lily
Andipodals
Central cell
polar nuclei
Egg and
synergids
Angiosperm life cycle
Fertilization
In angiosperms, sexual
reproduction involves double
fertilization.
Once the pollen tube enters the
micropyle it releases the male
nuclei into the embryo sac.
For double fertilization, one
male and one female egg nuclei
fuse to form the zygote and a
second male and two female egg
nuclei fuse to form the triploid
endosperm.
Pollen
Pollen tube
Micropyle
Integuments
Embryo
sac
Angiosperm life cycle
Fertilization
The female synergid cells are closely associated with the egg cell
and function to attract and guide male nuclei to the egg cell for
fertilization. Synergids produce a chemical that attracts the
pollen tube to the micropyle, arrests its growth, and ensures the
proper release of the sperm cells into the ovule.
Angiosperm life cycle
Fertilization
The exact function of antipodal cells is not completely
understood, but they disintegrate soon after fertilization
of the egg cell.
Angiosperm life cycle
Fertilization
The pollen tube enters the ovule through the micropyle and
deposits the two male nuclei into the embryo sac.
Female egg
nuclei
Embryo
sac
Pollen tube entering
the ovule
Angiosperm life cycle
Fertilization
Double fertilization
One generative nucleus
fuses with the egg cell to
form the zygote (2n
embryo)
Embryo sac
Central cell
Egg cell
The second generative
nucleus fuses with the
central cell and its two
polar nuclei to form the 3n
endosperm.
Double fertilization in lily.
Angiosperm life cycle
Fertilization
Most seeds of angiosperms mature
to include three basic parts.
The diploid embryo from the
fusion of male and female
gametes.
The triploid endosperm from the
fusion of one male and two female
gametes.
The diploid seed coat developed
from the integuments derived
from diploid maternal tissue.
Seed coat
Embryo
Endosperm